EPS200: Atmospheric Chemistry and Physics

Daniel J. Jacob, Harvard University

Fall 2013

Tropospheric gas-phase chemistry: ozone air quality

LONDON FOG

Aerosols a.k.a.particulate matter (PM) from domestic+industrial coal combustion

“Killer fog” of December 1952 resulted in 10,000 excess deaths

Coal combustion Temperature

Altitude

inversion

sulfate

organic carbon

black carbon

particles

< 1km

LOS ANGELES SMOG Respiratory problems, vegetation damage due to high surface ozone

troposphere

stratosphere

8-18 km

temperature

inversion

ozone

altitude

Nitrogen oxides (NOx ≡ NO + NO2)

Volatile organic compounds (VOCs)

UV radiation Ozone

(O3)

vehicles, industry, vegetation

produced by photolysis

of oxygen (O2)

AIR POLLUTION IN THE US TODAY: Ozone is the #1 pollutant

http://epa.gov/airtrends

75 ppb (8-h average)

4th-highest annual maximum of daily 8-h average ozone,

2008-2010

Current standard: 75 ppb

Proposed standard: 60-70 ppb

The 2012 ozone season

0 20 40 60 80 100 120 ppb

Europe AQS

(seasonal)

U.S. AQS

(8-h avg.)

U.S. AQS

(1-h avg.)

Preindustrial

ozone

background

Present-day ozone

background at

northern mid-latitudes

Europe AQS

(8-h avg.)

Canadian AQS

(8-h avg.) Mexican AQS

(1-h avg.)

Ozone air quality standards in the US and in the world

2008 2014? 1997

OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS

Air pollution model calculation for a typical urban airshed

NOx-

saturated

NOx-limited Ridge

LARGE SUPPLY OF BIOGENIC VOCs –

unrecognized until the 1990s

Isoprene (biogenic VOC) Anthropogenic VOCs

Jacob et al., 1993

Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!

recognized in Revised Clean Air Act of 1999

MAPPING OF VOC EMISSIONS FROM SPACE

using satellite measurements of formaldehyde

confirms dominance of biogenic over anthropogenic VOCs

Millet et al. [2008]

1970-2003 TREND OF U.S. EMISSIONS

Focus until past decade was on VOC emission controls

DECREASE OF POWER PLANT NOx EMISSIONS

OVER THE PAST DECADE

Decreasing US NOx emissions

from power plants

Decrease of North American NOx emisssions, 2005-2009

as seen with annual mean NO2 columns from the OMI satellite instrument

Shailesh Kharol (Dalhousie)

2009 2005

Decreases in both the eastern US and eastern Canada

Ozone trends in the Northeast, 1997-2012

EVEN IN NOx-LIMITED REGIME,

THE TOTAL O3 PRODUCED IS ONLY A WEAK FUNCTION OF NOx

NO NO2 HNO3 hv

HO2,RO2,O3 OH, O3

P(O3) L(NOx)

3 5 2 4

8 2 8 2

( ) 2 [ ][ ] 2 [ ]

( ) [ ][ ] [ ] OPE =

x

P O k HO NO k VOC

L NO k NO OH k NO

Emission Deposition

Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by

reaction with OH:

OPE m as NOx k e strong nonlinearity; in models, decreasing NOx emissions

by 50% reduces ozone only by ~15%

Define ozone production efficiency (OPE) as the total number of O3 molecules

produced per unit NOx emitted.

Trend in 95th percentile daytime ozone, 1990-2010

Spring

Summer

Cooper et al. [2012]

• Decrease in eastern US driven by NOx emission controls;

• Increase or flat in Intermountain West

4th-highest annual maximum for daily 8-h average ozone,

2008-2010 Intermountain West: The next ozone frontier!

High elevation, arid terrain → high ozone background

Current standard: 75 ppb

Proposed standard: 60-70 ppb

CASTNet surface ozone in Intermountain West (2006)

• Most of the ozone variability including high-events is due to background

• GEOS-Chem does not capture the highest events (stratosphere)

Zhang et al. [2011]

North American ozone background over the US

4th highest annual North American background ozone (GEOS-Chem model)

Zhang et al. [2011]

defined as the surface ozone concentrations that would be present in the

absence of North American anthropogenic emissions

Using OMI observations

to monitor growth in emissions from Canadian oil sands

Oil sand recovery

In Alberta

OMI NO2 columns, 2004-2010

NO2 increase of 10.4 ±3.5% per year

McLinden et al. [GRL 2012]

AQAST PI: DIckerson

Ozone trends in remote air at northern mid-latitudes

D.D. Parrish, NOAA

Rising surface ozone pollution in China

D.D. Parrish, NOAA

INTERCONTINENTAL OZONE POLLUTION INFLUENCES

Surface O3

enhancements from

North American

anthropogenic

emissions

from European

anthropogenic

emissions

from Asian

anthropogenic

emissions

Lin Zhang, Harvard

GEOS-Chem model results for 2006

Questions

1. Atmospheric measurements of the H2O2/HNO3 concentration ratio offer a

simple diagnostic of whether ozone production in a polluted environment is

NOx-limited or VOC-limited. Why?

2. Observations at polluted sites show a linear relationship between [O3] and

[NOy]-[NOx], where NOy is the sum of NOx and its oxidation products. The

slope of this relationship provides an estimate of the ozone production

efficiency. Why?

3. PAN formation decreases the ozone production efficiency in a NOx source

region but increases the ozone production efficiency on a global scale.

Explain why.