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
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
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
Oxidation
Reduced gas
EARTH
SURFACE
Oxidized gas/
aerosol
Uptake
Emission
Reduction
Example: Biogeochemical cycle of mercury
ANTHROPOGENIC
PERTURBATION:
fuel combustion
mining
VOLATILE
Hg(0)
oxidation
(months)
WATER-SOLUBLE
Hg(II)
volcanoes
erosion
ATMOSPHERE
OCEAN/SOIL
Hg(0)
Hg(II)
reduction
uplift
particulate
biological
uptake
Hg
burial
SEDIMENTS
CO and methane account for most of reduced gas
flux to atmosphere
• CO observed from
space: 50-200 ppb
• Methane observed from
space: 1650-1800 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 absorption
cross-section s
fsI
O(1D)
quantum
yield f
MEAN VERTICAL DISTRIBUTION OF ATMOSPHERIC OZONE:
only 10% is in the troposphere
OZONE CHEMISTRY IN STRATOSPHERE
O2  h  O  O
O  O2  M  O3  M
O3  h  O2  O (1 D )
O(1 D )  M  O  M
XO  O  X  O2
By contrast, in troposphere:
• no photons < 240 nm
no oxygen photolysis;
• neglible O atom conc.
no XO + O loss
O2+hv
O3+hv
UNTIL ~1990, PREVAILING VIEW WAS THAT
TROPOSPHERIC OZONE ORIGINATED MAINLY
FROM STRATOSPHERE…but that cannot work.
•
•
•
Estimate ozone flux FO3 across tropopause (strat-trop exchange)
– Total O3 col = 5x1013 moles
FO3 = 3x1013 moles yr-1
– 10% of that is in troposphere
– Res. time of air in strat = 1.4 yr
Estimate CH4 source SCH4:
– Mean concentration = 1.7 ppmv
SCH4 = 3x1013 moles yr-1
– Lifetime = 9 years
Estimate CO source SCO:
– Mean concentration = 100 ppbv
SCO = 9.7x1013moles yr-1
– Lifetime = 2 months
SCO+ SCH4 > 2FO3 e
OH would be titrated!
We need a much larger source of tropospheric ozone
CONSTRAINT ON CROSS-TROPOPAUSE OZONE FLUX
FROM OBSERVED OZONE-NOy CORRELATION
IN LOWER STRATOSPHERE
NOy / N2O  0.073 (  14%)
N2O  O(1 D)  2NO
NOy chemical family
Oxidation products (HNO3, etc.)
FN2O = EN2O
in lower strat.: NOy / O3  0.0033 ( 12%)
FO3 
EN2O = 13 Tg N yr-1 (±17%)
FN 2O (NOy / N2O)
NOy / O3
tropopause
 540  140 Tg O3 yr -1
OZONE LOSS IN TROPOSPHERE
FO3  540  140 Tg O3 yr
tropopause
-1
DO3  1000  200 Tg O3 yr
-1
deposition
Chemical loss:
O(1 D)  H 2O  2OH
OH  O3  HO2  O2
HO2  O3  OH  2O2
LO3  4600  700 Tg O3 yr -1
Ozone chemical loss is driven by photolysis
frequency J(O3 O(1D)) at 300-320 nm:

0
Closing the tropospheric ozone budget requires
a tropospheric chemical source >> FO3
dJ/d, 10-6 s-1 nm-1
J   q( )s ( ) I ( )d 
OZONE PRODUCTION IN TROPOSPHERE
Photochemical 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 VOC:
Oxidation of CO:
RH  OH  R  H 2O
CO  OH  CO2  H
H  O2  M  HO2  M
R  O2  M  RO2  M
HO2  NO  OH  NO2
RO2  NO  RO  NO2
NO2  h  NO  O
O2
NO2  h 
 NO  O3
O  O2  M  O3  M
Net: CO  2O2  CO2  O3
OH can also add to
double bonds of
unsaturated VOCs
RO can also
decompose or
isomerize; range of
carbonyl products
RO  O2  R ' CHO  HO2
HO2  NO  OH  NO2
Net: RH  4O2  R ' CHO  2O3  H 2O
Carbonyl products can react with OH to produce
additional ozone, or photolyze to generate more
HOx radicals (branching reaction)
GLOBAL BUDGET OF TROPOSPHERIC OZONE (Tg O3 yr-1)
IPCC (2007) average of 12 models
O2
h
Chem prod in
troposphere
4700
±700
Chem loss in
troposphere
4200
±500
Transport from
stratosphere
500
±100
Deposition
1000
±200
O3
Ozone lifetime: 24 ± 4 days
STRATOSPHERE
8-18 km
TROPOSPHERE
h
O3
Deposition
NO2
NO
OH
HO2
h, H2O
CO, VOC
H2O2
OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONS
Box model calculation
NOx-limited regime
Ridge
NOxsaturated
regime