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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY. The many faces of atmospheric ozone:. In stratosphere: UV shield. Stratosphere: 90% of total. In middle/upper troposphere: greenhouse gas. Troposphere. In lower/middle troposphere: precursor of OH, main atmospheric oxidant. - PowerPoint PPT Presentation
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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
TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes
at different temperatures
THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES
EARTHSURFACE
Emission
Reduced gas Oxidized 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
Example: Biogeochemical cycle of mercury
Hg(0) Hg(II)
particulateHg
burial
SEDIMENTS
uplift
volcanoeserosion
oxidation
Hg(0) Hg(II)reduction biological
uptake
ANTHROPOGENIC PERTURBATION:fuel combustion
mining
ATMOSPHERE
OCEAN/SOIL
VOLATILE WATER-SOLUBLE
(months)
CO and methane account for most of reduced gas flux to atmosphere
• Methane observed from space: 1650-1800 ppb
• CO observed from space: 50-200 ppb
THE TROPOSPHERE WAS VIEWED AS CHEMICALLY INERT UNTIL 1970
• “The chemistry of the troposphere is mainly that of of a large number of atmospheric constituents and of their reactions with molecular oxygen…Methane and CO are chemically quite inert in the troposphere” [Cadle and Allen, Atmospheric Photochemistry, Science, 1970]
• Lifetime of CO estimated at 2.7 years (removal by soil) leads to concern about global CO pollution from increasing car emissions [Robbins and Robbins, Sources, Abundance, and Fate of Gaseous Atmospheric Pollutants, SRI report, 1967]
FIRST BREAKTHROUGH:
• Measurements of cosmogenic 14CO place a constraint of ~ 0.1 yr on the tropospheric lifetime of CO [Weinstock, Science, 1969]
SECOND BREAKTHROUGH:• Tropospheric OH ~1x106 cm-3 predicted from O(1D)+H2O, results in tropospheric lifetimes of ~0.1 yr for CO and ~2 yr for CH4 [Levy, J. Geophys. Res. 1973]
THIRD BREAKTHROUGH:• Methylchlroform observations provide indirect evidence for OH at levels of 2-5x105 cm-3 [Singh, Geophys. Res. Lett. 1977]
…but direct measurements of tropospheric OH had to wait until the 1990s
WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm]
solar flux I
ozone absorptioncross-section
O(1D)quantumyield
I
MEAN VERTICAL DISTRIBUTION OF ATMOSPHERIC OZONE:only 10% is in the troposphere
OZONE CHEMISTRY IN STRATOSPHERE
2
2 3
13 2
1
2
( )
( )
O h O OO O M O M
O h O O D
O D M O MXO O X O
O2+hvO3+hv
By contrast, in troposphere:
• no photons < 240 nmno oxygen photolysis;
• neglible O atom conc.no XO + O loss
• Estimate ozone flux FO3 across tropopause (strat-trop exchange)– Total O3 col = 5x1013 moles– 10% of that is in troposphere– Res. time of air in strat = 1.4 yr
• Estimate CH4 source SCH4:– Mean concentration = 1.7 ppmv– Lifetime = 9 years
• Estimate CO source SCO:– Mean concentration = 100 ppbv– Lifetime = 2 months
UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work.
FO3 = 3x1013 moles yr-1
SCH4 = 3x1013 moles yr-1
SCO = 9.7x1013moles yr-1
SCO+ SCH4 > 2FO3 OH would be titrated!
We need a much larger source of tropospheric ozone
CONSTRAINT ON CROSS-TROPOPAUSE OZONE FLUXFROM OBSERVED OZONE-NOy CORRELATION
IN LOWER STRATOSPHERE
EN2O = 13 Tg N yr-1 (±17%)
FN2O = EN2O
12 ( ) 2N O O D NO
Oxidation products (HNO3, etc.)
2/ 0.073 ( 14%)yNO N O
3in lower strat.: / 0.0033 ( 12%)yNO O
2 2 -13 3
3
( / )540 140 Tg O yr
/N O y
Oy
F NO N OF
NO O
tropopause
NOy chemical family
OZONE LOSS IN TROPOSPHERE
tropopause-1
3 3540 140 Tg O yrOF 1
2
3 2 2
2 3 2
( ) 2
2
O D H O OHOH O HO OHO O OH O
Chemical loss:
-13 31000 200 Tg O yrOD
-13 34600 700 Tg O yrOL
deposition
Ozone chemical loss is driven by photolysis frequency J(O3 O(1D)) at 300-320 nm:
0
( ) ( ) ( )J q I d
dJ/d
, 1
0-6 s
-1 n
m-1
Closing the tropospheric ozone budget requires a tropospheric chemical source >> FO3
OZONE PRODUCTION IN TROPOSPHEREPhotochemical oxidation of CO and volatile organic compounds (VOCs)
catalyzed by hydrogen oxide radicals (HOx) and nitrogen oxide radicals (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 HH O M HO MHO NO OH NONO h NO OO 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 OR O M RO MRO NO RO NO
NO h NO ORO O R CHO HOHO NO OH NO
RH O R CHO O H O
Oxidation of VOC:
RO can also decompose or isomerize; range of carbonyl products
Carbonyl products can react with OH to produce additional ozone, or photolyze to generate more HOx radicals (branching reaction)
OH can also add to double bonds of unsaturated VOCs
GLOBAL BUDGET OF TROPOSPHERIC OZONE (Tg O3 yr-1)
O3
O2 h
O3
OH HO2
h, H2O
Deposition
NO
H2O2
CO, VOC
NO2
h
STRATOSPHERE
TROPOSPHERE8-18 km
Chem prod in troposphere 4700
±700Chem loss in troposphere 4200
±500Transport from stratosphere 500
±100Deposition 1000
±200
IPCC (2007) average of 12 models
Ozone lifetime: 24 ± 4 days
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation
NOx-saturatedregime
NOx-limited regime Ridge