TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY Troposphere Stratosphere: 90% of total The many faces of...
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Transcript of TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY Troposphere Stratosphere: 90% of total The many faces of...
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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY
Troposphere
Stratosphere:90% of total
The many faces of atmospheric ozone:
In stratosphere: UV shield
In middle/upper troposphere: greenhouse gas
In lower/middle troposphere: precursor of OH, main atmospheric oxidant
In surface air: toxic to humans and vegetation
00
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The atmosphere as an oxidizing medium
EARTHSURFACE
Emission
Reduced gasOxidized gas/aerosol
Oxidation
Uptake
Reduction
Atmospheric oxidation is critical for removal of many pollutants, e.g.• methane (major greenhouse gas)• Toxic gases such as CO, benzene, mercury…• Gases affecting the stratosphere
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CO and methane account for most of reduced gas flux to atmosphere
• Methane observed from space: 1700-1900 ppb
• CO observed from space: 50-200 ppb
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Production of O(1D) in troposphere takes place in narrow band [290-320 nm]
solar flux Iat sea level
ozone absorptioncross-section s
O(1D)quantumyield f
fsI
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Stratospheric ozone mechanism doesn’t apply to troposphere
2
2 3
13 2
1
2
( )
( )
O h O O
O O M O M
O h O O D
O D M O M
XO O X O
O2+hvO3+hv
By contrast, in troposphere:
• no photons < 240 nm no oxygen photolysis;
• neglible O atom conc.gno XO + O loss
In stratosphere:
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Ozone production in tropospherePhotochemical oxidation of CO and volatile organic compounds (VOCs)
catalyzed by HOx and NOx
HOx ≡ H + OH + HO2 + RO + RO2
NOx ≡ NO + NO2
Oxidation of CO:
2
2 2
2 2
2
2 3
2 2 3Net: 2
CO OH CO H
H O M HO M
HO NO OH NO
NO h NO O
O O M O M
CO O CO O
2
2
2 2
2 2
2 3
2 2
2 2
2 3 2
'
Net: 4 ' 2
O
RH OH R H O
R O M RO M
RO NO RO NO
NO h NO O
RO O R CHO HO
HO NO OH NO
RH O R CHO O H O
Oxidation of VOC:
Carbonyl products can react with OH to produce additional ozone.
• CO emitted by combustion• VOCs emitted by biosphere,
combustion, industry• NOx emitted by combustion,
biosphere, lightning
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Chemical mechanism for tropospheric ozone and OH
O3
HO2 H2O
NO
H2O2
CO
NO2
hn
O(1D)
hn
M OH
HNO3
NOx-limited regime: ~ [O3]1/2[H2O]1/2[NO] ~ [O3]1/2[H2O]1/2[NO]/[CO]
NOx-saturated regime: ~ [O3][H2O][CO]/[NO2] ~ [O3][H2O]/[NO2]
P(O3) [OH]
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GLOBAL BUDGET OF TROPOSPHERIC OZONE
O3
O2 hn
O3
OH HO2
hn, H2O
Deposition
NO
H2O2
CO, VOC
NO2
hn
STRATOSPHERE
TROPOSPHERE
8-18 km
Chem prod in troposphere,Tg y-1
43001600
Chem loss in troposphere,Tg y-1
40001600
Transport from stratosphere,Tg y-1
400400
Deposition,Tg y-1
700400
Burden, Tg 360230
Lifetime, days 2842
Present-day Preindustrial
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ATMOSPHERIC CARBON MONOXIDE
Source: incomplete combustionSink: oxidation by OH (lifetime of 2 months)
Satellite CO data at 500 hPa
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ATMOSPHERIC METHANE
LIVESTOCK80-90
LANDFILLS40-70
GAS50-70
COAL30-50RICE
30-110
TERMITES20-30
WETLANDS100-230
BIOMASSBURNING10-90
2005 methane emissions
Global sources,Tg a-1 Sink: oxidation by OH
(lifetime of 10 years)
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METHANE: #2 ANTHROPOGENIC GREENHOUSE GASGreenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]
Referenced to concentration Referenced to emission
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HISTORICAL TRENDS IN METHANE
The last 1000 years
The last 30 years
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Methane column observed by SCIAMACHY, 2003-2009
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NOx EMISSIONS (Tg N a-1) TO TROPOSPHERE
FOSSIL FUEL 23.1
AIRCRAFT 0.5
BIOFUEL 2.2
BIOMASSBURNING 5.2
SOILS 5.1
LIGHTNING 5.8
STRATOSPHERE 0.2
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LIGHTNING FLASHES SEEN FROM SPACE (2000)
DJF
JJA
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SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2
SCIAMACHY data. May-Oct 2004
(R.V. Martin, Dalhousie U.)
detectionlimit
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TROPOSPHERIC NO2 FROM THE OMI SATELLITE INSTRUMENT (MARCH 2006)
March 2006
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NOx EMISSION TRENDS SEEN FROM SPACE
Hillboll et al. [2012]
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GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE
Zhang et al. [2010]
TES thermal IR satellite observations for 2006, seasonal means at 500 hPa
• Maximum values at northern mid-latitudes in spring-summer due to anthropogenic pollution;
• High values in tropical regions affected by seasonal biomass burning;
• Minimum values over tropical oceans due to chemical loss
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OBSERVED RISE IN TROPOSPHERIC OZONEOVER 20th CENTURY
Observations at mountain sites in Europe [Marenco et al., 1994]
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GLOBAL OZONE AND OH TRENDSMean mass-weighted tropospheric values computed from GEOS-Chem model
for present-day (PD), pre-industrial (PI), and last glacial maximum (LGM)
Ozone, ppb
OH, 105 molecules cm-3
PD PI LGMtwo different climatereconstructionsLee Murray, Harvard
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Inferring recent OH trends from methylchloroform data
Global trendsin methylchloroform concentrations
Global trendsin OH concentrations
Montzka et al. [2011]
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LONDON FOGAerosols a.k.a.particulate matter (PM) from domestic+industrial coal combustion
“Killer fog” of December 1952 resulted in 10,000 excess deaths
Coal combustionTemperature
Altitude
inversion
sulfateorganic carbonblack carbon
particles
< 1km
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LOS ANGELES SMOGRespiratory problems, vegetation damage due to high surface ozone
troposphere
stratosphere8-18 km
temperature
inversionozone
altitude
Nitrogen oxides (NOx ≡ NO + NO2) Volatile organic compounds (VOCs)
UV radiation Ozone (O3)
vehicles, industry, vegetation
produced by photolysisof oxygen (O2)
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AIR POLLUTION IN THE US TODAY:Ozone is the #1 pollutant
75 ppb (8-h average)
http://epa.gov/airtrends
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4th-highest annual maximum for daily 8-h average ozone,2008-2010
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The 2012 ozone season
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0 20 40 60 80 100 120 ppb
Europe AQS(seasonal)
U.S. AQS(8-h avg.)
U.S. AQS(1-h avg.)
Preindustrialozone
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
20082014? 1997
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OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed
NOx-saturated
NOx-limited Ridge
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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
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MAPPING OF VOC EMISSIONS FROM SPACEusing satellite measurements of formaldehyde
confirms dominance of biogenic over anthropogenic VOCs
Millet et al. [2008]
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1970-2003 TREND OF U.S. EMISSIONS
Focus until past decade was on VOC emission controls
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OZONE TRENDS IN U.S. http://www.epa.gov/airtrends/
Boston trend
National trend
• The national trend is heavily weighted by California sites;
• other parts of the country have seen marginal improvement
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DECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE
Decreasing US NOx emissions from power plants
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Decrease of North American NOx emisssions, 2005-2009
as seen with annual mean NO2 columns from the OMI satellite instrument
Shailesh Kharol (Dalhousie)
20092005
Decreases in both the eastern US and eastern Canada
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Ozone trends in the Northeast, 1997-2012
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EVEN IN NOx-LIMITED REGIME,THE TOTAL O3 PRODUCED IS ONLY A WEAK FUNCTION OF NOx
NO NO2 HNO3hv
HO2,RO2,O3 OH, O3
P(O3) L(NOx)
3 2
2 2
( ) [ ][ ] [ ]~ ~
( ) [ ][ ] [ ]OPE =
x
P O HO NO VOC
L NO NO OH 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 emissionsby 50% reduces ozone only by ~15%
Define ozone production efficiency (OPE) as the total number of O3 molecules produced per unit NOx emitted.
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Median surface ozone, 2006-2010:emerging problem in the Intermountain West
Cooper et al. [2012]
Spring
Summer
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Daytime surface ozone trends, 1990-2010
Spring SummerCooper et al. [2012]
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8-h daily maximum ozone in the intermountain West (2006)
56.3 ppb 53.2 ppb12.3 ppb
58.1 ppb 56.4 ppb
15.0 ppb
GEOS-Chem model US pollutioncontribution
Zhang et al., 2011
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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
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Growing NOx emissions from Canadian oil sands
Oil sand recoveryIn Alberta
Satellite NO2 columns, 2004-2010
NO2 increase of 10.4 ±3.5% per year
McLinden et al. [GRL 2012]
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Ozone trends in remote air at northern mid-latitudes
D.D. Parrish, NOAA
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Rising surface ozone pollution in China
D.D. Parrish, NOAA
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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