EU ACCENT Plus and ICACGP Workshop...ACCENT Plus has focussed on some key selected issues of...

PRESCRIBE workshop Bremen, May 15 –16, 2013 1 / 66

Report from the

EU ACCENT Plus and ICACGP

Workshop

Pollution studied by REmote Sensing

of Conurbations/urban conglomerations/

megacities

and Retrieved from observations

made by Instrumentation

on space BasEd platforms

-

PRESCRIBE

Bremen, May 15 –16, 2013


Preface

ACCENT Plus (Atmospheric Composition Change: the European NeTwork-Policy Support and Science)

is an Instrument within the European Union, EU Framework Programme 7, FP7, being a coordination

and support action. It ran from 2010 to the end of 2014 and follows on its successful and larger

predecessor the ACCENT (Atmospheric Composition Change: the European NeTwork) project.

ACCENT Plus has focussed on some key selected issues of particular significance for environmental

policymaking.

iCACGP (international Commission on Atmospheric Chemistry and Global Pollution) is one of the

Commissions in IAMAS (International Association of Meteorology and Atmospheric Sciences), which

in turn is one of the associations within IUGG (International Union of Geodesy and Geophysics)

under the non-governmental ICSU (International Council for Science) family. iCACGP was initiated in

the international Geophysics year in 1957. iCACGP promotes research on chemistry and the

composition of troposphere related to global pollution and climate change. It aims to initiate,

facilitate research programs which by necessity require international cooperation and collaboration.

It co-sponsors the international research project IGAC (International Global Atmospheric Chemistry)

together with the International Geosphere- Biosphere Programme (IGBP), and the international

SOLAS (Surface Ocean Lower Atmosphere Study) together with IGBP (International Geosphere

Biosphere Programme), the Scientific Committee on Oceanic Research (SCOR) and the World Climate

Research Programme (WCRP). SOLAS and IGAC are currently in the process of migrating to Future

Earth. The latter is a new transdisciplinary research initiative building on the Earth System Science

Partnership (ESSP), which under the auspices of the International Council for Science (ICSU)

addressed, the integrated study of the Earth System, the ways that it is changing, and the

implications for global and regional sustainability.

The rapid growth of population since the industrial revolution and in particular since the second

world war has the growth of conurbations, urban conglomerations and mega cities, or major

pollution centres, MPC, coupled with an increasing standard of living. MPC are an increasingly

important source of global air pollution. The population is predicted to reach around 10 Billion with

75% living in urban areas by 2050. Our knowledge and understanding of the impact of MPC on air

pollution and feedback with climate change is not adequate. An adequate knowledge of the

emissions of key pollutants and their precursors by MPC is a pre requisite to improve our

understanding of the processes, which determine the transport and transformation of pollution

within the troposphere, and provides an important part of the evidence base required for

policymaking. The remote sounding of trace atmospheric composition from space based platforms

provides a unique and only feasible approach to deliver global comparable knowledge about the

tropospheric trace constituents yield in top down estimates of emissions from MPC and subsequent

transport and transformation.

The development of and use of remote sensing to quantify atmospheric trace constituents and

assess both natural phenomena and tropospheric pollution has been one of the milestones of the

past two decades in atmospheric research for the provision of the data for numerical environmental

and climate predictions. This began in Europe with the preparation form 1984 to 1988 of the

SCIAMACHY (SCanning Imaging Absorption spectrometer for Atmospheric ChemistrY) project and

subsequent successful proposal, submitted in July 1988, for a passive solar remote sensing


instrument for the ESA’s first Polar Orbiting Earth Observation Mission, POME-1: the research part of

the later renamed Envisat. The spin off originally called SCIA-mini was selected for ERS-2 and after

descoping became GOME (Global Ozone Monitoring Experiment), which began measurements on

board ERS-2 after its launch in 1995. SCIAMACHY flew aboard Envisat, which operated successfully

from 2002 to 2012. Subsequently GOME-2 was developed and flies on the EUMETSAT/ESA Metop

series: the first being launched in 2006 and the second in 2012. The successful retrieval of cloud and

aerosol parameters from space based platforms also developed rapidly over the past two decades

using a variety of instruments. The nadir sounding thermal infrared instruments such as the AIRS

(Atmospheric Infrared Sounder), on NASA AQUA in 2002 and later the IASI (Infrared Atmospheric

Sounding Interferometer) on MetOp series have as their primary objective the measurement of the

temperature but also yield some unique trace gas data products from the mid and upper

troposphere. Driven by the need to observe the diurnal variation of pollution, and the global

tropospheric composition, the field is growing. New missions are planned e.g. the realisation of the

GeoSCIA concept and first geostationary the EU Copernicus/ESA/EUMETSAT Sentinel 4 and the

follow on to GOME-2, the EU Copernicus/ESA/EUMETSAT Sentinel 5 and the ESA Sentinel 5

Precursor.

It was therefore very appropriate and timely that ACCENT Plus as a European contribution to the

iCACGP, commission a workshop with the title: “Pollution studied by REmote Sensing of

Conurbations/urban conglomerations/ megacities and Retrieved from observations made by

Instrumentation on space BasEd platforms – PRESCRIBE. This workshop was organised at the

University of Bremen for ACCENT Plus and was attended by an international team of scientific

experts, taking place on the 15 and 16th May 2015. The organisation team led by myself and

Andreas Richter had key support work from Geraldine Schmiechen, Petra Horn, Heiko Schellhorn,

Enno Peters, Folkard Wittrock and Lars Jeschke at IUP-UB Bremen and the ACCENT Plus project

office. The report was collated and written by Andreas Richter and I with contributions from all

PRESCRIBE participants and support from Folkard Wittrock, Enno Peters, and Geraldine Schmiechen

from the Institute of Environmental Physics/Institute of Remote Sensing of the University of Bremen.

My thanks go to all who worked hard to facilitate the smooth running of the PRESCRIBE meeting

and the report. The resulting report is a uniquely valuable and comprises a status reviewing the

progress made in remote sensing the pollution from megacities and making proposals for the future.

John P. Burrows Bremen 31st December 2014


Executive Summary

As one of the key activities of ACCENT Plus and as a European contribution to the International

Commission on Atmospheric Chemistry and Global Pollution, ICACGP, a workshop was held in

Bremen on the 15th and 16th of May 2013 with the title: “Pollution studied by REmote Sensing of

Conurbations/megacities and Retrieved from observations made by Instrumentation on space BasEd

platforms – PRESCRIBE”. The objective of the PRESCRIBE workshop was to establish the status of

current and planned measurements from space, the development of retrieval algorithms and their

data products, as well as our understanding of the needs for the research community and

policymakers. At the workshop, the current status of research was reviewed and the successes were

assessed. Some key requirements, which will guide the future evolution of space based observations

of pollutants for numerical environmental prediction, monitoring environmental and climate change,

and the verification of international environmental policy, were established. Specifically, the

following was reviewed:

the relevant observational capabilities available in the past, present and in the near future

from planned missions;

some outstanding achievements thus far in terms of measurement and quantification of

the outflow and increasing emissions of pollutants and trace constituents (gases and

aerosols);

the current state of pollution / tropospheric chemistry observations from space

instrumentation;

the use of satellite data for attribution of pollution sources and their changes;

the potential global and regional impacts resulting from further industrialisation,

urbanisation, and land use change etc.

Highlights from some of the recent retrieved data products and applications were presented. It was

recognised that the past three decades have been a golden age of development of passive and

active remote sensing of atmospheric constituents. The challenge now is to achieve an adequate fit

for purpose global observing system. In principle the ground work and definition of needs has been

developed under the auspices of the WMO. The main challenge is to achieve an adequate temporal

sampling at adequate spatial resolution. In this context there is a clear need for new satellite

platforms driven by the scientific needs and for the development of the new and improved remote

sensing instrumentation required for future generations of observation systems. The latter are

needed for example to meet the objectives of the EU Copernicus programme. In particular the

recent establishment in late 2014 by the EU of the next phase of its Copernicus Atmospheric

Monitoring Service and the Climate Change Service, which are to be managed by ECMWF, is an

important step. In this context the specific use of the International space station, ISS, as an

international Atmospheric Observatory from the ISS, iAOBISS, was recommended. With respect to

the observation of the emissions from megacities, urban conurbations and agglomerations, the

group identified that the current and planned systems with their limited spatial resolution and

sampling have significant limitations. In summary much higher spatial and temporal sampling are

required for the next generation of instrumentation. New scientific missions and use of platforms

such as the ISS are required to demonstrate the capability of high resolution measurements.


Table of Contents

1 Introduction......................................................................................................................................... 7

1.1 The pre-industrial evolution of atmospheric composition and the biosphere ...................... 7

1.2 The early use of fossil fuels by mankind and the first air pollution ..................................... 10

1.3 Population growth .............................................................................................................. 10

1.4 Impact of industrialisation on agriculture and emissions from agriculture ......................... 11

1.5 Impact of industrialisation on the troposphere and air quality ........................................... 11

1.6 Impact of industrialisation on the upper atmosphere ........................................................ 13

1.7 Impact of industrialisation on climate, biodiversity and desertification ............................. 13

1.8 The ACCENT Plus Project and the PRESCRIBE Workshop .................................................... 14

2 Objectives and Scope of PRESCRIBE ........................................................................................... 17

3 Instrumentation for space-borne observations of megacity pollution ........................... 18

3.1 Recent Scientific Highlights ................................................................................................. 18

3.1.1 Carbon Monoxide ........................................................................................................ 18

3.1.2 Nitrogen dioxide .......................................................................................................... 19

3.1.3 Sulphur Dioxide ........................................................................................................... 21

3.1.4 VOCs and OVOCs ......................................................................................................... 21

3.1.5 Ozone (O3) ................................................................................................................... 22

3.1.6 Methane (CH4) ............................................................................................................. 23

3.1.7 Carbon Dioxide (CO2)................................................................................................... 24

3.1.8 Aerosols....................................................................................................................... 26

3.2 Optimising the observing system ........................................................................................ 29

3.2.1 Definition of needs for an integrated global observing system ................................... 30

3.2.2 The evolution of European GMES/Copernicus and the Sentinels ................................ 36

3.2.3 The UVN instrument Sentinel-4 and MTG-IRS ............................................................. 38

3.2.4 The Sentinel 5 Precursor ............................................................................................. 40

3.2.5 Sentinel-5 and IASI-NG ................................................................................................ 41

3.2.6 CarbonSat .................................................................................................................... 41

3.3 Geophysical Validation of Satellite Data ............................................................................. 42

3.4 Scientific Exploitation of the Sentinel Programme and beyond .......................................... 44

3.5 Scientific Missions ............................................................................................................... 44

3.6 Platforms ............................................................................................................................. 45

3.6.1 The International Space Station .................................................................................. 45

3.6.2 Unmanned Aerial Vehicles (UAV) ................................................................................ 46


4 Current achievements of remote sensing of areas of high population density and

their emissions from space and our interpretative capability ................................................... 46

4.1 Forward Modelling .............................................................................................................. 46

4.2 Data Assimilation ................................................................................................................ 47

4.3 Inverse modelling ................................................................................................................ 48

4.4 Identifying gaps - making recommendations for the way forward ..................................... 52

4.4.1 Model improvements .................................................................................................. 52

4.4.2 Specific modelling needs from satellite data products ................................................ 53

5 Conclusions ........................................................................................................................................ 55

6 References .......................................................................................................................................... 56

7 Agenda of the PRESCIBE Workshop............................................................................................ 63

8 Participants of the PRESCRIBE workshop ................................................................................ 66


1 Introduction

1.1 The pre-industrial evolution of atmospheric composition and the

biosphere The earth is approximately 4.5 billion years old with the first single cell life resembling stromatolites

and possibly appearing as early as about 3.5 billion years ago. The composition of the earth’s

atmosphere, prior to its having a biosphere was very different from the current atmosphere. The

development of the biosphere resulted in the release of molecular oxygen, O2, by photosynthesis,

and molecular nitrogen, N2, and nitrous oxide, N2O by bacteria. This changed the bulk atmospheric

composition and is clear after 2.7 billion before present. The release of O2 resulted in its photolysis

by vacuum ultraviolet radiation, the production of O-atoms and the formation of ozone, O3. The

absorptions of short wave ultraviolet solar radiation by N2, the solar vacuum UV by O2 and N2O

above the mesopause at around 85 km, and the biologically damaging short wave solar ultraviolet B

radiation by O3 above the tropopause warms the upper atmosphere. This created the vertical

structure of atmospheric temperature shown in Figure 1. The absorption by O3 creates the

temperature inversion we know as the stratosphere.

Figure 1: The temperature structure of the atmosphere at different latitude bands.

The gas layers protecting the earth’s surface from short wave ultraviolet radiation impacted the

evolution of life. The lack of short wave ultraviolet radiation at the surface of the earth enabled life

to leave the oceans and inhabit the earth with species evolving and disappearing for different

reasons. In the geological record the oldest fossils of the skulls of Homo Sapiens are recent, dating

back to approximately 160,000 years ago. This species dominated both its environment and

contemporaries and as a result began to grow in number. The hunter gatherer culture sustained a

population of 4-10 million about 10 000 years ago. Around this time the Neolithic Revolution took

place. This was characterised by the first permanent settlements being established. This way of life

and culture gave way to that of villages, then towns, later cities and more recently urban

conglomerations and mega cities. Initially the associated change in diet resulted in people becoming

smaller but, as affluence increased, humans grew in both size and population. This resulted in wide

spread land use change and man began significantly to modify the earth’s environment.


The odour associated with electrical discharge was noted by van Marum in the 17 th century. The

origin of this smelly gas was attributed by Schönbein to its being O3 around 1839. Schönbein

developed a chemical detection system, which has similarities to the modern electrochemical cells

used in balloon sondes for O3 measurements but used colour change as its metric.

2KI + O3 + H2O 2KOH + O2 + I2

I2 + starch Blue or Purple colour

This chemical approach enabled tropospheric O3 to be measured well before stratospheric O3 was

discovered. In 1858 André Houzeau (Rouen, France) developed a quantitative method (involving a

mixture of iodine and arsenic) to measure ozone, and discovered that ozone is present in ambient

air. French Chemist Albert Levy used this chemical method to observe the abundance of O3 almost

continuously from 1877 to 1907 at the municipal Observatory of Parc Montsouris in Paris. In 1879,

Marie Alfred Cornu observed a sharp cut-off around 300 nm in the ultraviolet (UV) solar spectrum.

This was closely followed in 1881 by the measurement of the ozone absorption cross section in the

laboratory by Walter Noel Hartley and his recognition that this UV cut-off is produced by the

presence of ozone in the atmosphere. Initially and erroneously, tropospheric O3 was thought to be

healthy. However, for certain diseases and conditions, O3 is still used in medicine.

Following the discovery of winter smog and the importance of aerosol in the late 19th century in

cities and the subsequent discovery of summer smog in the late 1940’s in Los Angeles, our

knowledge of tropospheric chemistry has developed rapidly. The production of O3 in the

troposphere by catalytic cycles involving a) oxides of nitrogen, NOx (NO and NO2), and b) the

oxidation of volatile organic compounds, VOC, was recognised. Similarly the catalytic destruction of

O3 in in remote regions with low NOx by reactions of HOx (OH and HO2) was identified. The

importance of halogens in tropospheric O3 chemistry was initially disputed and remains an

important research topic. They participate in catalytic reactions which deplete O3. In addition In the

case of iodine, higher oxides are formed, which are also acid anhydrides. These hygroscopic

molecules lead to the formation of aerosol and cloud condensation nuclei. The importance of

heterogeneous multi-phase chemistry in the troposphere is now well recognised.

A schematic diagram of the current understanding of tropospheric chemistry is provided in Figure 2.

This describes schematically our understanding of the processes, which create and destroy ozone

and aerosol. These comprise primarily photochemical, gas and multiphase chemical reactions. These

reactions are influenced by changing temperature with, for example, the amount of O3 produced

empirically being found to be proportional to the third power of the change in temperature at the

surface. For this reason typical tropical mid- and high-latitude temperature profiles and the pressure

profiles are provided.


Figure 2: A schematic diagram of the formation and removal of tropospheric O3 and aerosols.

Changes in temperature, which result from the release of short-lived and long-lived climate

pollutants, are changing the dynamics and chemistry conditions within the troposphere and upper

atmosphere. As a result the surface fluxes of species, stratosphere troposphere exchange, and

tropospheric composition and chemistry change. In turn, tropospheric chemistry feeds back and

impacts on climate change by changing the rate of loss of long-lived greenhouse gases, such as

methane (CH4), shorter lived climate pollutants such as aerosols, and the hydrological cycle, in

particular clouds.

The rise of mankind from one minor subspecies to its becoming the dominant animal has resulted in

its polluting and changing atmospheric composition at all scales from the local to the global within

the earth system, which comprises the sun, the earth’s atmosphere and surface. As a result of

industrialisation, intensive agriculture, modern land use and land management practises, and the

exploitation of the oceans, anthropogenic activity is now modifying the earth’s surface, its

ecosystems and its biodiversity on an unprecedented scale. Similarly the world’s oceans, through

water pollution and the current fishing practices, and the cryosphere, through climate change are

being altered.

The global extent and impact of anthropogenic activity on the earth’s atmosphere, environment,

ecosystems and biodiversity is such that earth’s lithosphere is now no longer in the Holocene but

rather in a new geological epoch, defined as the Anthropocene. This term was coined by Stroemer in

the 1980s but since 2000 the meaning and our understanding of this new epoch has been advanced

and popularised by the Nobel Prize winning scientist Paul J. Crutzen (Crutzen 2002).


1.2 The early use of fossil fuels by mankind and the first air pollution The use by mankind of fossil fuels, in addition to biofuels, for domestic heating and cooking and later

the production of metals began after the Neolithic revolution. There is archaeological evidence from

China after approximately 3490 BC, that surface mining of coal and the domestic use of coal was

taking place. An early reference to the use of coal in metalworking is found in the geological treatise

on stones by the Greek scientist Theophrastus (c. 371–287 BC). The earliest known use of coal in the

Americas was by the Aztecs who used coal for fuel and jet (a type of lignite) for ornaments. In

Roman Britain, the Romans were exploiting all the major coalfields except those of North and South

Staffordshire by the late 2nd century BC. While much of its use remained local, a lively trade

developed along the North Sea coast supplying coal to Yorkshire and London. This also extended to

the continental Rhineland, where bituminous coal was already used for the smelting of iron ore.

Air pollution and poor air quality in towns and cities is not a new phenomenon. Complaints were

recorded in the 13th century when coal was first used in London. In 1273 the use of coal was

prohibited in London because of its being "prejudicial to health". In 1306 towards the end of the

reign of Edward I there was a royal proclamation, which prohibited artificers (craftsmen) from using

sea-coal (a soft coal) in their furnaces. In the 16th century, the diarist John Evelyn described air

pollution. Evelyn correctly identified the cause of pollution. His solution was based on an apparently

erroneous assumption that commercial, rather than domestic fires were the chief cause of dirty air.

Evelyn’s solution was to remove all of the shops and industries that burned coal from the city to a

suburban location, and would have created gardens throughout the city and a belt of fragrance

around it. Evelyn’s description and proposals will seem familiar to those who recall London before

the clean air act of 1956.

1.3 Population growth Starting from an estimated 4-10 million people at the Neolithic Revolution, the population grew and

rose to approximately 1 billion people worldwide in 1750. With the industrial revolution came the

ability to exploit fossil fuels for energy use in industry and transportation. As a consequence the

human population, its standard of living and the rate of urbanisation have grown much more rapidly.

Currently the population is increasing at a rate of around 1.14% per year i.e. an average increase of

around 80 million per year. Population growth rates maximised in the late 1960s, reaching 2% per

annum. From a peak of 2.19 percent per year in 1963, the annual growth rate has now halved and is

projected to continue to decline. The United Nations estimate that the rate of growth of population

will become less than 1% by 2020 and less than 0.5% by 2050 and world population is predicted to

stabilize at just above 10 billion persons after 2062 with 75% of humans living in urban areas.

(http://esa.un.org/unpd/wpp/Documentation/publications.htm)

Urbanisation in particular during the Middle Ages, often produced conditions, which are favourable

for the outbreak of diseases such as cholera, typhoid fever and others. These epidemics were

directly related to unsanitary conditions caused by human and animal wastes, and garbage. For

example in 1347, the bacterium Yersinia pestis, carried by rats and spread by fleas, caused the "Black

Death", an outbreak of bubonic plague. Unsanitary conditions provided the perfect environment for

the deadly bacteria to flourish. Plagues and Pandemics have subsequently continued to modulate

the growth of human population. For example the outbreak of the Spanish flu beginning in 1918

killed 50 to 100 million people or 3-5% of the world's population.

http://esa.un.org/unpd/wpp/Documentation/publications.htm


1.4 Impact of industrialisation on agriculture and emissions from

agriculture Agricultural practices have altered dramatically as a result of the industrial revolution. Machines

driven by domesticated animals gave way to those driven by fossil fuel combustion engines. In

addition the availability of cheap fossil fuel driven power facilitated the invention and realisation of

the Haber Bosch process, which fixes nitrogen, N2, with hydrogen, H2, to produce ammonia, NH3.

Haber demonstrated the process in 1909 in the laboratory and Bosch developed a commercial scale

production beginning in 1913. The resultant availability of inexpensive NH3 and also of nitric acid,

HNO3, which is made by several processes including the Ostwald process, which uses NH3, resulted in

the wide spread use an inexpensive source of ammonium nitrate, NH4NO3, as a fertiliser. In soils

fertilisation results in the release of NH3, the oxidation of ammonium ions, NH4+, and the reduction

of nitrate ions, NO3-. This leads to significant and important surface fluxes of the long lived pollutant

nitrous oxide, N2O, and the short lived pollutant NO as well as nitrogen, N2, to the boundary layer.

The development of pesticides by the modern agro-chemical industry also relied on cheap fossil fuel

power. Initially in a "first generation", compounds, such as arsenic and hydrogen cyanide pesticides

were used. As they were either too ineffective or too toxic, their exploitation was limited. The

"second generation" pesticides include synthetic organic compounds. One of the most infamous

pesticides which are produced commercially, is 1,1,1-trichloro-2,2-di(4-chlorophenyl)ethane,

C14H9Cl5 better known as DDT and was discovered in 1939 by a Swiss chemist Paul Muller. DDT is

toxic to a wide range of insect pests ("broad spectrum") yet appeared to have low toxicity to

mammal. It was persistent (didn't break down rapidly in the environment) so that it didn't have to be

reapplied often. It was not water soluble (insoluble), so didn't get washed off by rain. It success led

to Muller receiving the Nobel Prize in 1949. However, in 1962 Rachel Carson published her book

“Silent Spring”. This milestone book pointed out the direct and indirect consequences of using DDT,

which led to the demise of birds, mammals and insects around the globe.

The Unite Nations Environmental Programme now defines Persistent Organic Pollutants, POPs, of

which DDT was the first identified member, as “chemical substances that persist in the environment,

bioaccumulate through the food web, and pose a risk of causing adverse effects to human health

and the environment”. With the evidence of long-range transport of these substances to regions

where they have never been used or produced and the consequent threats POPs pose to the

environment of the whole globe, the international community has on several occasions called for

urgent global actions to reduce and eliminate releases of these chemicals.

A co benefit of the availability of cheap energy from fossil fuels has been the combined use of

artificial fertilisers and pesticides, which has enabled mankind to feed its growing primarily urban

population. However it has thereby also contributed to large changes in land use and land use

management practices. The run-off from agricultural areas has polluted the rivers and estuaries.

Overall emissions to the planetary boundary layer have been modified and increased.

1.5 Impact of industrialisation on the troposphere and air quality Following the industrial revolution, fossil fuel combustion became the dominant source of energy for

mankind, being used for power generation, heating, cooking and transport, and releasing both short

lived and long lived pollutants to the atmosphere. Sulphur dioxide, SO2, nitric oxide, NO, which is

rapidly converted by reaction with O3, to nitrogen dioxide, NO2, carbon monoxide, CO, and a variety


of hydrocarbons, are examples of short lived pollutants. Aerosols of different types, comprising black

and brown carbon, are released directly to the atmosphere. They are also produced indirectly

through the oxidation of SO2 to sulphuric acid, H2SO4, and the production of aerosol condensation

nuclei.

The increasing magnitude and affluence of the world population, coupled with the industrial

production delivering its standard of living, have led to increasing emissions of both short lived and

long lived pollutants to the atmosphere. “Smog” was first used to describe the toxic, reducing and

acidic atmospheric conditions, which were found in London and other cities emitting much smoke,

during fog episodes in autumn, winter and spring and is attributed to Dr. Henry Antoine Des Voeux

in his 1905 discussion paper at a health congress. These conditions were first identified in London air

of the 19th century.

The phenomenon of winter smog coupled with the transport and transformation of this pollution

has resulted in acid deposition across the globe. In Europe this led to the acidification of the lakes in

Scandinavia. Recently this type of smog has become prevalent in Asia, where the rapid economic

growth in the past 30 years in both India and China has resulted in poor air quality on

unprecedented scales. A milestone occurred in the United Kingdom where the increase in the

number of deaths and respiratory disease in the winter of 1952 led to the first air pollution control

legislation, the clean air act of parliament in 1956. Similar legislation followed throughout Western

Europe and North America. This ultimately led in Europe amongst other measures to scrub sulphur

dioxide, SO2, from power stations.

The phenomenon of summer smog was first identified in the late 1940s in Los Angeles. Subsequently

this has been observed through the globe. These conditions are oxidising and O3, peroxyacetyl

nitrate, PAN (CH3CO.O2.NO2), and related compounds and aerosols are produced in large amounts.

These are all generally toxic to a greater or lesser extent. O3 maxima are often downwind from urban

areas. O3 impacts negatively agriculture and is estimated to cost Europe about 8 billion € per year in

lost production.

As a result of the use of fossil fuels for transportation by motor vehicles, shipping and aircraft,

previously remote pristine tropospheric regions are being, or have been already, impacted by air

pollution, e.g. the free troposphere, the stratosphere, and the planetary boundary layer above the

savannahs, the boreal and tropical forests, the oceans and the cryosphere. In addition

anthropogenic activities such as biomass burning, deforestation and changes in land management

practice are further impacting on the natural terrestrial eco systems and their ecosystem services.

The growing recognition of the importance of atmospheric pollution and its trans-boundary impacts

led to the creation of the United Nations Economic Commission for Europe, UNECE. Since 1979 the

UNECE Convention on Long-range Trans-boundary Air Pollution, UNECE LRTAP has addressed some

of the major environmental problems of the UNECE region through scientific collaboration and

policy negotiation. This has focused on controlling trans-boundary pollution in the European region

where its parties primarily sit. More recently in 2005 UNECE LRTAP organised its Task Force on

Hemispheric Transport of Air Pollution (TF HTAP). This is an international scientific cooperative effort

to improve our understanding of the intercontinental transport of air pollution across the Northern

Hemisphere. HTAP reports to the Convention's EMEP Steering Body with participation being open to

all interested experts, both inside and outside the UNECE region.


1.6 Impact of industrialisation on the upper atmosphere Surprisingly perhaps the upper atmospheric ozone, O3, which is located some distance from the

surface, is also negatively impacted by human behaviour. In the late 1960s the impact of high flying

aircraft on stratospheric O3 was raised. More recently the increasing release of N2O and methane

(CH4) by human behaviour has been recognised and assessed. For a variety of industrial applications

mankind began to release chlorofluorocarbon compounds, CFCs, (e.g. CFC-11, CF3CL und CFC-12,

CF2CL2) halons (Halon 1211, CF2ClBr, carbon tetrachloride, and methyl bromide, CH3Br. The CFCs and

halons were not previously present in the atmosphere. CH4, N2O, the CFCs and halons all have long

tropospheric lifetime and are transported to the stratosphere. The release of CFCs, Halons, CH3Br

and other ozone depleting substances has caused significant global depletion of stratospheric ozone

and the phenomenon known as the “ozone hole” over Antarctica in spring identified in the last

quarter of the 20th century. This resulted in the United Nations Vienna Convention on Ozone

Depleting Substances and its Montreal Protocol in 1987, which banned the production of ODS. The

parties to the Montreal Protocol have committed themselves to the assessment of the state of

atmospheric ozone by experts selected by WMO and UNEP ozone. The most recent of the ozone

assessments were published by the UNEP and the WMO in 2011 and 2015.

1.7 Impact of industrialisation on climate, biodiversity and desertification Global climate change is now recognised as one of the key scientific issues and societal challenges of

the 21st Century. The greenhouse effect was first discussed by Fourier and Tyndall in the 19th

century. Gases such as water vapour, H2O, carbon dioxide, CO2, CH4, N2O and sulphur hexafluoride,

SF6, which absorb strongly in the thermal infrared but are relatively transparent in the solar spectral

region, comprising parts of the longer wavelength ultraviolet, visible and near infrared, are termed

greenhouse gases. The presence of current amounts of H2O in the earth’s atmosphere results in the

surface temperature on earth being approximately 40 K higher than the temperature of the earth

observed from space. Arrhenius around the turn of the 19th century calculated that a doubling of CO2

would produce an increase of about 4 K at the surface.

After some early inaccurate measurements of the mixing ratio of CO2, accurate ground based

measurements of CO2 at selected locations began in the International Geophysics Year, IGY in 1957.

These were first made at Mauna Loa. The plot of the mixing ratio of CO2 versus time from this site is

known as the Keeling curve in honour of C. D. Keeling who, supported by R. Revelle initiated the

measurements. Later CH4 and a list of other greenhouse gases have been added to the targeted

gases. Highly precise measurements of these species are now made by a sparse in situ measurement

network.

The observed increase of greenhouse gases was considered to be a result of the use of fossil fuel

combustion for energy. This led to scientific discussion and public concern. As a result, the

Intergovernmental Panel on Climate Change, IPCC, was created in 1988. It was set up by the World

Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) to

prepare, based on available scientific information, assessments on all aspects of climate change and

its impacts, with a view of formulating realistic response strategies. The initial task for the IPCC as

outlined in UN General Assembly Resolution 43/53 of 6 December 1988 was to prepare a

comprehensive review and recommendations with respect to the state of knowledge of the science

of climate change; the social and economic impact of climate change, and possible response

strategies and elements for inclusion in a possible future international convention on climate. Today


the IPCC's role is as defined in Principles Governing IPCC Work, "...to assess on a comprehensive,

objective, open and transparent basis the scientific, technical and socio-economic information

relevant to understanding the scientific basis of risk of human-induced climate change, its potential

impacts and options for adaptation and mitigation. IPCC reports should be neutral with respect to

policy, although they may need to deal objectively with scientific, technical and socio-economic

factors relevant to the application of particular policies."

The scientific evidence, described in the first IPCC Assessment Report of 1990, underlined the

importance of climate change as a challenge requiring international cooperation to tackle its

consequences. It therefore played a decisive role in leading to the creation of the United Nations

Framework Convention on Climate Change (UNFCCC), the key international treaty to reduce global

warming and cope with climate change.

The UNFCCC is a “Rio Convention”, i.e. one of three measures adopted at the “Rio Earth Summit” in

1992: the other two being the Conventions on Biological Diversity and to Combat Desertification.

The three are intrinsically linked. It is in this context that the Joint Liaison Group was set up to boost

cooperation among the three Conventions, with the ultimate aim of developing synergies in their

activities on issues of mutual concern. This now also incorporates the Ramsar Convention on

Wetlands. The Convention on Biological Diversity (CBD) entered into force on 29 December 1993. It

has 3 main objectives:

i) The conservation of biological diversity

ii) The sustainable use of the components of biological diversity

iii) The fair and equitable sharing of the benefits arising out of the utilization of genetic

resources.

Established in 1994, UNCCD is the sole legally binding international agreement linking environment

and development to sustainable land management. The Convention addresses specifically the arid,

semi-arid and dry sub-humid areas, known as the drylands, where some of the most vulnerable

ecosystems and peoples can be found. In the 10-Year Strategy of the UNCCD (2008-2018) that was

adopted in 2007, Parties to the Convention further specified their goals: "to forge a global

partnership to reverse and prevent desertification/land degradation and to mitigate the effects of

drought in affected areas in order to support poverty reduction and environmental sustainability".

The UNFCCC entered into force on 21 March 1994. Today, it has near-universal membership. The

195 countries that have ratified the Convention are called Parties to the Convention.

1.8 The ACCENT Plus Project and the PRESCRIBE Workshop Recently a group of scientists have reviewed our understanding of the impact of emissions from

mega cities on the air pollution and climate for the Global Atmosphere Watch, GAW, of the World

Meteorological Organisation, WMO, and the International Global Atmospheric Chemistry, IGAC,

Project of the international Geosphere Biosphere Programme, IGBP, (Zhu et al 2012). The prediction

that by 2050 the earth population will rise to 9-10 billion is coupled with the expectation that the

urban population will then be 75%. Thus the emissions from megacities and urban conglomerations

will increase.

The remote sensing of tropospheric composition from passive and active remote sensing

instrumentation on satellite platforms is particularly challenging. It is a fairly new science and its


evolution has recently been described elsewhere (Burrows et al 2012). Briefly since the dawn of the

space age, scientists have been developing and using remote sensing instrumentation for the study

of the earth’s atmosphere, its surface, and its interior. The development of the remote sensing of

tropospheric gases was initiated in earnest by the proposals of the SCIAMACHY (SCanning Imaging

spectroMeter for Atmospheric CHartographY) Project for ESA Envisat (Burrows et al 1995 and

Bovensmann et al 1999), which later comprised the spin off GOME (Global Ozone Monitoring

Experiment, Burrows et al 1999) on ESA ERS-2, a descoped version of SCIA-mini, and the follow on

GOME-2, which flies on the three EUMETSAT/ESA Metop series of platforms, and the CSA/NASA

Mopitt (Drummond and Mand 1996) for NASA Aura in 1989. These built on the heritage of NASA

TOMS and SBUV, which focussed on stratospheric O3 but were also used for the detection of SO2 and

the Measurement of Air Pollution from Satellites (MAPS) experiment, which flew on STS-2 in

November 1981 and on STS-41G in October 1984 and STS-68 in 1994. Later the OMI experiment,

which spun out of the SCIAMACHY project, was provided by The Netherlands to the NASA AURA.

The nadir sensing instruments AIRS on NASA AQUA and TES on NASA AURA also focussed on

tropospheric retrieval of trace constituents by using thermal infrared radiation. These were later

complemented by the ESA/EUMETSAT operated Infrared Atmospheric Sounding Interferometer IASI

which represents a significant advance in the quality of the measurements injected into

meteorological models but also provides measurements of many trace gases. It uses original

technologies for a new European contribution to polar meteorology.

The successes of SCIAMACHY and GOME and later IASI have placed Europe at the forefront of the

development of atmospheric, and in particular tropospheric, composition measurements from

space. This was complemented by the scientific Project TROPOSAT, essentially a bottom up initiative,

which focussed on the exploitation of the European remote sensing data for tropospheric data

products. This facilitating instrument was supported initially by ESA, and then became part of

European Union, EU, project EUROTRAC. From 2004 to 2010, the Project Troposat 2 became project

within ACCENT (Atmospheric Composition Change European Network) and known as AT-2. This

project facilitated much collaboration on data products and the further development of passive

remote sensing data for use in tropospheric science and applications.

ACCENT brought together the atmospheric science community engaged in global change and air

pollution studies. ACCENT Plus is a smaller effort and aims to reach out to the policy community,

facilitating the transfer of research results into policy/decision making. The success of ACCENT and

AT-2 led to a remote sensing theme becoming part of the ACCENT Plus project. The ACCENT-Plus

project builds on the successful efforts of the EU (Network of Excellence).

ACCENT Plus has several motivations to hold a focussed workshop on the influence of megacities on

tropospheric chemistry, air pollution and science, which are listed below:

1) The need for continuity and evolution of the earth observation system.

In general the success of the first pioneering phase of the remote sensing of

tropospheric constituents from space has highlighted that the lessons learned need to

be documented and an optimisation undertaken for the development of a global

observation system.


2) The re-evaluation of the needs for global measurements of atmospheric composition.

As articulated in for example the CEOS (Committee on Earth Observation Satellites) -

IGOS (Integrated Global Observing Strategy) – IGACO (Integrated Global Atmospheric

Chemistry Observations) document

http://www.eohandbook.com/igosp/Atmosphere.htm), these needs aimed at the

provision of data for chemical weather and numerical environmental prediction, require

re-evaluation. This evolving measurement system is required to provide

a. the data needed by scientists to understand better the earth system and its

response to change in the anthropocene

b. an improved predictive capability of our earth system models, with respect to air

quality, tropospheric pollution, and climate change

c. the evidence base for the development of international environmental policies

designed to minimise the impact of man and help to achieve sustainable economic

activity.

3) The recognition of the role of urban conglomerations and megacities in global pollution,

as highlighted for example by the recent EU research project CityZen, which was a 3-year

research project focusing on the impacts of megacities and emission hot spots. It ran

from September 2008 to August 2011, including the preparation of some policy

documents and was funded like ACCENT by the European Commission through

Framework Programme 7.

4) The end of a pioneering age and the need for transition to an evolving and innovative

operational global observing system. A pioneering age of discovering how to make

atmospheric observation from space is coming to an end. Technologies have been

successfully tested and the first global decadal or longer data sets of atmospheric

composition have been made.

5) Future measurements

It has long been recognised that the requirements for the measurement of tropospheric

composition include the observation of diurnal variation. The GeoSCIA and GeoTROPE

concepts and proposals, which were developed between 1997 and 2005, in response to

the calls for the ESA Explorer Missions and national German missions will now be

realised in part through the EU/EUMETSAT/ESA Sentinel 4 on the EUMETSAT Meteosat

Third Generation from 2018 onwards. The in orbit configuration will consist of two

parallel positioned satellites, the MTG-I imager (a 3-tonne satellite with 16 nominal

channels) and the MTG-S sounder. MTG-I satellites will fly the Flexible Combined Imager

(FCI) and an imaging lightning detection instrument the Lightning Imager (LI). The MTG-S

will include an interferometer the Infrared Sounder (IRS), with hyper-spectral resolution

in the thermal spectral domain, and the Sentinel-4 instrument, the high resolution

Ultraviolet Visible Near-infrared (UVN) spectrometer. Technologies continue to evolve in

particular in the big data aspects.

6) Recent loss of atmospheric observations and plans to evolve the system

The sudden and unexpected loss of ESA Envisat in April 2012 means that there will be a

significant lack of data over the next decade. The European nadir remote sensing from

space in the near infrared and shortwave infrared spectral regions from 800 to 2400 nm

and limb remote sensing in the solar and thermal infrared are lost with currently no

follow-on planned. The NASA AURA is now well beyond its planned lifetime. This shows

http://www.eohandbook.com/igosp/Atmosphere.htm


how little redundancy exists in the current generation of sensors and the shortcomings

of the plans of the space agencies for atmospheric trace constituent measurements.

For the above reasons a dedicated meeting to review the previous and current capability and to

assess the needs for the measurement of the impact of evolving mega cities and urban

conglomeration in the next phase of the anthropocene was required. As part of the European Union,

EU, sponsored network of excellence called ACCENT Plus Project and scientifically sponsored by the

international Commission on Atmospheric Chemistry and Global Pollution (iCACGP also abbreviated

CACGP), as a European led ICACGP activity, a workshop was held in Bremen, Germany on the 15th

and 16th of May 2013, entitled “Pollution studied by REmote Sensing of Conurbations/megacities

and Retrieved from observations made by Instrumentation on space BasEd platforms – PRESCRIBE”.

2 Objectives and Scope of PRESCRIBE The objective of the PRESCRIBE workshop was to establish the status of current and planned

measurements, the development of retrieval algorithms and their data products, as well as our

understanding of the needs for the research community and policymakers.

The Workshop built on three decades of international efforts to understand the impact of megacities

on tropospheric chemistry, and specifically on remote sensing research, which has pioneered the

study of tropospheric trace constituents and aerosols from space. The latter comprises the initiation

and development of TOMS, MOPITT, GOME, SCIAMACHY, GOME-2, IASI, OMI, TES, SeaWiFS, MERIS,

MODIS, POLDER, ATSR-2, AATSR, small satellite constellations and the use of their data products. It

also includes the work done in a series of successful European research projects including

TROPOSAT, ACCENT TROPOSAT II, CITYZEN and MEGAPOLI.

The scope of the PRESCRIBE covered both existing measurement and retrieval capabilities and the

needs for a fit for purpose future observing system. At the workshop, the current status of research

was reviewed, the successes were assessed. Some key requirements, which will guide the future

evolution of space based observations of pollutants of both numerical environmental prediction,

monitoring environmental and climate change, and the verification of international environmental

policy, were established. Specifically, the following were reviewed:

the relevant observational capabilities available in the past, present and in the near future

from planned missions; some outstanding achievements thus far in terms of measurement

and quantification of the outflow and increasing emissions of pollutants and trace

constituents (gases, aerosol, and cloud);

the current status of pollution / tropospheric chemistry observations from space

instrumentation;

the use of satellite data for attribution of pollution sources and their changes

the potential global and regional impacts resulting from further industrialisation,

urbanisation, land use change etc.

In the following sections, a brief summary is given of the main outcomes of the workshop. All

presentations given at the workshop can be found at

http://www.doas-bremen.de/prescribe_2013.htm

http://www.doas-bremen.de/prescribe_2013.htm


3 Instrumentation for space-borne observations of megacity pollution

Over the last three decades the science and technology of space based remote sensing for

atmospheric observations has been transformed. Early in this period, the notion that we had the

capability to study the lower atmosphere was treated with suspicion and sensors were optimized to

study the stratosphere. Since then, instruments for studying the troposphere have been launched

and have seen widespread validation resulting in an evolving observing system with capabilities for

studying some of the most pressing issues facing humankind, especially climate change and air

pollution. In addition, we are developing the capacity to address fundamental science questions

about the composition of atmosphere, for example, describing and explaining the role of lightning

and convection on the distribution of gases and aerosol or understanding other natural sources of

organic molecules and nitrogen oxides.

With improving spatial resolution and coverage, data from these sensors is becoming applicable to

studies of large urban areas and their specific problems of pollution and air quality. However, much

higher temporal and spatial sampling than currently available is required to constrain adequately our

knowledge of the emissions from such regions.

In the following we first describe some of the scientific highlights obtained using existing space

based sensors in combination with a multifaceted ground-based and aircraft observing program and

a suite of modelling tools ranging from simple conceptual models to the most sophisticated coupled

chemistry climate models available today. We then discuss opportunities for research and needs for

the development to produce new scientific breakthroughs and new day-to-day operational

capabilities in the future. We emphasize the role of megacities since urban centres are the places,

where most of humanity is now living and are the locations responsible for the majority of human

emissions but we also include significant advances in our understanding of the global background

and its variability for context.

3.1 Recent Scientific Highlights

3.1.1 Carbon Monoxide

The MOPITT instrument on board the Terra satellite launched in 1999 has provided scientists and

policy-makers with a long-term (14+ years) dataset on carbon monoxide. From this dataset we have

come to understand the global aspects of pollution fuelled by local sources. In the case of carbon

monoxide these sources, principally uncontrolled combustion in fires, are somewhat random in

both space and time making each annual cycle unique and prediction, other than in very general

terms, very difficult. Using the significant lifetime of carbon monoxide in the atmosphere which

allows tracking of plumes over large distances, we have visualised the transport of pollution

between continents which has highlighted the need to include transport in and out of study regions

for any regional pollution studies. Trend analysis has been enhanced by the care taken with both

calibration within the instrument and validation using external comparisons such as in situ aircraft

measurements. The MOPITT data set is still being extended and is complemented and continued by

observations from the European IASI instrument operating on the MetOp satellite series and other

instruments.


On individual observations and under favourable thermal conditions, CO boundary layer

concentrations in and around pollution hotspots can be accurately measured. A typical example

includes IASI observations during the severe pollution event of January 2013 that occurred in the

North China Plain showed high concentrations of key trace gases including CO (see Figure 3 – left).

By examining the averaging kernels (AK) within the plume (see Figure 3 – right), which represents

the IASI sensitivity to a given altitude, we clearly see that IASI measurements are more sensitive to

the surface than to the FT (see red curve), which is due the presence of a large negative thermal

contrast (−10 K). These findings demonstrate the ability of thermal infrared instrument such as IASI

to monitor boundary layer CO, which can support air quality evaluation and management.

Figure 3: (left) Spatial distribution of IASI nighttime retrievals of CO total columns on January 12th, 2013. (right) IASI averaging kernels (AK) obtained for a pixel located in the pollutant plume. The colored dots correspond to the retrieval altitude levels. From Boynard et al. (2013).

3.1.2 Nitrogen dioxide

NO2 observations from space performed over the last 16 years have not only provided detailed

global spatial distributions of this short lifetime marker for pollution but also shown rapid and

systematic changes in pollution levels over just a few years. Analysis of measurements from GOME,

SCIAMACHY, OMI, and GOME-2 shows strong and significant increases over China and the rest of the

emerging world (Asia, India, Middle East, Northern Africa, Central and Southern America). While NO2

columns over eastern China have more than tripled since 1996, most emission inventories fail to

show an increase in NOx emissions before 2002. This is in contrast to the satellite measurements

(see Fig. 1 in Hilboll et al., 2013 see Figure 4). On the other hand NO2 columns over the developed

world were shown to strongly decline, with decreases over the eastern U.S. reaching 40% relative to

1996. This means that both the effect of economic growth and the success of emission control

legislation can be observed from space. These changes have also been shown to be influenced by

targeted emission control strategies, such as installation of more effective catalytic convertors on

passenger vehicles and introduction of control measures on heavy duty trucks and power plants,

regionally targeted emission reduction measures (such as in Beijing during the Olympics). The

amount of NO2 also changes with the amount and type of fuel used as economies grow or shrink

(e.g. Greek recession period, Vrekoussis et al., 2013) and also technologies for shifting energy

production from coal to natural gas emerge.


Figure 4: Slope of a linear fit to changes of tropospheric NO2 in a combined data set of GOME and SCIAMACHY observations from 1996 – 2011 at 0.25° x 0.25° resolution. From Hilboll et al., 2013.

In some locations NO2 columns show distinct weekly patterns which reveal information on the

relative contributions of different types of anthropogenic sources (e.g. cars vs. trucks) for the region.

Specific identification of individual sources including power plants, international shipping lanes, and

flaring at oil and gas production facilities has been examined. Some preliminary studies have been

performed on investigating diurnal patterns in atmospheric NO2 chemistry and NOx emissions using

instruments in morning and afternoon orbits (SCIAMACHY and OMI), but current platforms cannot

provide a good sampling of the diurnal cycle.

Figure 5: Relative trend of tropospheric NO2 for the period 2002 to 2012 over the world's 66 largest urban agglomerations as derived from SCIAMACHY data. Based on the methodology described in Schneider and van der A (2012).

In combination with atmospheric models, the NO2 data has been inverted to produce spatial maps of

emission strengths by sector and their change over time at resolutions of the order of 1° x 1°. Both

emission estimates and temporal trends have also been derived on city level as shown in Figure 5,

but current sensors do not have enough spatial resolution to fully resolve individual megacities.

More information on the N-cycle is added by the NH3 distributions retrieved from IASI observations

(Clarisse et al., 2009), highlighting the effects of intense agriculture and livestock breeding. While not

specific to megacities, these observations are relevant to understand the overall anthropogenic

impact on the N-cycle.


3.1.3 Sulphur Dioxide

Observations of SO2 from space using TOMS, GOME, SCIAMACHY, OMI, GOME-2 and IASI provide

unique information on SO2 emissions from volcanoes, both from eruptive events and from

degassing. The transport of SO2 can be tracked over time, providing information about SO2 emissions

and lifetime. Using the strong UV absorption, the synergy between UV and IR observations or

backward modelling, vertical profiles of SO2 can be retrieved which can be relevant information in

support of aviation safety during volcanic eruptions.

With respect to anthropogenic emissions, long-term observations (monthly to annual averages) of

SO2 performed with UV sensors also show pollution from large coal fired power plants and smelters

and their change over time. Examples include expanding tar oil mining in Canada, the increasing use

of coal in China and the introduction of flue gas desulphurization, first in Europe and the US and

later in China, leading to significant reductions in SO2 levels. In case of large pollution and favourable

thermal contrast situations, infrared instruments have recently also shown potential for measuring

SO2 from anthropogenic activities. This was demonstrated for instance by measuring daily SO2

concentrations around the industrial complex of Norilsk in the Arctic cycle, exploiting the large

temperature inversions persisting there throughout the winter, or during an intense smog event in

China. With the development of more sophisticated retrieval approaches, it is anticipated that such

observations could be extended to SO2 pollution hotspots around the globe, complementing the

observations from the UV sounders.

3.1.4 VOCs and OVOCs

Volatile Organic Compounds (VOCs) play a significant role in several important environmental issues

such as photochemical smog, the production of ozone (O3) and the secondary organic aerosol (SOA)

formation thus impacting quality of life and human health. Studying VOCs is a particularly difficult

task as these organic compounds are numerous and their chemistry is complex. The oxidation of

both anthropogenic and biogenic VOCs species gives different yields of oxygenated products

(OVOCs) and ultimately carbon monoxide (CO).

Over the last few years, significant progress has been made in retrieving the global distribution of

two of the key members of the OVOC family, namely formaldehyde (HCHO) and glyoxal (CHOCHO)

from four satellite instruments GOME, SCIAMACHY, GOME2 and OMI (Wittrock et al., 2006,

Vrekoussis et al., 2009,2010, Lerot et al., 2010, Alvarado et al., 2014). It was found, for the first time

that HCHO and CHOCHO (see Figure 6) present their highest levels over the tropical and sub-tropical

regions, associated with high biological activity and the plumes from vegetation fires. The regions

with enhanced amounts of HCHO and CHOCHO, the photochemical active hot-spots, are

characterized by a well-defined seasonality with the highest values being observed during the warm

and dry periods. This is another indication of their main source, the biogenic emissions, of primarily

isoprene, and biomass burning from natural and man-made fires. Interestingly, regions influenced by

strong anthropogenic pollution also encounter enhanced amounts of formaldehyde and glyoxal and

notably over China an increase in glyoxal values has been recorded for the period 2003 to 2007

(Vrekoussis et al., 2009).


Figure 6: Multiannual composite map of the glyoxal vertical column densities retrieved from the radiance measurements from the SCIAMACHY instrument. The largest amounts of CHO.CHO are found over the tropical and subtropical latitudes which are characterized by vegetation and fire emissions of volatile organic compounds. Due to the short lifetime of CHO.CHO of about 2–3 h, these high values are expected to originate mostly from regional sources of the precursor VOCs.

Based on modelling studies, Myriokefalitakis et al., (2008) and Fu et al., (2008) found that around

55% of glyoxal is produced from the various biogenic precursors (mainly by isoprene), 20% from

biomass burning, 17% from biofuel use, and 8% from other anthropogenic emissions. More recently,

model simulations of the above measurements revealed a large unknown source of CHOCHO over

China (Liu et al. (2012)). This missing source is most likely caused by substantially underestimated

aromatics emissions in the VOC emission inventories over China used in current regional and global

models.

These data have been used to infer biogenic isoprene emissions, their seasonality and spatial

patterns. For example, the impact of humidity and temperature was analysed during the European

heat wave and long-term changes were identified, linking VOC levels and their changes to

anthropogenic emissions, biomass burning and climate.

Thermal infrared measurements have in addition allowed provided global distributions of methanol

(CH3OH), formic acid (HCOOH) and more recently acetylene (C2H2), which have provided new insights

onto biogenic and pyrogenic emissions (Razavi et al., 2011, Stavrakou et al., 2011, Duflot et al.,

2013). For fire emissions specifically, IASI IR observations of a larger number of VOCs enabled

process studies of the composition and temporal evolution of plumes as they are transported away

from the sources, for example during recent fire events in Greece.

3.1.5 Ozone (O3)

Infrared sounders, such as IASI, are shown to be able to detect the tropospheric O3 column seasonal variation globally as well as around cities since high thermal contrast and thus more information in the boundary layer, is usually associated with the photochemical pollution events (Eremenko et al., 2008).


Figure 7: Seasonal distribution of the IASI 0-8 km tropospheric O3 in selected urban regions. Results are shown for the period 2008-2011. The shaded regions show the minimum and maximum values recorded during this period. (From Safieddine et al., 2013)

Figure 7 shows the seasonal variation of tropospheric (0-8 km) O3 column over nine cities of the Northern Hemisphere for the period 2008-2011. IASI depicts a typical seasonal behavior of tropospheric O3, with a first maximum reached in late spring because of stratospheric intrusion mainly, and a continuous rise till summer because of the anthropogenic based ozone production. Over the East Asian cities (panel b), a decrease in the O3 tropospheric column is detected during monsoon period.

3.1.6 Methane (CH4)

Methane is an important anthropogenic greenhouse gas and contributes to global warming.

SCIAMACHY on ENVISAT permitted to retrieve near-surface-sensitive column-averaged mole

fractions of methane denoted XCH4. As an example, Figure 8 shows annual and seasonal global maps

and time series as a function of latitude. Clearly visible are strong methane source regions such as

China (e.g., rice paddy emissions), Siberia (e.g., wetland emissions) and parts of the US (e.g.,

emissions from natural gas exploitation, coal mining and wetlands). These data can be used to derive

emissions via inverse modelling schemes (e.g., Bergamaschi et al., 2013, and references given

therein). As can be concluded from Figure 8 and similar figures (e.g., Schneising et al., 2011),

methane increases by about 7-8 ppb/yr since 2007 after years of stability. At the end of 2005, a

SCIAMACHY detector in the spectral region used for methane retrieval after being impacted by a

solar proton has much higher noise. This results in higher noise in the CH4 data after 2006. The

reason for the unexpected recent increase after this period have been investigated (Schneising et al.,

2011). According to Bergamaschi et al., 2013, the main reason appears to be increasing

anthropogenic emissions with wetlands and biomass burning emissions being primarily responsible

for significant year-to-year variations.

Since 2009, the SCIAMACHY methane time series is being continued with GOSAT (e.g., Buchwitz et

al., 2013b, 2013c, and references given therein) and in the near future with Sentinel-5 Precursor.


Figure 8: Multiannual composite global maps (top left and right) and latitude-time-series (bottom left) of column-averaged dry-air mole fractions of atmospheric methane, XCH4, and as retrieved from SCIAMACHY on ENVISAT (Schneising et al., 2012).

Recently the emissions of CH4 from fracking in selected sites in North America have been identified

from SCIAMACHY measurements (Schneising et al. 2014b). This article shows that the amount of CH4

lost to the atmosphere (leakage rate) is of the order of 10%.

Figure 9: Image of nighttime lights assembled from data acquired by the Visible Infrared Imaging Radiometer Suite on board the Suomi National Polar-Orbiting Partnership satellite in 2012 overlaid with changes of methane anomalies during the periods 2006–2008 and 2009–2011 over the continuously growing oil and gas production regions Bakken, Eagle Ford, and Marcellus derived from the measurements of the SCIAMACHY satellite instrument on Envisat (Schneising et al., 2014b).

3.1.7 Carbon Dioxide (CO2)

Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas (e.g., Ciais et al., 2014,

and references given therein). SCIAMACHY on ENVISAT permits to retrieve near-surface-sensitive


column-averaged mole fractions of CO2, denoted XCO2. Several algorithms have been developed to

retrieve XCO2 from SCIAMACHY (e.g., Schneising et al., 2011, 2012, 2013, and Reuter et al., 2010,

2011). The corresponding data products have been compared with ground-based retrievals from the

TCCON network, with global models and with GOSAT satellite data (e.g., Buchwitz et al., 2013b,

2013c, Reuter et al., 2013, and references given therein). The SCIAMACHY and GOSAT XCO2 data

products have been used to address a number of scientific applications related to natural (e.g.,

Schneising et al., 2014a, and Basu et al., 2013) and anthropogenic (e.g., Schneising et al., 2013,

Figure 10) applications. An overview about major achievements is given in Buchwitz et al., 2013c.

The SCIAMACHY/ENVISAT time series ends early April 2012 with the loss of ENVISAT. Currently (since

2009) the SCIAMACHY XCO2 time series is being continued with GOSAT and with OCO-2, launched in

2014 and hopefully later with CarbonSat (Bovensmann et al., 2010, Buchwitz et al., 2013a).

Figure 10: Regional maps of SCIAMACHY XCO2 (left) for three major anthropogenic source regions (from top to bottom: Central Europe, US East Coast, China) compared with anthropogenic CO2 emissions from EDGAR (v4.2, middle). On the right, the corresponding regional enhancements are shown for individual years during 2003-2009 (black: SCIAMACHY, red: EDGAR). The enhancement trend is shown at the bottom. As can be seen, the trends derived from EDGAR agree with the satellite data within the error bars of the satellite retrievals. As can also be seen, the CO2 trend is close to zero for Europe and the US East Coast but approximately +10%/year for the Yangtze River Delta region in China (from: Schneising et al., 2013).

Recently Reuter et al 2014 could show that the ratio of NO2 to CO2 has changed in recent years over

the Beijing Mega city region. This is attributed to changing NOx to CO2 ratio in the fossil fuel

combustion. This observation is most likely explained by changing traffic and power station


technologies with perhaps lower NOx emissions. In the same manuscript the interesting observation

was made that whilst North America and Europe show a clear weekly cycle in the amounts of NO2

and CO2, with a minimum on Sundays, in the selected region of East Asia no such weekly cycle is

found.

3.1.8 Aerosols

Satellite observations of aerosol parameters have greatly improved over the last decade, with a

series of instruments using both passive and active techniques. Passive sensors include instruments

designed to observe aerosol and cloud parameters using multi-spectral observations (MODIS)

combined with multiple viewing angles (MISR) and both of these combined with polarization

measurements (POLDER / PARASOL). In addition instruments primarily designed for other purposes

(ocean, land surfaces, etc.) which spectral (MERIS, SeaWIFS) and viewing information (ATSR-2,

AATSR) matching that of the dedicated instruments. Together these instruments span a period of

almost two decades (starting with ATSR-2 in 1995) of daily global aerosol and cloud observations

with a high utility for climate studies, complementing and improving earlier data from AVHRR and

TOMS/OMI which go back to the early 1980’s. Algorithm development has been progressing over

the same time period and satellite-retrieved information has significantly improved. Today, aerosol

products from several sensors are of similar quality in a statistical sense based on validation versus

ground-based observations. For similar overpass times, information from different instruments

could potentially be used to reduce the uncertainties of the measurements from single satellite

instrument. Figure 11 is an example of integration use of several AOD datasets as well as PM2.5 data

to study a series of wildfires broke out in western Russia starting in late July of 2010. The results

show that the PM2.5 concentration is 3–5 times the normal amount based on both satellite data and

in situ values with peak daily mean concentrations of approximately 500 μgm−3. Also, the visibility

of many parts of Russia, even Moscow, was less than 100m; in some areas, the visibility was less

than 50m. Additionally, the possible impact on neighbouring countries due to the long-transport

effect was also analysed during 31 July and 15 August 2010. A comparison of the satellite aerosol

products and ground observations from the neighbouring countries suggests that wildfires in

western Russian have had little impact on most European and Asian countries, the exceptions being

Finland, Estonia, Ukraine and Kyrgyzstan. However, a possible impact on the Arctic region was also

identified; such an effect would have a serious influence on the polar atmospheric environment and

on animals such as polar bears.

Some of these data have been used with different degree of success for trend analysis over

megacities. Progress is being made to use satellite data for measurement-based estimates of the

aerosol direct radiative effect on climate and the effect of assumptions used in the retrievals are

being evaluated. An important issue, and one of the largest unknowns in climate studies, is the

indirect radiative effect of aerosols, i.e. the effect of aerosol particles on cloud radiative properties

and the hydrological cycle. Methods using satellite information are being developed and used to get

a better handle on these. Multiple viewing angles allow for the retrieval of plume height and extent,

such as for forest fires and volcanic ash plumes. The additional polarization information from

PARASOL is being used to develop high-quality aerosol information over ocean and over land,

including information on size distribution and chemical composition which offers unprecedented

possibilities to obtain such information on global scale with spatial resolution similar to that of

current AQ models (on the order of 10 km).


Figure 11: AOD (at 0.55 μm) distribution over the study area for the period from 31 July to 15 August 2010. The AOD was obtained from integration of the AOD retrieved from three different methods as described in the text by Mei et al. 2012

Satellite aerosol remote sensing over urban areas is still a difficult task because of the high

reflectance of the underlying surface. Many aerosol retrieval algorithms are appropriate for ‘dark’

pixels and provide aerosol products with low resolutions. Li et al. (2012) presented a new aerosol

retrieval algorithm that applies the synergetic use of small satellite data and Moderate Resolution

Imaging Spectroradiometer (MODIS) data (see Figure 12). The algorithm was applied to data from

the China HJ-1A/1B of the Environment and Disasters Monitoring Microsatellite Constellation

Charge-Coupled Device (CCD) camera and Terra MODIS data. By applying this algorithm to aerosol

retrieval over Beijing City, they obtained the aerosol optical depth (AOD) with a 100m x 100m


resolution. The algorithm could potentially be useful for other small satellite constellation data.

High-resolution AOD is very useful and powerful for urban air quality monitoring and other

applications.

A drawback of passive sensors is the lack of height information (except for plumes as indicated

above). This gap is in part filled by active sensors (LIDAR), such as CALIOP which, in addition to height

information, also provide information on aerosol type. The disadvantage of LIDAR is the narrow

swath which prevents global coverage at any reasonable time scale and makes it hard to find

collocated measurements with megacities, instruments on different platforms, or ground-based

observations. However, when such collocations occur, a wealth of 3D information is available.

Figure 12: Satellite RGB images and AOD maps over the Beijing area on April 5, 2010. (a) 500 m x 500 m Terra MODIS RGB image (R: Band 2, G: Band 1, B: Band 4); (b) 10 km x 10 km AOD from the MOD04_L2 product; (c) HJ-1 CCD RGB image (R: Band 4, G: Band 3, B: Band 2); (d) 100 m x 100 m AOD retrieved by the model. The regions in (c) and (d) are contained in (a) and (b) and are shown by a red block in (a) and (b). This Figure is taken from Li et al., 2012.

Satellite observations of aerosol properties have been used to provide information on health effects.

This works through a semi-empirical relation between AOD and PM 2.5 which however varies

between different locations. Van Donkelaar et al. (2010) evaluated these relations and used them to

provide global PM 2.5 maps which in turn were related to health effects.


Aerosol information is needed to correct for atmospheric effects on observations of land or ocean

surface properties using UV-VIS-NIR wavelengths, the retrieval of atmospheric trace gases, for

atmospheric chemistry studies (with the stratosphere as an important application area which is hard

to sample with other than satellite instruments). In addition, satellites can provide useful

information on aerosol transport and deposition to land (e.g. black carbon on snow leading to

albedo changes) or ocean (e.g. desert dust deposition contributing to acidification) surfaces. Satellite

observations are used to constrain atmospheric transport models by data assimilation, can be used

to infer source strengths of aerosols with high temporal resolution (as opposed to inventories) or to

provide information on parameters forcing sea spray aerosol production and thus improving

estimates of these particles which govern aerosol direct radiative effects on a global scale as well as

play a role in many other over-ocean processes. An important issue for natural aerosol particles is

that they are often formed from their precursor gases but these new particles are too small to be

observed by electro-optical instruments. Hence proxies are being developed to provide information

on such particles which constitute the natural background in the remote atmosphere over land, such

as over the boreal forest.

In summary, satellite observations of aerosols and their physical and chemical properties have

strongly evolved in the last two decades with instruments launched in Europe and the USA,

accompanied by algorithm development and new and exciting applications providing a wealth of

information contributing to scientific understanding as well as policy-related issues.

3.2 Optimising the observing system In addition to these measurement and analysis highlights, it is important to recognize that we have

learned much about how to build the comprehensive observing system needed to take optimal

advantage of space-based observations, which could then be used for studies of megacities. This

system, developed by combining satellite based observations with a mix of ground-based and

aircraft-based instruments has been essential to the successes described above and will continue to

be key to maximizing return on investments in space-based sensors in the future. The observing

system works best when space-based sensors are combined with a long-term measurement vision

and with focused shorter term experimental campaigns (at the surface and/or from aircraft) that

both provide direct ground truth for evaluation of the satellite observations and contextual data to

help interpret them. It is also essential that sensors with different principles of detection be

employed to identify biases in interpretation of the space based (or ground based) measurements.

However, there also are weaknesses in the currently available space borne observing system,

particularly in view of the increasing needs for monitoring and understanding changing air

composition in large conurbations.

Most importantly, the best spatial resolution of current sensors (~10 km) is not yet adequate for

observations of individual cities and pollution sources, whose spatial scales are ~1km and smaller.

This severely limits our ability to identify and quantify pollution sources and hot-spots as well as

their impact on public health. Technical benefits of higher spatial resolution include an increase in

the fraction of cloud free observations which leads to lower uncertainties and the ability to better

resolve non-linear plume chemistry (for example for NO2).


Temporal resolution is another limitation of the current system – only early morning and early

afternoon measurements are available. Thus it is not possible to constrain the complex interaction

of diurnal variations in emissions, chemistry, and dynamics in heavily polluted environments and

their impact on the population.

Synergies between space instruments covering the IR and the UV/visible spectral ranges have been

demonstrated, but so far mainly in theoretical studies. By better integrating such instruments from

the design phase to operations and data analysis, the theoretical benefits should be realised. The

results will be improved vertical resolution and accuracy. The same holds true for the improvement

of the integration of ground-based and space borne measurement systems. This is often hindered by

the disjointed nature of funding for both types of missions and projects.

Most of the advances in satellite remote sensing over the last two decades have focused on global

and regional aspects of atmospheric science. In the coming decade, new instruments in low earth

orbit and for the first time ones that will be launched to geostationary orbit will provide the spatial

and temporal resolution that will create capabilities to observe cities at an unprecedented level of

detail. These will include the European instruments Sentinel 5-Precursor, Sentinel 4, Sentinel 5, and

the EE8 proposed mission, CarbonSat. The higher spatial resolution and the continuous observations

open the possibility of obtaining new insights into the role of megacities in the chemistry and

climate of the Earth’s atmosphere. There may also be value in tailoring a satellite platform to the

requirements for complete urban monitoring and management. This platform would address issues

of land use, surface temperature, and local meteorology, in addition to atmospheric composition,

climate and local air quality. Careful measurement of the temporally-variable surface properties

which in turn will assist with understanding variations in surface emissions is of key significance to

this new topic-specific mission agenda.

3.2.1 Definition of needs for an integrated global observing system

The development of space based remote sensing of atmospheric constituents and parameters, one

of the areas of earth observation, is a relatively new science. It began with measurements in the

exosphere of the van Allen belts by the first earth observation experiments by NASA in 1958. It has

in part been driven by

a) the operational needs for accurate near real time data for numerical weather prediction.

b) Scientific curiosity of the workings of the earth system.

Over the first four decades following the start of the space age in 1957 with the launch of Sputnik,

exploration of the atmosphere from space had a large technology aspect and was driven by the need

to identify which species and parameters can be measured. It thus often had a technological focus.

The improvement of technology remains a driver, as this enhances the capability of space based

measurements. However, as the result of the rapid growth of human population and its standard of

living in the new geological epoch Anthropocene, a new driver is to understand and assess the

impact of man on the atmosphere, environment, and climate.

There has been a consistency in the requirements given by the user community to the space

agencies, when asked. For example the User Consultation meeting organised by ESA in 1991, the

issues of continuity of data sets and accuracy were clearly raised.


In the past two decades there have been a number of international efforts to define the needs for a

global observing system. One of the first was the WMO GAW report 140 (WMO report 140)

WMO/CEOS REPORT on a strategy for integrating satellite and ground based observations of Ozone

published in January 2001. This dealt for the first time with global requirements for atmospheric

trace gas measurements, focussing on O3 but also taking all relevant trace gases into account. “The

recommendations contained in the report make specific proposals for remedying the missing

components of the upcoming systems. They also describe improvements that are required in existing

systems and current procedures. The following is a summary of these recommendations:

Establish a co-ordinated validation activity that extends over the entire lifetime of satellite

sensors that encompasses all elements of the IGOS system and takes maximum advantage of

concurrent national validation activities.

Extend the coverage of ground-based (WMO-GAW and NDSC) systems particularly in the

tropics and the Southern Hemisphere and designate a carefully selected subset thereof as

permanent, long term ground "truthing" facilities.

The space agencies that require validation data must provide sustained support for the

ground networks to insure data availability and quality.

Improve and/or provide additional measurements resulting from a survey of existing and

planned measurements. There is a particular need for measurements in the lower

stratosphere and troposphere.

The validation process is iterative and resources for reprocessing data must be made

available to ensure that users have access to the highest quality data.

Standardise data formats and encourage the synergistic use of data supported by accessible

archives and proper provision for reprocessing.

Improve national radiometric standards and sensitise the user community to calibration

issues.

Encourage international co-operation in the development of algorithms employed by similar

instruments and pool knowledge of radiative transfer physics.

Establish a body of scientists, engineers and managers to provide technical support to

funding agencies to ensure compatibility and completeness of the systems.

There is also a practical incentive for swift action. Several satellite missions with ozone instruments

on board are scheduled for launch during this decade. The recommendations in this report attempt to

co-ordinate these missions and to remedy those areas that remain deficient in the present and

planned observing systems. Data collected following this approach will have the necessary quality to

enable the state of the atmosphere to be reliably monitored and changes understood, thereby

providing a basis for formulating sound environmental policies.” (WMO report 140)

The Committee on Earth Observation Satellites, CEOS, was established in 1984 following a G7

economic summit of industrial nations, which recognized the multidisciplinary nature of space-based

Earth observations and the value of coordinating international earth observation efforts to benefit

society. One of its activities was the creation of the Integrated Global Observing Strategy, IGOS. This

produced theme documents. IGOS approved the following themes: Global Carbon Cycle,

Geohazards, Ocean Water Cycle and Atmospheric Chemistry. In addition the following were in

preparation: Coastal observations, Coral Reefs Sub-Theme, Land, and Cryosphere. The two most


relevant documents from the theme studies for this report are the atmospheric chemistry and global

carbon cycle.

The report of the atmospheric chemistry theme was entitled “an integrated global atmospheric

chemistry observation theme for the IGOS partnership” and was published by WMO GAW in

December 2004. The objective of this report was “to initiate a process leading towards a globally

coordinated development of future observation and integration programmes, whose components are

either in place or, with careful planning, can be implemented within the next 10 years. The report

identifies the current major societal and scientific issues associated with atmospheric

composition change;

establishes the requirements for observations of atmospheric composition and their analysis,

integration and utilisation;

reviews the existing observational systems, including data processing and distribution, and

validation programmes vis-à-vis these requirements;

proposes an implementation plan to adapt the systems to meet the identified requirements.

The emphasis of the report is on the need for long-duration integrated observations and their societal

and scientific applications. The focus will therefore be on operational systems providing continuity

and reliability, and on setting priorities, in order to establish a technically and programmatically

feasible long-term solution. It should be noted that the report addresses the needs for a global

observation system and the value-added benefit that comes from integration. The schedule for

implementation is divided into short- and long-term actions. One thing is clearly evident: with the

lead times for deploying satellites and for developing ground stations and routine aircraft

programmes, planning for funding and implementation of both stages has to begin immediately if

the aims of the report are to be fulfilled. “

The iGACO report went on to make the following general recommendations:

“GR1 Establishment: an Integrated Global Atmospheric Chemistry Observation System (IGACO)

should be established for a target list of atmospheric chemistry variables and ancillary

meteorological data.

GR2 Continuity: the data products from satellite and non-satellite instruments, which are to be

integrated into a global picture by IGACO, must have assured long-term continuity.

GR3 Management of IGACO: the responsibility for the co-ordination and implementation of the

IGACO should rest with a single international body. International and national agencies responsible

for aspects of IGACO should be committed partners and agree on their appropriate responsibilities.

GR4 Gaps in observational coverage: for each target species and variable, the present gaps in the

current spatial and temporal coverage should be filled by extending the existing measurement

systems.

GR5 Long-term validation of satellite observations: in order to ensure the accuracy and consistency of

satellite measurements, sustained quality-assurance measures, over the entire lifetime of satellite

sensors, are essential.


GR6 Validation of vertical profile data from satellite observations: a set of high-performance

scientific instruments using ground, aircraft and balloon platforms, possibly operated on campaign

basis, must be maintained to provide the crucial validation data.

GR7 Comparability: the ability to merge observations of different types must be ensured by insisting

that appropriate routine calibration and comparison activities linking diverse measurements together

are part of an individual measurement programme.

GR8 Distribution of data: universally recognised distribution protocols for exchange of data on

atmospheric chemical constituents should be established.

GR9 Multi-stake holder World Integrated Data Archive Centres (WIDAC) should be established for the

targeted chemical variables.

GR10 Storage for raw data should be established so that they can be re-interpreted as models and

understanding improve.

GR11 The development of comprehensive chemical modules in weather and climate models with

appropriate data assimilation should be an integral part of IGACO.

GR12 Strong coordination with the meteorological services is essential so that the ancillary

meteorological data, required by IGACO, is accessible.”

The report defined two groups of trace gases and parameters comprising stratospheric and

tropospheric constituents for both remote sensing form space based platforms and also for

measurements by sub orbital aircraft or balloon platforms:

Group 1: H2O, O3, CH4, CO2, NO2, CO, BrO, ClO, HCI CFC-12

Group 2: NO C2H6 CH3Br Halons, HNO3 ClONO2 HCHO SO2, UVA j(NO2) UVB j(O1D).

It did not consider aerosol and cloud although recognising their importance for atmospheric

chemistry. The report went on to make specific recommendations (SR1 to SR7) for the

implementation of IGACO:

“SR 1 Establishment of an IGACO system for selected Group 1 species encompassing data collection,

harmonisation, QA/QC, data archiving and model-based integration.

Aerosol optical properties, stratospheric and tropospheric O3 and water vapour are ripe for

demonstration projects in this regard since many components already exist. For the remaining Group

1 species, observational system gaps are considerably greater and recommendation SR4 should be

implemented first.

SR2 Initiate immediately the planning and implementation of a network of satellite platforms to be

launched in the long term, with consideration of geostationary as well as enhanced low-Earth-orbit

capabilities. To address climate-chemistry interaction in the UT/LS and stratospheric ozone depletion,

high-vertical resolution profiles are obtained, using the limb sounding technique which requires a low

Earth-orbit. Two satellites are sufficient to achieve 12-hour time sampling. For air quality, oxidising

efficiency and climate, observations of the troposphere down to the surface are made. This is

achieved in nadir viewing geometry which provides total-column information or low resolution


profiles. These measurements are required with much better temporal and horizontal resolution than

any existing atmospheric chemistry satellite data.

In the long term, three to four geostationary satellites or, alternatively, a number of polar-orbiting

satellites will be needed. The trade-off between the two options is determined by the performance

with respect to temporal resolution, geographical coverage, horizontal resolution, signal-to-noise

ratio, pointing stability, and necessarily by technical and cost considerations.

SR3 Develop the necessary data-harmonisation, quality/control and data-exchange protocols using

the demonstration projects in SR1.

SR4 Upgrade the missing ground-based (in situ, total column, active and passive profiling, and

balloon sonde) measurements to measure the Group 1 variables, and, where feasible, some of those

from Group 2.

SR5 Develop a sustainable routine aircraft measurement programme based on the considerable

experience obtained in ongoing projects.

Most of the existing aircraft programmes are operated as short-term research projects and have no

secure future. In order to develop the required capacity for a global routine aircraft programme, it is

necessary to expand the fleet of aircraft appreciably, so as to provide global coverage;

measurements are particularly needed for the Pacific and the southern hemisphere.

This requires the development and the certification of smaller instrumentation packages under

aeronautical rules, both for implementation on civil aircraft and for the maintenance in an

operational system. The current suite of measurements should be extended as much as possible, in

particularly for NOx (NO and NO2), aerosol and H2O in the lower stratosphere.

SR6 Develop the necessary algorithms and associated calibration/validation procedures to retrieve

operationally, total-column and vertical-profile concentrations from existing and planned satellites

for as many of the Group 1 and 2 variables as possible.

SR7 Develop chemical transport modelling and data assimilation so as to accommodate data from

the various measurement components.”

The report by the Integrated Global Carbon Observation Theme was entitled “A Strategy to Realize a

Coordinated System of Integrated Global Carbon Cycle Observations” and was finalised in April 2005.

The entire Carbon Cycle is a broad field involving measurements of land, ocean and atmospheric

parameters. For this ACCENT Plus report the focus is on the determination of the surface fluxes of

carbon dioxide, CO2, and methane, CH4, and their atmospheric amounts and distributions.

The objectives and activities of the IGOS Theme teams are now being pursued within the framework

of the Group on Earth Observation, GEO: http://www.earthobservations.org/cop.shtml. Building on

the IGOS theme report, GEO published the GEO Carbon Strategy in 2010. This calls for an Integrated

Global Carbon Observing system (IGCO) to meet pressing needs for policy-relevant scientific

information about the carbon cycle. Carbon observations deserve very special attention because the

increasing concentrations of atmospheric CO2 and CH4 play a central role in driving global climate

change. Carbon cycling is also fundamental to the Earth system because of its intimate coupling

http://www.earthobservations.org/cop.shtml


across the land, oceans and inland waters, and atmosphere domains, and with earth’s climate. As

the nations of the world experience the impacts of climate change and act in response to those

changes, their needs will include observations and monitoring of the effects of their actions – and

the knowledge to distinguish the effects of those actions (“anthropogenic”) from those of other

changes (“natural”) in the system. In no area is this more evident than in global carbon cycling.

Information about carbon cycle changes will be absolutely essential for climate policy development,

implementation, and verification.

The GEO Carbon Strategy clearly explains the limitations of our current knowledge of the global

carbon cycle and explains why improved scientific understanding will be essential to underpinning

societal responses to global climate change. The report unequivocally states that “a key reason for

our lack of understanding of the global carbon cycle is the dearth of global observations,” and calls

for “an increased, improved and coordinated observing system for observing the carbon cycle as a

prerequisite to gaining that understanding.” CEOS recognizes that the GEO requirements for carbon

observations from space are well judged and technically feasible, but challenging in terms of a

complete, sustained and coordinated response.

At its 24th plenary meeting in Rio de Janeiro, Brazil in 2010, CEOS charged its Carbon Task Force

(CTF) to develop a response to the GEO Carbon Strategy, describing the approach CEOS will take in

meeting the GEO requirements for space-based observations of carbon. In response the report

entitled “CEOS Strategy for Carbon Observations from Space: The Committee on Earth Observation

Satellites (CEOS) Response to the Group on Earth Observations (GEO) Carbon Strategy” was

published in 2014. It details the adequacy of past, present, and planned satellite measurements of

carbon in the land, oceans and inland waters, and atmosphere domains to support GEO, and it

identifies important challenges that CEOS must face and actions CEOS and its agencies must take to

meet needs for carbon observations from space. This report was written by an international team of

scientists from a range of research institutions and CEOS agencies that were recruited by the CEOS

CTF. In directly responding to the GEO Carbon Strategy, the authors felt it important to provide

updates on scientific developments and measurement capabilities that occurred since the 2010

publication of the GEO Carbon Strategy and to anticipate the carbon information needs for climate

policy (e.g., United Nations Framework Convention on Climate Change (UNFCCC) and

Intergovernmental Panel on Climate Change (IPCC)). This report also takes account of, and attempts

to be consistent with, the Global Climate Observing System (GCOS) Implementation Plan and its

requirements for Essential Climate Variables (ECVs).

The authors of this report have identified high-priority needs for decisions, resources, and actions

that go well beyond the scope of what CEOS alone can do and that exceed the mandates and current

capacities of many of its agencies. The relevant CEOS Actions recommended are summarized as

follows:

“Ensure the continuity of satellites and established time series data records for carbon-

related measurements of land surface properties, ocean colour and related physical

properties, coastal and inland water properties, and atmospheric column measurements of

carbon dioxide and methane. (5 CEOS Actions)

Develop and deploy new missions to acquire high priority measurements for carbon science

and policy, including new observations to estimate aboveground biomass and its carbon


content, geostationary observations of carbon-containing constituents in coastal ocean

waters, improved resolution ocean salinity measurements, and measurements of

atmospheric carbon dioxide and methane from complementary Low Earth Orbit (active and

passive) and geostationary (passive) satellite constellations. (5 CEOS Actions)

Improve satellite data products, including establishment of standard formats and protocols,

enhanced validation, securing access to essential in situ data, merger of data from multiple

sensors and platforms into enhanced products, and rigorous intercomparison of data

products. (9 CEOS Actions)

Produce new data products from existing missions, including maps of wetlands, inundated

areas and small water bodies, ocean colour products for inland water bodies, ocean carbon

pool products, river discharge and sediments, and anthropogenic emissions of carbon. (4

CEOS Actions)

Improve the accessibility and utility of the satellite data and carbon data products derived

from them, including transparency in data processing procedures, complete documentation,

long-term archive, and provision of products in forms scientists and policy makers will use. (1

CEOS Action)

Continue and enhance calibration and validation activities, including expanded quality

assessments, cross-calibrating additional sensors (e.g., for carbon dioxide and methane),

securing access to essential in situ validation data, expanding the number of land variables to

be validated, and establishing an ocean product validation subgroup. (10 CEOS Actions)

Improve institutional arrangements, communications, and joint activities with the carbon

community and organizations with carbon interests. (3 CEOS Actions, plus numerous

references to such linkages in other actions)

Improve or establish CEOS Mechanisms to implement this report’s recommendations or to

engage in the future planning activities called for in it.”

In addition to the above documents, the National Research Council, NRC of the national Academies

published a report by its Committee on Earth Science and Applications from Space entitled “A

Community Assessment and Strategy for the Future” in 2007. This document had a national focus

but many of the missions planned reflected the strategy outlined in the IGACO report. In years since

the publication of the strategy NASA and NOAA have made progress to meet the needs but few of

the new planned missions have been manifested as yet.

Overall the increasing influence of man on the earth’s system in the Anthropocene has been

recognised. After 5 decades of pioneering development in earth observation since the start of the

space age, the national, European and international expert bodies, have specified the requirements

to achieve an adequate integrated global observing system. These systems are feasible with current

technologies but the decision to provide the resources to realise and implement a global observing

system, having an adequate space segment, has not been yet made.

3.2.2 The evolution of European GMES/Copernicus and the Sentinels

As a result of reconstruction after the Second World War, Europe started to develop earth

observation later than the USA or the Soviet Union (see Chapter 1 of Burrows et al 2012). In 1964

the European Launcher Development Organisation, ELDO, and the Space Research Organisation

ESRO were created. The European Space Agency formed by merging these two organisations in


1976. An early focus was the successful development of the observations required for Numerical

Weather Prediction. In parallel national space agencies in Europe continued their own programmes.

The European institutions involved in the development of space activities in Europe gave birth to the

Global Monitoring for Environment and Security (GMES) programme through a declaration known as

the "Baveno Manifesto" in May 1998. This Manifesto called for a long-term commitment to the

development of space-based environmental monitoring services, making use of, and further

developing, European skills, and technologies. The EU, ESA and EUMETSAT and the national space

agencies in Europe participated in GMES.

As part of this activity and of relevance to this report, the EU GMES Working group 4 on atmospheric

composition recommended the manifestation of a geostationary UV visible NIR sensor on the

EUMETSAT Meteosat Third Generation. This became the EU supported ESA developed Sentinel 4

which will fly on MTG form 2021.

In October 2010, the programme entered a new phase with the adoption of the Regulation

911/2010 on GMES and its Initial Operations, which provided for an initial operational governance

framework and funds (EUR 107 million). This Regulation paved the way for the evolution of

GMES/Copernicus to a fully operational programme from 2014.

Copernicus has ground based and space segments. ESA is currently developing six types of new

satellites called Sentinel to meet the needs of the Copernicus programme. The Sentinel missions

include radar and super-spectral imaging for land, ocean and atmospheric monitoring. The Sentinel

missions will have the following objectives:

Sentinel-1 will provide all-weather, day and night radar imaging for land and ocean services.

The first Sentinel-1A satellite was launched on 3 April 2014, by an Arianespace Soyuz, from

the Guyana Space Centre;

Sentinel-2 will provide high-resolution optical imaging for land services (e.g. imagery of

vegetation, soil and water cover, inland waterways and coastal areas). Sentinel-2 will also

provide information for emergency services. The first Sentinel-2 satellite is planned for

launch in 2015;

Sentinel-3 will provide ocean and global land monitoring services. The first Sentinel-3

satellite is planned for launch in 2015;

Sentinel-4, embarked as a payload upon a Meteosat Third Generation Satellite, will provide

data for atmospheric composition monitoring. It will be launched in 2021;

Sentinel-5 will also provide data for atmospheric composition monitoring. It will be

embarked on a post-EUMETSAT Polar System (EPS) spacecraft and launched in 2021.

Sentinel-6 is the intent to sustain high precision altimetry missions following the Jason-2

satellite.

For the atmospheric composition and global pollution research Sentinel 4 and Sentinel 5 are the

most important elements, with Sentinel 3 providing some unique observations of aerosols and

clouds. The sounding part of MeteoSat Third Generation, MTG-S is the first geostationary

measurements of atmospheric composition and comprises

(http://www.eumetsat.int/website/home/Satellites/FutureSatellites/MeteosatThirdGeneration/inde

x.html) :

http://www.eumetsat.int/website/home/Satellites/FutureSatellites/MeteosatThirdGeneration/index.html

http://www.eumetsat.int/website/home/Satellites/FutureSatellites/MeteosatThirdGeneration/index.html


The Infrared Sounding (IRS) mission focusing on operational meteorology (water vapour

tracking & profiling, and temperature profiling), with some relevance to atmospheric

chemistry as a secondary application (thanks to the UVN contribution);

The UV/VIS/NIR sounding (UVN) mission (also called Sentinel 4) dedicated to atmospheric

chemistry and air quality.

In combination with the IRS, the UVN mission will complement the atmospheric chemistry mission

needed by the users. The UVN instrument targets its observations of the upwelling solar radiation at

the top of the atmosphere in latitude range 30° N/65°N, longitude 30°o W/45° E with a repeat cycle

of 0.5 hour as a goal and 1 hour as the threshold. Unfortunately measurements of Africa cannot be

performed simultaneously. It may be possible when the second MTG is launched, provided that the

instrumentation on the first MTG is in good health to reposition the first MTG to make observations

over Africa. Alternatively in the future a geostationary constellation can be built.

The Sentinel 5, which flies as part of MetOp Second generation builds on the heritage of GOME,

SCIAMACHY and OMI. The current atmospheric component of the Sentinel programme, whilst being

an important step forward, does not make adequate fit for purpose measurements of the

greenhouse gases and does not have any limb observations. One key area of tension in the current

Copernicus is the role of research. Whilst there has been an important strong emphasis within

Copernicus on the generation of operational systems and services, these require an adequate,

continuing and evolving research base to improve the technology to exploit optimally the

observations and to develop and educate the human capacity needed to be able to implement and

evolve an adequate global observation system. At present the provision of the necessary research

base to ensure the success and evolution of Copernicus has not been identified.

3.2.3 The UVN instrument Sentinel-4 and MTG-IRS

The Copernicus Sentinel-4 UVN instrument, scheduled to be launched in 2021 on board EUMETSAT’s

MTG, is an imaging spectrometer designed to monitor air quality over Europe hourly from

geostationary orbit (http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/

Sentinels_-4_-5_and_-5P). Sentinel-4 UVN is being built under ESA lead as part of the core payload

of the Meteosat Third Generation, MTG, which is the next generation operational meteorological

satellite system managed by EUMETSAT. The Sentinel-4 UVN instrument builds on the heritage of

the Low Earth Orbit instrumentation SCIAMACHY (Burrows and Chance 1991, Burrows et al 1995,

Bovensmann et al., 1999), GOME on ERS-2 (Burrows et al 1999) and the GeoSCIA and GeoTROPE

concepts (see, e.g. Bovensmann et al., 2002, 2004, Burrows et al 2004, and Flaud et al 2004).

The Sentinel-4 mission comprises an Ultraviolet Visible Near-infrared (UVN) spectrometer and data

from Eumetsat's thermal Infrared Sounder (IRS), both embarked on the MTG-Sounder (MTG-S)

satellite. After the MTG-S satellite is in orbit, the Sentinel-4 mission also includes data from

Eumetsat's Flexible Combined Imager (FCI) embarked on the MTG-Imager (MTG-I) satellite.

The main purpose of the Sentinel-4 mission is to monitor the air quality by measurements of

tropospheric O3, NO2, SO2, HCHO, CHO.CHO and aerosol quantities. The Copernicus Sentinel-4

mission will consist of two instruments, the first one to be launched in 2019 on board the MTG-S1

satellite. In combination with the second instrument to be launched around 2026 on MTG-S2 it is

planned to cover a period of 15 years. Similar to Sentinel-5P, UVN will use 2-dimensional CCD

detectors. It measures direct as well as backscattered solar irradiance in two spectral bands, the UV-

http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinels_-4_-5_and_-5P

http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Sentinels_-4_-5_and_-5P


VIS (305–500 nm, spectral resolution 0.5 nm) and the NIR (750–775 nm, spectral resolution 0.12

nm). The long side of the instrument slit is oriented in N–S direction. Continuous scans in E–W

direction are performed to obtain a spectrally resolved image of Europe on an hourly scale – i.e. they

will be able to record temporal (~hourly) changes during the sunlit period of the day. The typical

UVN ground pixel size over Europe is 8×8km2, which is considerably smaller than currently

operational similar Earth atmosphere observation instrumentation in low-earth orbits and of the

same order as Sentinel-5P and Sentinel-5 described in the next section. In addition to its unique

standalone measurement capabilities, Sentinel-4 will also profit from synergies utilising data from

the other sensors on the MTG system, namely the Infrared Sounder (IRS) on the same platform as

well as the Flexible Combined Imager (FCI) and the Lightning Imager (LI) on the MTG Imager (MTG-I)

platforms.

Sentinel-5 mission together with Sentinel 4 is dedicated to monitoring the composition of the

atmosphere for Copernicus Atmosphere Services. Both missions will be carried on meteorological

satellites operated by Eumetsat. Sentinel 5 builds on the heritage of SCIAMACHY, GOME and the

GOME-2, which is currently flying on the first MetOp series of platforms. It will fly on the polar-

orbiting MetOp Second Generation satellite. The Sentinel-5 mission overall comprises an Ultraviolet

Visible Near-infrared Shortwave (UVNS) spectrometer and data from Eumetsat's IRS, the Visible

Infrared Imager (VII) and the Multi-viewing Multi-channel Multi-polarization Imager (3MI).

The Sentinel-4 and -5 missions will provide information on atmospheric variables in support of

European policies. Services will include the monitoring of air quality, stratospheric ozone and solar

radiation, and some aspects of climate monitoring. However Sentinel 4 does not measure Carbon

dioxide, CO2, or methane, CH4. ESA has decided not to give the measurements of CO2 on Sentinel 5,

the highest priority as a result of budget limitations.

The Korea Aerospace Research Institute (KARI), which is the aeronautics and space agency of South

Korea, is developing the geostationary orbit GEO-KOMPSAT-2 satellite. This satellite is planned for

launch in 2018 as twin satellites, 2A as weather and 2B as atmospheric environment and ocean

satellite, with a 16-channel AMI (Advanced Meteorological Imager), a UV-Visible scanning

spectrometer, GEMS (Geostationary Environment Spectrometer), and GOCI-2 (Geostationary Ocean

Color Imager-2). GEMS measurements will yield data products such as O3, aerosol and their

precursors including NO2, and SO2. Synchronous measurements of atmospheric composition

together with the meteorological variables and ocean colour information are expected to contribute

to better understanding of the distribution and transboundary transportation of air pollution, and of

interactions between meteorology and air chemistry in the Asia-Pacific region.

Tropospheric Emissions: Monitoring of Pollutions, TEMPO was selected in 2012 by NASA as the first

Earth Venture Instrument, for launch circa 2018. It will measure atmospheric pollution for greater

North America from space using ultraviolet and visible spectroscopy. TEMPO plans to measure from

Mexico City to the Canadian tar sands, and from the Atlantic to the Pacific, hourly and at high spatial

resolution (~2 km N/S×4.5 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement

suite that includes the key elements of tropospheric air pollution chemistry.

The constellation of the GeoKOMPSAT with the NASA (TEMPO) over North America and the ESA

Senteniel-4 UV-Visible-NIR (UVN) over Europe in 2020 - 2025 period offers synergistic opportunities.

Unfortunately, even if fully deployed, these instruments will cover only a limited part of the tropics.


Thus, a great opportunity is to be missed for the investigation of predominantly tropical key

phenomena such as biomass burning or lightning. Nevertheless the combination of these three

sensors will cover significant parts of the globe (save for the Polar Regions) from geostationary orbit

and together with instruments on the LEO satellites will provide unique insights into the diurnal

variability of tropospheric composition and the transport and transformation of pollution.

3.2.4 The Sentinel 5 Precursor

The Sentinel 5 Precursor (S5P), scheduled for launch in mid-2016, is the first of the sentinel satellite

series dedicated to monitoring of atmospheric composition. The main application areas of the

mission are air quality, climate and the ozone layer. The single payload of the S5P mission is the

TROPOspheric Monitoring Instrument (TROPOMI) (Veefkind et al., 2012). TROPOMI is a nadir

viewing spectrometer that will measure in the UV-visible wavelength range (270-500 nm), the near

infrared (710-770 nm), and the shortwave infrared (2314-2382 nm). TROPOMI will have a spatial

resolution of about 7x7 km2 at nadir. The spatial resolution is combined with a wide swath to allow

for daily global coverage. The TROPOMI/S5P geophysical (Level 2) data products include nitrogen

dioxide, carbon monoxide, ozone (total column, tropospheric column & profile), methane, sulphur

dioxide, formaldehyde and aerosol and cloud parameters.

The improved spatial resolution serves two goals: (1) emission sources can be detected with more

accuracy and (2) the number of cloud-free ground pixels will increase substantially. Both these

aspects will contribute to the monitoring of megacities from space, as illustrated in Figure 13 which

shows an OMI observation in the zoom-in mode which has a spatial sampling of 12x13 km2. It is

noted that TROPOMI will have a three times better spatial sampling and also a higher signal-to-noise

ratio. Figure 13 covers several highly urbanized regions, including the Po Valley, the Rotterdam-

Antwerp region, the Ruhr area, and Paris. It provides a preview of the amount of detail on source

regions as well as transport of pollutants that is expected from the TROPOMI observations.

Sentinel-5 Precursor flies in an orbit with an equator crossing time of 13:30. This is similar to the

NASA AURA orbit. In comparison GOME, SCIAMACHY, and the GOME-2 instruments fly in the early

morning orbits with equator crossing times of 10:30, 10:00, and 09:30, respectively. The Sentinel-5

instruments on the Metop Next Generation polar system, to be launched in 2021, will also be in the

early morning orbit. This will result in a long time series of comparable measurements of

tropospheric composition going back to the launch of GOME in 1995 and including the SCIAMACHY

data sets.


Figure 13: Spatially variability of the NO2 columns (molecules/cm2) over Europe for 12 September 2006, measured by

OMI in the spatial zoom mode. In this measurement mode the ground pixels are approximately 13x12 km2 (along x

across track) at nadir. The white areas in the figure have too much cloud contamination for accurate NO2 retrieval. The absolute NO2 columns are likely to be underestimated, because a single NO2 profile shape was used in the retrieval.

3.2.5 Sentinel-5 and IASI-NG

EUMETSAT is currently preparing the next polar-orbiting program (EPS-SG) with the MetOp-SG

satellite series that should be launched around 2020. On the first of the two envisaged platforms,

the UVNS/Sentinel-5 will be operating, an imaging UV/vis/NIR/SWIR spectrometer that will have

similar capabilities as the S5P Precursor with daily global coverage, a spatial resolution of 7x7 km2

and spectral coverage facilitating retrieval of O3, NO2, HCHO, CHOCHO, SO2, H2O, CO, CH4, and

aerosols.

Also in this framework, CNES is studying the concept of a new instrument, the IASI-New Generation

(IASI-NG), characterized by an improvement of both spectral and radiometric characteristics as

compared to IASI (Clerbaux et al., 2013; Crevoisier et al., 2013), with three objectives: (i) continuity

of the IASI/MetOp series; (ii) improvement of vertical resolution; (iii) improvement of the accuracy

and detection threshold for atmospheric and surface components. An improvement of spectral

resolution and radiometric noise fulfill these objectives by leading to (i) a better vertical coverage in

the lower part of the troposphere, thanks to the increase in spectral resolution; (ii) an increase in the

accuracy of the retrieval of several thermodynamic, climate and chemistry variables, thanks to the

improved signal-to-noise ratio as well as less interferences between the signatures of the absorbing

species in the measured radiances.

3.2.6 CarbonSat

The CarbonSat and CarbonSat Constellation concepts were developed in the 2008 to 2010 period at

the University of Bremen together with academic, industrial and agency partners. This building on

the heritage of SCIAMACHY, which was the first instrument to deliver measurements of the up

welling radiation in the near and short wave infrared spectral regions of an adequate precision to

enable accurate measures of the dry mole fractions of methane, XCH4 and carbon dioxide XCO2 to be

delivered. The CarbonSat mission (Bovensmann et al. 2010) aims to deliver highly accurate XCO2

and XCH4 column mixing ratios at a high spatial resolution of 2x3 km2 and with good spatial coverage


via continuous imaging across a 240 km swath width (goal: 500 km). In 2010 the CarbonSat and FLEX

proposals were selected for Phase A B1 studies by ESA for its 8th Earth Explorer Mission.

The imaging of XCO2 and XCH4 distributions above strong source regions (megacities, volcanoes,

strongly emitting industrial areas, etc.) enables the determination of the source strength of those

targets by using inverse-modelling techniques. This has been demonstrated by aircraft

measurements using the MaMap (Methane and carbon dioxide Mapper), which was also developed

at the University of Bremen (Gerilowski et al., 2011, Krings et al., 2011, 2013).

On local regional and global scales, the CarbonSat data, when coupled with knowledge of wind, yield

the CO2 and CH4 fluxes down to the seasonal and even monthly time scale. CarbonSat is planned to

fly in sun-synchronous orbit with an equator crossing time around 11:30 a.m. This equator crossing

time was decided on the basis of a trade-off between maximising the measurements and cloud free

conditions. The latter maximise in the early afternoon around 10:00 hrs. CarbonSat’s main

observation mode will be nadir, but solar spectra will also be obtained as well as observations in

(near) sun-glint mode. The CarbonSat imaging spectrometer will cover three spectral bands to

accurately determine XCO2 and XCH4: NIR (O2 A-band) 747 – 773 nm at 0.1 nm spectral resolution,

SWIR-1 (weak CO2 and CH4) 1590 – 1675 nm at 0.3 nm spectral resolution, and SWIR-2 (strong CO2

and H2O) 1925 – 2095 nm at 0.55 nm spectral resolution. Sensitivity studies (Buchwitz et al. 2013a)

indicate that systematic errors are mostly (~85% of all scenes) below 0.3 ppm for XCO2 (< 0.5 ppm:

99.5%) and below 2 ppb for XCH4 (< 4 ppb: 99.3%) with single measurement precision of typically

around 1.2 ppm for XCO2 and 7 ppb for XCH4 (1-sigma). This data quality will allow the quantification

of city emissions with errors on the order of 10%-20% for single overpasses, as estimated in a case

study for Berlin (Buchwitz et al. 2013a).

The selection for the Earth Explorer 8 Mission is planned by ESA for 2015 with a launch planned for

around 2020. The limitation of a single CarbonSat is that it will have only 10 day coverage at the

equator. The selection of CarbonSat is an important and essential step forward in the establishment

of an adequate global observing system. As surface fluxes vary significantly from day to day and

during a day, a constellation of CarbonSat satellites is required to achieve daily coverage or twice

daily coverage. This is the spatial and temporal sampling and coverage, which meets the

requirements of the Carbon Task Force (2010) and the CEOS strategy for carbon observations from

Space (CEAS 2014).

3.3 Geophysical Validation of Satellite Data Remote sensing uses both passive and active sources of radiation. Geophysical data products from

satellite based remote sensing are generated by mathematically inverting observations made at the

top of the atmosphere of solar radiance, infrared or microwave emission. The mathematical

inversion utilises algorithms, which utilise parameters measured in the pre-flight and in-flight

characterisation and calibration of the instrument. In addition as these algorithms are often ill

defined mathematically and thus use a priori information to constrain the values of the data

products.

Verification is the process by which the algorithms are tested. This uses both synthetic and real data.

Geophysical validation is the process by which the satellite data products are compared to an


independent data set. These have preferably an accuracy that is higher than the satellite data and is

produced by instruments using different measurement principles.

In North America, the need for extensive verification and validation was established with the

development of the ozone, trace gas and aerosol data products from the measurements of BUV

(Backscattered UltraViolet Spectrometer) on Nimbus 4 and the Atmospheric Explore E, SBUV (Solar

backscattered Ultraviolet Spectrometer) and TOMS (Total Ozone mapping Spectrometer) on Nimbus

7, SBUV-2 on board NOAA-9, NOAA-11, NOAA-16, NOAA-17, and NOAA-18, the SAMII (Stratospheric

Aerosol Measurement II) on Nimbus-7, SAGE (Stratospheric Aerosol and Gas Experiment) -I, -II and

III series of occultation instruments, which flew on Flew on the Explorer 60 satellite, the Earth

Radiation Budget Satellite (ERBS), and Meteor-3M, the LIMS infrared sounder and also with the data

products from the instruments aboard, and trace gas and radiation measurements made from the

UARS (Upper Atmospheric Research satellite) payload: Halogen Occultation Experiment HALOE,

Cryogenic Limb Array Etalon Spectrometer (CLAES), High-Resolution Doppler Imager (HRDI),

Improved Stratospheric and Mesospheric Sounder (ISAMS), Microwave Limb Sounder (MLS), Particle

Environment Monitor (PEM), Solar-Stellar Irradiance Comparison Experiment (SOLSTICE), Solar

Ultraviolet Spectral Irradiance Monitor (SUSIM), Wind Imaging Interferometer (WINDII).

In Europe the template for the calibration and characterisation, data algorithm development, and

verification and validation of atmospheric data products was established in the 1990s within the

selection of SCIAMACHY and GOME for flights on ENVISAT and ERS-2. Verification and Validation

requirements documents were written, establishing the approach. These documents identified the

needs for level 1 and level 2 data products. These documents described in detail all aspects of

validation. In summary the need for four phases of activity was established:

i) Pre-flight activity involving instrument calibration and characterisation and retrieval

algorithm development;

ii) Commissioning phase, where the level 1 and level 2 products are initially verified and

validated;

iii) Main Phase of Validation, where the objective is to achieve a validation of all level 1 and

level 2;

iv) Long term Validation, which addresses the degradation of the instrument and eh

changes in calibration parameters during flight in space.

These resulted in extensive commissioning phase and main phase activities. For example the DLR in

Germany invested over 11 M€ for the SCIAMACHY in the period 2002 to 2008.

There has been an evolution from the need for stations measuring stratospheric constituents to

those measuring tropospheric composition. This is reflected in the evolution of the Network for the

Detection of Stratospheric Change, NDSC, to the Network for the Detection Atmospheric

Composition Change, NDACC (see http://www.ndsc.ncep.noaa.gov/ and all related sites). The WMO

Global Atmosphere Watch GAW is another important source of data for long term validation

(http://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.html).

An area of particular difficulty is with respect to long term validation. The specific issue is the

maintenance of adequate ground based capability for long term measurements and who pays for

this. At present the national and international space agencies typically pay for added costs,

http://www.ndsc.ncep.noaa.gov/

http://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.html


associated with validation, but use data from existing long term measurements stations without

contributing to their upkeep and maintenance.

In recent years several validation campaigns have been performed (e.g. Brinksma et al, 2008; Piters

et al., 2012), however most of the effort was put on background conditions and clear skies, a

consequence of the need to compensate for the very different spatial sampling of the satellite and

ground-based systems. For the upcoming space missions that are primarily driven by requirements

for tropospheric monitoring and research, it is essential that validation is performed under more

realistic and thus more challenging conditions. In the U.S.A. the DISCOVER-AQ campaigns

(http://discover-aq.larc.nasa.gov/science.php) are an important step in that direction.

There are many new challenges of emission from mega cities and urban conglomerations. In the

future validation of satellite data will be needed within urban environment where it is known that

concentrations of trace gases are variable, inhomogeneous and are coupled with high aerosol

loadings. It is important to note that ground-based networks, which are essential for long-term

validation, are under threat worldwide, because of a lack of funding for these activities. As satellite

data without continuous validation is of unknown accuracy and thus of limited use, a sustained

effort in realistic validation exercises is needed as integral part of the future observing system.

3.4 Scientific Exploitation of the Sentinel Programme and beyond Although the Sentinel 4/5/5P programme is primarily driven by operational and monitoring

requirements, it will also deliver answers to many science questions. To exploit this, adequate

funding is needed to develop new retrieval techniques focussing on synergy between different

observations (e.g. combinations of shortwave and thermal infrared), and interpretation of different

trace gases and aerosol parameters simultaneously.

While we anticipate tremendous progress using the new Sentinel instruments, the long lead time for

developing new capabilities requires that we already begin to look to a next generation of satellite

instruments now. We recognize four primary opportunities for significant scientific advances in the

future:

1) The capability for observing the vertical structure of the troposphere remains extremely

limited. The development of LIDAR and DIEL instrumentation, targeted for selected trace

gases and aerosol and multi-angle viewing for aerosol would be an important potential

advance.

2) Observing greenhouse gases (CO2 and CH4) with high spatial resolution and complete daily

global coverage are not possible today but are of great importance for climate science and

policy.

3) Air pollution and tropospheric research needs data with high spatial resolution to pin-point

down local sources and identify and understand transformation processes.

4) Understanding unique aspects of the tropics, where many of the emerging megacities are

located.

3.5 Scientific Missions Over the past decades there has been a trend towards more expensive satellite missions with longer

lead times. This has a number of drawbacks including


i) the inability to include new technology in instrumentation,

ii) the resultant lack of overlap between missions to provide continuity to datasets and the

increased probability of gaps in the datasets through instrument failures or satellite

launch delays.

The pressure on Satellite payload development is for smaller and more affordable missions, while at

the same time improving the science output. However, even for this scenario the launch

opportunities are limited and need to be increased. Another approach that can be followed is to

launch constellations of smaller instruments instead of large missions.

However, different types of missions are clearly needed to fulfil different needs. These comprise

a) rapid, dedicated, and more experimental missions designed to address specific questions

and to accelerate technological and scientific progress and

b) long-term missions built and operated to provide consistent long-term data sets and

foundation to our increased understanding.

The experience of the past decade is that agencies have are not being granted by government the

strategically important combination of both.

3.6 Platforms Traditionally the choice for regional and global remote sensing of the atmosphere has been

dedicated scientific payload space on LEO or GEO platforms. However, new opportunities may

appear as instruments are reduced in scale for constellations of smaller satellites, piggyback

opportunities on larger private and government platforms, or sub-orbital vehicles. Many synergies

will be available between these new platforms and more traditional platforms, including the areas of

data interpretation, nested spatial scales and validation.

3.6.1 The International Space Station

One option for additional space instrumentation in the near future is the International Space Station

(ISS). Its low orbit would allow measurements with unprecedented spatial resolution. The ISS is in a

51.6 degree inclination orbit. Of the top 1000 cities by population (total 1.33 billion) 930

representing 95% of the population lie directly under the ISS track. With a 550km (~60 degree) cross-

track-scan this rises to 985 cities and 99% of the city population. The station is not perfectly stable in

orbit altitude or orientation, but this should not pose too much of a problem for nadir sounders with

appropriate hardware or software compensation. There is potential to place a number of

instruments simultaneously on the ISS in various locations and these instruments could be large by

current standards. The major restriction on instruments is in operation time: there will be scheduled

times when the instrument will not be operational and it is intended that instruments should only

operate for an agreed mission time. Some instrumentation has already been placed on the station

including the Japanese SMILES instrument.

One important aspect of the ISS orbit is the large inclination which results in gradual changes of the

local overpass time. This can be seen as both, an advantage and a disadvantage. In the short run, the

change in overpass time complicates interpretation of tropospheric measurements of short lived

species as both emissions and photochemistry vary with time. On the other hand, a drifting orbit


allows successive sampling of tropospheric composition at all local times providing insight into the

diurnal variability.

3.6.2 Unmanned Aerial Vehicles (UAV)

In addition to traditional satellite platforms, alternative carriers such as high altitude UAVs have

great potential for efficient and high-spatial monitoring of megacities and regions with many

urbanized regions like Europe. However, such platforms currently put severe limitations on

instrument size and power and also face difficult problems with respect to flight acceptance over

regions with civil or military air traffic.

4 Current achievements of remote sensing of areas of high population density and their emissions from space and our interpretative capability

During the past two decades several high quality satellite products have been made available for

tropospheric chemistry and climate studies. In parallel, significant advances have been made in the

field of air pollution and climate modelling and the use of satellite products. Satellite observations of

the troposphere and of surface characteristics have pushed forward our understanding of

atmospheric composition changes (e.g. observations of trends due to human activities). Satellite

data are now routinely used to improve input data for climate and chemistry-transport models. They

are also used for model evaluations, or assimilated in models for weather and air pollution

forecasting and in inverse modelling techniques to estimate emissions. All these applications have

advanced greatly in recent years.

The major areas of synergistic use of models and retrievals from satellite observations can be

classified in three main categories: Forward modelling, data assimilation, and inverse modelling (see

Kanakidou et al. (2011) for a short summary of synergistic use of satellite data with models for

tropospheric studies).

4.1 Forward Modelling Climate/chemistry/transport models use (pre- or online calculated) emissions to compute the

distribution of atmospheric constituents and their impact on air quality and climate. Satellite

observations are used to improve the input data to these models (e.g. surface characteristics,

vegetation types, chlorophyll-a content, temperature – Figure 14) or for model evaluation (Figure

15. In particular significant advances have been made with regard to the observation of trends of

atmospheric constituents and the ability of the models to reproduce these. Models have also moved

from a climatological (static) representation of the atmosphere to time/case specific simulations and

the description of gas and aerosol chemistry has been improved.


Figure 14: Synergistic use of satellite observations and forward modelling

Satellites and models now provide a consistent description of global pollutant distributions. They can

also provide information on emission inventory deficiencies (inverse modelling, see below).

As models achieve higher and higher resolution to resolve features on smaller scales down to the

size of individual cities, the spatial resolution and temporal sampling of the data products from

space based remote sensing instrumentation must keep pace and improve in order to be optimally

exploited and to constrain our understanding of surface fluxes and transport and transformation of

pollution.

Figure 15: Comparisons of SCIAMACHY observations of NO2 vertical columns (left) with WRF/CMAQ mesoscale model results (right) over Europe for 2008. Differences point to the shipping tracks in the West Mediterranean (not shown in the observations) and to anthropogenic emission hot spots at the coast of the East Mediterranean (not shown in the observations), indicating problems in used emission data or other representations in the model. Figure from Im et al., 2014.

4.2 Data Assimilation Data assimilation techniques have for many years been used in numerical weather prediction modelling to improve the quality of weather forecasts. During the last decade also air quality forecasting has evolved significantly. Especially in the European projects GEMS, MACC and MACC-II, satellite observations of chemical species have been further developed and are now routinely used in chemical weather forecasts (http://macc-raq-op.meteo.fr/). Figure 16 shows the production chain

http://macc-raq-op.meteo.fr/


of the EMEP MSC-W model, which is part of the MACC-II air quality forecasts. Meteorology is retrieved from ECMWF automatically each morning (1st column), then the air quality model is run without data assimilation (2ndcolumn) and with assimilation of satellite data (3rd column - analysis) and the results are further processed placed on the MACC-II data server (4thcolumn). This chain proceeds daily and operationally for MACC-II. The quality of forecasts improves through assimilation of observed data in the models as this brings the initial state of the atmosphere (at the start of the model run) closer to reality. When dealing with air quality forecasts the need for real time emissions is evident. Such data are rarely available, but can be improved through satellite observations and inverse modelling techniques, as mentioned in the next section.

Figure 16: Work flow in the EMEP chemical weather forecasting chain (Alvaro Valdebenito, Norwegian Meteorological Institute). Up to now NO2 columns from OMI, in situ NO2 observations, and AOD from MODIS have been assimilated in the system. The chemical weather forecast done by the model can thus start from an initial state of the atmosphere, which is closer to reality than in those cases where only climatological data sets are used.

For this important application of satellite data in chemical weather forecasts, near-real time

provision of data and high spatial and temporal resolution are important. In real-world applications,

the use of satellite data will be complemented by in situ observations to allow for finer resolutions

and to fill gaps when satellite observations are not available i.e. when surface air pollution is masked

by clouds).

In the context of emission data, there is an ESA project called GLOBEMISSION

(http://www.globemission.eu/) that aims to provide fast emissions for weather forecast. This could

be extended to provide constituent emission data for ingest into these forecasting schemes.

4.3 Inverse modelling Long term satellite measurements are required for studies of the evolution of emissions for areas

where emission inventories are unreliable or out of date. This is important information for policy


makers and numerical prediction of air pollution and chemical weather. Emissions, however, cannot

be observed directly. A chemical transport model is used to deduce the emissions from the

concentration data. The differences between observed and simulated concentrations contain

information on how to update the underlying emissions.

Figure 17: The nitrogen oxide (NOx) emissions over Beijing are estimated using OMI and GOME-2 observations of tropospheric NO2. The drop in emissions before and during the 2008 Olympic Games shows the effectiveness of the air quality measures taken by the local authorities. After the Olympic events, however, NOx emissions started to rise again gradually. (Bas Mijling, KNMI, figure from Mijling, B. and R.J. van der A (2012))

The inverse modelling technique (see Burrows et al 2012 ACCENT-AT2 book - Chapter 9) is a method

for estimating the emissions of atmospheric pollutants, by adjusting the emission fields used in a

chemistry-transport model in order to minimize the discrepancy between the model predictions and

a set of atmospheric observations. The adjustment requires defining the emission parameters to be

optimized, and to minimize a scalar function of these parameters, often termed as “cost function”,

which quantifies the discrepancy between the model predictions and the observations. This

approach relies on the assumption that the model adequately describes the relation between

emissions and distributions, so that the model/data differences can be mostly attributed to errors in

emissions rather than to errors in the model or the data itself. For that, state-of-art knowledge

about the physical and chemical processes of the atmosphere should be included in the models and

updated to account for the latest developments. Inverse modelling applications concern the

emissions of NOx (Figure 17), CO, CH4, CO2, which are compounds directly observed from satellite.

Furthermore, HCHO and CHOCHO satellite retrievals have been used to derive VOC emission

estimates (Figure 18), while NO2 retrievals have been used to infer emissions of some species co-

emitted with nitrogen oxides. For this later application, the relationship between the investigated

pollutant and the proxy species has to be established by using measurements, a model or an

emission inventory.


Figure 18: Left panel : GOME-2 and modelled HCHO columns before and after inversion in July 2008. Right panel: GOME-2 derived isoprene emission estimates in July 2008. Emissions are freely distributed at http:// www.globemission.eu.

In particular, satellite NO2 measurements can be used to infer multiannual changes in CO2 emissions

Figure 19). Thus tropospheric NO2 measurements provide useful (indirect) information about

anthropogenic CO2 emission sources collocated with the sources of NOx emissions. Satellite

measurements confirm the accelerating and strongly nonlinear CO2 emission trend in China, which is

manifested by the emission inventory data. However, strong quantitative differences are revealed

between the top-down and bottom-up emission estimates for the first time period (1996-2001)

evaluated. These differences may be indicative of major flaws in the emission inventories.

Figure 19: NO2 measurement-based estimates of the multi-annual trends in CO2 emissions for the different variants (A, B and C) of the estimation procedure in comparison with corresponding bottom up CO2 emission data of the EDGAR, GCP and PKU emission inventories. Figure from Berezin et al. (2013).

Early inverse modelling methods addressed either inert (CO2) and long-lived gases (CH4), or

compounds exhibiting weak non-linearities between their atmospheric abundances and their

emissions (e.g. CO). However, the techniques developed for those gases were no longer exact when

applied to reactive trace gases, like NOx and HCHO, due to their short lifetime and presence of


strong chemical feedbacks. New, more powerful schemes were thus developed to account for

chemical dependencies between species, and, in particular, the impact of the predicted emissions

changes on the chemical lifetime of the compound, which was usually neglected. Multi-compound

inversions have been performed using different species sharing common sources in a way that

observations for a given compound can help constrain the sources of other compounds that are

emitted but not necessarily observed. An important advance in inverse modelling, eased by the

increase in computational resources becoming available, was to switch from “big-region” inversions,

where only the emission strength was optimized in large, predefined world areas, but not the spatial

distribution of the source inside the areas, to “grid-based” inversions, where the updated emissions

are inferred at the resolution of the underlying model. More recently, innovative methods enable re-

allocation of sources together with improvements in the determination of emission strength (Figure

20).

Figure 20: Emission inventories of nitrogen oxides (NOx) of the populated and industrialized Highveld area in South Africa projected on a 0.25 degree grid. Emissions are dominated by coal-fired power plants, indicated by the yellow markers. The EDGAR v4.2 emission inventory is generally wrong in location and strength of these hot-spots (left panel). Emission estimates with the using NO2 observations from the OMI instrument relocate the emissions to a more plausible position (right panel). (Bas Mijling, KNMI)

The inferred emission estimates depend on the quality and resolution of the satellite observations.

New satellite missions with better signal-to-noise ratios and higher spatial and temporal resolution

are expected to further improve emission estimates.

Beirle et al. (2011) applied a novel interpretation of satellite NO2 data to derive emission estimates.

This is applicable to strong point sources inside a low NO2 background region. From the analysis of

the downwind decay of NO2 columns, the effective lifetime and the emissions of NOx can be

determined simultaneously (see Figure 21). Such estimates can be improved with geostationary

satellites with regional focus. In addition, observations with improved spatial resolution will allow for

investigations of non-linear chemistry inside the pollution plume.


Figure 21: NOx emissions and effective lifetimes for the considered megacities and power plants estimated by Beirle et al (2011) based on OMI NO2 observations and comparison with the EDGAR emissions. (EDGAR version 4.1. http://edgar.jrc.ec.europa.eu/)

Concerning our knowledge of ship emissions of NOx, during the past decade, ship tracks have been

identified from space. However, the strength of NO2 column detected from space in the ship tracks is

lower than that modelled. In addition to uncertainties in our knowledge of shipping emissions, these

differences can be related to the presence of clouds or to the direction of the wind over the shipping

tracks as well as to the non-linear behaviour of chemistry in the shipping plumes that is not

accurately reproduced by the models. Higher spatial and temporal resolution studies, involving both

modelling and satellite observations are need to resolve this issue. Figure 15 shows the issue and the

inability of current models to determine shipping NOx. In summary this issue is an important

research question, which addresses the impact of shipping on both coastal and pristine marine

regions. In summary it shows the current limitation of either models or measurements.

4.4 Identifying gaps - making recommendations for the way forward

4.4.1 Model improvements

Chemistry: Recent studies point to flaws in the current mechanisms regarding the representation of

NOx and VOC chemistry in models. In particular, modelling work has shown limitations in

reproducing the oxidant levels in the troposphere due to missing or not sufficiently represented

chemistry (e.g. isoprene/biogenic volatile organic chemistry and HOx, heterogeneous reactions,

NO/NO2 reactions). The importance of these model deficiencies differs in space and in time due to

the spatial variability of emissions and the non-linearity in chemistry. Recent developments in

understanding of oxidant chemistry have a direct impact on the accuracy of the inverse modelling

estimates. In addition, expected future model chemistry improvements will be achieved by focusing

on heterogeneous/multiphase chemistry.

Emissions: Isoprene is one of the major sources of uncertainty in models. Arneth et al. (2011)

reported strong sensitivity in current estimates of global isoprene emissions (factor of 3-4),

depending mostly on the isoprene emission algorithm used. Accurate knowledge of the vegetation

coverage is also required to improve these emission estimates. Langner et al. (2012) reported that


four different state-of-the-art regional models give very large differences in seasonality and strength

of isoprene emissions. Yet, isoprene is critical for oxidant concentrations, as illustrated in Figure 22.

Inversion schemes constrained by satellite HCHO observations have proved their capabilities to

constrain isoprene emissions in a number of past studies (e.g. Palmer et al. 2003, Stavrakou et al.

2009, Marais et al. 2012). At high northern latitudes, however, current satellite observations have

larger errors and data gaps, and therefore, provide weaker constraints on isoprene emissions over

boreal forests, which are nevertheless believed to be strong emitters of biogenic compounds.

Furthermore, only very few isoprene flux measurement data are available at these latitudes.

Figure 22: Left: Isoprene emissions in July calculated with MEGAN model (Muller et al., 2008) expressed in 1010

molec.cm

-2 s

-1. Right: Boundary layer OH concentration predicted by the IMAGESv2 model (Stavrakou et al. ACP, 2010).

Surface OH concentrations are strongly depressed over isoprene-rich areas.

4.4.2 Specific modelling needs from satellite data products

The requirements for satellite data products are often case study i.e. molecule or parameters

specific for particular events. The creation in late 2014 by the EU of the next phase of its Copernicus

Atmospheric Monitoring Service and the Climate Change Service, which are to be managed by

ECMWF, is a statement of intent. To meet the ultimate goals of this system much improved

observations from ground and space segments are required. In particular global measurements of

short lived and long lived atmospheric constituents (trace gases, aerosol and clouds) from the local

to the global scale. These are needed to improve our understanding of surface fluxes and the

transport and transformation of pollutants, stratospheric tropospheric exchange and both dry and

wet deposition.

Below we make an indicative list of the satellite data and ancillary information required for air

quality and climate studies required for assessing the impact and predicting of the emission from

large megacities and urban agglomerations. These are prioritised at this stage.

1. Operational near-real time satellite observations of temperature, wind, and composition for

data assimilation. Improved emissions estimates (mainly anthropogenic) for weather and air

pollution forecasts.

2. Error estimates of satellite retrievals and averaging kernels, spatially and temporally resolved.

3. Long term continuity of satellite datasets with careful handling of the handover between one satellite and the next.


4. Information on horizontal vertical profiles of pollutants and aerosol properties at locations of megacities to incorporate into models to improve their predictions.

5. Information on the diurnal variation of short-lived chemical species for air quality studies

and monitoring - geostationary satellites would greatly improve this, or a constellation of

low orbiting satellites. Improves our understanding of short timescale events.

6. Higher spatial and temporal sampling/resolution of most constituents to the spatial scale of a city and a temporal scale of fractions of hours. Allows more realistic assessment of current conditions and better input to predictive models.

7. Reliable satellite retrievals of HCHO/CHOCHO at high northern latitudes, since biogenic organic compounds are strongly emitted by boreal ecosystems. Correct estimation of biogenic VOCs and, in particular, isoprene, play also an important role for climate studies, due to the feedbacks between warming climate and biogenic aerosols (Paasonen et al. (2013)). This requires high signal-to-noise from the instrumentation.

8. Integrated approach to measurements including laboratory, surface, local, sub-orbital and orbital measurements. Allows effective use and increases reliability of all the assets.

9. Need to train a new generation of scientists to face new challenges - new instruments bring better resolution but also new challenges (do we have enough modelling tools to exploit/interpret the upcoming data? Consider computational/storage issues)

10. Geostationary observations for the tropics where there is significant lightning and biomass burning emissions and regions that are the drivers of global tropospheric chemistry which then influences regional issues. To date all the planned geostationary observations focus on the Northern Hemisphere (Europe, N America, Korea) with limited coverage of the equatorial regions

11. Improved spatial resolution and temporal sampling of trace gases, aerosols and cloud parameters: specifically horizontal resolutions of ~ 1km or better and as much vertical profile information ideally separating the planetary boundary layer form the free troposphere: temporal sampling of the order of 20 minutes are required to match the most significant tropospheric processing times. This yields improved knowledge of the following

deposition on ecosystems and BVOC emissions, land use, vegetation types, 1-5 km of resolution to better constraint the models

fields of air pollutant measurements for urban scale air pollution forecasting:

resolving aerosol/clouds interactions specifically addressing such issues as the invigoration or suppression of cloud etc., precipitation rates etc.

Table 1 shows a list of chemical species and parameters that were identified as important for

megacity studies at the PRESCRIBE workshop. The list is not exhaustive.

Table 1: Molecules and parameters that are considered essential to observe from space

Gases: O3, NO2, CO, CH4, HCHO, CHOCHO, CH3COCHO, CH3OH, HONO, halogens, SO2, NH3, HNO3, organic acids (HCOOH, CH3COOH), vertically resolved measurements of primary gaseous species

Aerosols: AOD, extinction coefficients, absorbing aerosol, BC, SSA, vertically resolved size distribution, polarisation measurements, aerosol speciation

Atmospheric Structure: boundary layer height (preferably with the same resolution and coverage as the trace gas measurements)

Surface characteristics: BRDF, vegetation types, chlorophyll-a, plankton types, spatial & temporal resolution (pollen emissions)


5 Conclusions The ACCENT Plus Prescribe meeting was very successful. It enabled an international team of experts

to come together and assess

i) the relevant observational capabilities available in the past, now, and in the near future

from planned missions,

ii) the achievements thus far in terms of measuring and quantifying the outflow and

increasing emissions of pollutants and trace constituents (gases and aerosols),

iii) the current state of pollution / tropospheric chemistry observations from space

instrumentation,

iv) the use of satellite data for attribution of pollution,

v) the potential global and regional impacts resulting from further industrialisation,

urbanisation, land use change etc.,

vi) retrieved data products of trace atmospheric constituents,

vii) the needs for improved assessment of the contributions of anthropogenic activity and

natural phenomena to atmospheric pollution and climate change;

viii) the specific needs for the improvement of our knowledge of the impact of mega cities

and large urban agglomerations on air quality and climate change.

The dramatic improvement of tropospheric remote sensing from space, since the launch of the

GOME instrument was documented with some recent highlights being reported.

It was recognised that the past three decades have been a golden age of development of passive

and active remote sensing of atmospheric constituents. The challenge now is to achieve an adequate

fit for purpose global observing system for the future. In principle the ground work and definition of

needs has been developed under the auspices of the WMO. The main challenge is to achieve an

adequate temporal sampling at adequate spatial resolution, resulting in a clear need for new

satellite platforms driven by the scientific needs and for the development of the new and improved

remote sensing instrumentation required for future generations of observation systems. The latter

are needed for example to meet the objectives of the EU Copernicus programme. In this context the

specific use of the International space station, ISS, as an international Atmospheric Observatory from

the ISS, iAOBISS, was recommended.

With respect to the observation of the emissions from megacities, urban conurbations, and

agglomerations, the group identified that the current and planned systems with their limited spatial

resolution and sampling have significant limitations. In summary, much higher spatial and temporal

sampling is required. To achieve the high spatial (~ 1km) and temporal (~ 20 minutes) sampling and

global coverage, constellations of instruments in LEO and GEO will be required.


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7 Agenda of the PRESCIBE Workshop Day 1, 15th May 2013

09:00 – 09:30 Arrival, Registration, Coffee

09:30 – 10:00 Opening, Welcome and Introduction

10:00 – 10:30 Overarching Objectives of the Workshop

- Presentation (John P. Burrows)

- Formation of the two working groups

- Discussion

10:30 – 11:00 Coffee Break

Block 1: Status of Current Space Based Research on Atmospheric Composition of

Conurbations / Megacities

11:00 – 11:20 J. Drummond: Insights from Long Term Measurements of CO from Space

11:20 – 11:40 A. Boynard: How able is IASI for tracking pollution?

11:40 – 12:00 G. de Leeuw: Aerosol retrieval using satellite data

12:00 – 12:20 Y. Xue: Multi-scale AOD Retrieval from Satellite Data for Beijing Air Pollution Study

12:20 – 12:40 M. Vrekoussis: On the impact of the economic recession on urban air quality:

Trends in air pollution levels

12:40 – 13:00 P. Valks: GOME-2 observations of air quality in Chinese Megacities

13:00 – 14:00 Lunch Break

Block 1 continued

14:00 – 14:20 U. Platt: Ground truth for flux measurements from Urban Areas

14:20 – 14:40 P. Schneider: A global SCIAMACHY-based trend analysis of tropospheric NO2 over

megacities

14:40 – 15:00 A. Hilboll: Changes in tropospheric NO2 over megacities: A multi-instrument approach

15:00 – 15:20 S. Beirle: From columns to emissions - how much a-priori do we need?

15:20 – 15:40 T. Wagner: The potential of cloud slicing to derive profile information from Nadir looking instruments


Block 2: Perspectives for future Space Based Research on Megacities


16:00 – 16:20 R. Cohen: A Space Based Perspective on Urban Emissions and Photochemistry:

Winds, Spatial Resolution and perspectives on Future progress

16:20 – 16:40 R. Leigh: Remote sensing of NO2: Integrating slant column measurements into

operational air quality management systems.

16:40 – 17:00 J.P. Veefkind: TROPOMI on the Sentinel 5 Precursor: global urban-scale monitoring

of air quality and climate

17:00 – 17:20 H. Bovensmann: Hourly geostationary observations of key constituents to constrain air pollution and tropospheric chemistry at the Urban scale: GMES Sentinel-4

17:20 – 17:40 M. Buchwitz: Carbon gases (CO2, CO) over anthropogenic source regions: From

SCIAMACHY to CarbonSat

17:40 – 18:30 Wrap-up session, Day 1

Teaming of the Break out Working Groups:

20:00 Dinner

Day 2, 16th May 2013

Block 3: Use of Remote Sensing for Megacity Observations coupled with Models

08:20 – 08:40 M. Gauss: Use of satellite observations in EMEP modelling

08:40 – 09:00 M. Kanakidou: Synergistic use of chemistry-transport modelling and satellite

observations for air pollution control.

09:00 – 09:20 B. Mijling: Fast emission estimates in China and South Africa constrained by

satellite observations

09:20 – 09:40 T. Stavrakou: Addressing the role of major chemical uncertainties on top-down

NOx and VOC emission estimates

09:40 – 10:00 I. Konovalov:Using satellite NO2 measurements to infer multiannual changes in

CO2 emissions in China

10:00 – 10:30 Discussion

10:30 – 11.00 Coffee Break

11:00 – 12:30 Block 4: Working Groups

- A Requirements for evolution of Instrumentation

- B Requirements for Modelling and Inversion

12:30 – 13:30 Lunch Break


13:30 – 14:30 Block 4 continued

14:30 – 15:00 Presentation from working groups


15:15 – 16:00 Planning of the review

Assignment of writing tasks

16:00 End of meeting


8 Participants of the PRESCRIBE workshop Lola Andrés Hernández, IUP Bremen, Germany

Steffen Beirle, MPI Mainz, Germany

Heinrich Bovensmann, IUP Bremen, Germany

Anne Boynard, LATMOS/IPSL, France

Michael Buchwitz, IUP Bremen, Germany

John Burrows, IUP Bremen, Germany

Ron Cohen, UC Berkeley, USA

Gerrit de Leeuw, FMI, Finland

James Drummond, Dalhousie University, Canada

Michael Gauss, MetNo, Norway

Andreas Hilboll, IUP Bremen, Germany

Maria Kanakidou, University of Crete, Greece

Alexander Kokhanovsky, IUP Bremen, Germany

Igor Konovalov, Institute of Applied Physics RAS, Russia

Annette Ladstätter-Weißenmayer, IUP Bremen, Germany

Roland Leigh, University of Leicester, UK

Bas Mijling, KNMI, The Netherlands

Enno Peters, IUP Bremen, Germany

Ulrich Platt, IUP Heidelberg, Germany

Andreas Richter, IUP Bremen, Germany

Cornelia Schlundt, IUP Bremen, Germany

Philipp Schneider, NILU, Norway

Trissevgeni Stavrakou, IASB, Belgium

Pieter Valks, DLR Oberpfaffenhofen, Germany

Pepijn Veefkind, KNMI, The Netherlands

Mihalis Vrekoussis, The Cyprus Institute, Cyprus

Thomas Wagner, MPI Mainz, Germany

Folkard Wittrock , IUP Bremen, Germany

Yong Xue, London Metropolitan University, UK

Jongmin Yoon, MPI Mainz, Germany

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