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

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

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 [290-320 nm] solar flux I ozone absorption cross-section s fsI O(1D) quantum yield f

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

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

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

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

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

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

SATELLITE OBSERVATIONS OF BIOMASS FIRES (1997)

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?

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

GLOBAL METHANE SOURCES, Tg a-1 [IPCC, 2007] BIOMASS BURNING 10-90 ANIMALS 80-90 WETLANDS 100-230 LANDFILLS 40-70 GAS 50-70 TERMITES 20-30 COAL 30-50 RICE 30-110

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

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

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

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

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

NITROGEN DIOXIDE FROM THE OMI SATELLITE (MARCH 2006)

LIGHTNING FLASHES SEEN FROM SPACE (2000) DJF JJA

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

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]

TROPOSPHERIC OZONE COLUMN DATA FROM SPACE June-August 2006 observations

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

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]

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

RECENT TRENDS IN TROPOSPHERIC OH inferred from methylchloroform observations