1. THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES
Atmospheric oxidation is critical for removal of many pollutants, e.g.
methane (major greenhouse gas)
CO (toxic pollutant)
HCFCs (Clx sources in stratosphere)
Oxidation
Oxidized gas/
aerosol
Reduced gas
EARTH
SURFACE
Emission
Uptake
Reduction

2. 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, Science, 1971, 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

3. WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT
WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT? Production of O(1D) in troposphere takes place in narrow band [ nm]
solar flux I
ozone absorption
cross-section s
fsI
O(1D)
quantum
yield f

4. TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE
~tropopause
10 ppmv
40 ppbv

5. 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
10% of that is in troposphere
Res. time of air in strat = 0.7 yr
Estimate CH4 source SCH4:
Mean concentration = 1.7 ppmv
Lifetime = 9 years
Estimate CO source SCO:
Mean concentration = 100 ppbv
Lifetime = 2 months
FO3 = 3x1013 moles yr-1
SCH4 = 3x1013 moles yr-1
SCO = 8x1013moles yr-1
SCO+ SCH4 > 2FO3 e OH would be titrated!
Recycling of OH involving NOx is critical, and this recycling drives tropospheric ozone production

6. RADICAL CYCLE CONTROLLING TROPOSPHERIC OH AND OZONE CONCENTRATIONShn O3
STRATOSPHERE
8-18 km
TROPOSPHERE hn
NO2 NO O3
hn, H2O OH
HO2
H2O2
Deposition
CO, CH4
SURFACE

7. GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL)
Chem prod in troposphere,
Tg y-1
4300
1600
Chem loss in troposphere,
4000
Transport from stratosphere,
400
Deposition,
700
Burden, Tg
360
230
Lifetime, days 28 42
Present-day
Preindustrial O2 hn O3
STRATOSPHERE
8-18 km
TROPOSPHERE hn
NO2 NO O3
hn, H2O OH
HO2
H2O2
Deposition
CO, VOC

8. CARBON MONOXIDE IN ATMOSPHERE
Source: incomplete combustion
Sink: oxidation by OH (lifetime of 2 months)

9. SATELLITE OBSERVATION OF CARBON MONOXIDE
MOPITT
CO columns
(Mar-Apr 01)

10. SATELLITE OBSERVATIONS OF BIOMASS FIRES (1997)

11. SHORT QUESTIONS
How does a thinning of the stratospheric ozone layer affect tropospheric OH concentrations?
2. If the CO source to the atmosphere were to double, would the CO concentration (a) double, (b) less than double, (c) more than double?
3. Methylperoxy radicals produced from methane oxidation can self-react to form methanol:     CH3O2 + CH3O2 g CH3OH + CH2O + O2 What is the effect of this reaction on OH levels?

12. METHANE: #2 ANTHROPOGENIC GREENHOUSE GAS
Greenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]
Referenced to emission
Referenced to concentration

13. GLOBAL METHANE SOURCES, Tg a-1 [IPCC, 2007]
BIOMASS
BURNING
10-90
ANIMALS
80-90
WETLANDS
LANDFILLS
40-70
GAS
50-70
TERMITES
20-30
COAL
30-50
RICE
30-110

14. GLOBAL DISTRIBUTION OF METHANE NOAA/CMDL surface air measurements
Sink: oxidation by OH (lifetime of 10 years)

15. HISTORICAL TRENDS IN METHANE
The last 20 years
The last 1000 years
IPCC [2007]

16. IPCC [2001] Projections of Future CH4 Emissions (Tg CH4) to 2050
Scenarios
900
A1B
A1T
A1F1 A2 B1 B2
IS92a
800
700
600
2000
2020
2040
Year

17. NOx EMISSIONS (Tg N a-1) TO TROPOSPHERE
STRATOSPHERE
0.2
LIGHTNING
5.8
SOILS
5.1
FOSSIL FUEL
23.1
BIOMASS
BURNING
5.2
BIOFUEL
2.2
AIRCRAFT
0.5

18. USING SATELLITE OBSERVATIONS OF NO2 TO MONITOR NOx EMISSIONS
SCIAMACHY data. May-Oct 2004
(R.V. Martin, Dalhousie U.)
detection
limit

19. NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006)

20. LIGHTNING FLASHES SEEN FROM SPACE (2000)
DJF
JJA

21. PEROXYACETYLNITRATE (PAN) AS RESERVOIR FOR LONG-RANGE TRANSPORT OF NOx

22. NOAA/ITCT-2K2 AIRCRAFT CAMPAIGN IN APRIL-MAY 2002 Monterey, CA
Asian pollution plumes transported to California
May 5 plume at 6 km:
High CO and PAN,
no O3 enhancement CO
PAN O3
HNO3
NOx
PAN
May 17 subsiding
plume at 2.5 km:
High CO and O3,
PAN gNOxgHNO3 O3
HNO3 CO
NOx
Hudman et al. [2004]

23. TROPOSPHERIC OZONE COLUMN DATA FROM SPACE
June-August 2006 observations

24. TROPOSPHERIC OZONE: #3 ANTHROPOGENIC GREENHOUSE GAS
Greenhouse radiative forcing of climate between 1750 and 2005 [IPCC, 2007]
Referenced to emission
Referenced to concentration

25. IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC OZONE (0
IPCC RADIATIVE FORCING ESTIMATE FOR TROPOSPHERIC OZONE (0.35 W m-2) RELIES ON GLOBAL MODELS
…but these underestimate the observed rise in ozone over the 20th century
Fitting to observations would imply a radiative forcing of 0.8 W m-2
Preindustrial
ozone models }
Observations at mountain sites in Europe
[Marenco et al., 1994]

26. 1996-2005 NOx EMISSION TREND SEEN FROM SPACE
Van der A et al., 2008

27. RECENT TRENDS IN TROPOSPHERIC OH inferred from methylchloroform observations