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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY

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.

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TROPOSPHERIC OZONE AND OXIDANT CHEMISTRY

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  1. 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

  2. TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes at different temperatures

  3. 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 THE ATMOSPHERE: OXIDIZING MEDIUM IN GLOBAL BIOGEOCHEMICAL CYCLES Oxidation Oxidized gas/ aerosol Reduced gas Uptake EARTH SURFACE Emission Reduction

  4. Example: Biogeochemical cycle of mercury ANTHROPOGENIC PERTURBATION: fuel combustion mining WATER-SOLUBLE VOLATILE oxidation Hg(II) Hg(0) (months) volcanoes erosion ATMOSPHERE OCEAN/SOIL Hg(0) Hg(II) particulate Hg reduction biological uptake burial uplift SEDIMENTS

  5. CO observed from space: 50-200 ppb CO and methane account for most of reduced gas flux to atmosphere • Methane observed from space: 1650-1800 ppb

  6. 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

  7. 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

  8. MEAN VERTICAL DISTRIBUTION OF ATMOSPHERIC OZONE:only 10% is in the troposphere

  9. O2+hv O3+hv OZONE CHEMISTRY IN STRATOSPHERE • By contrast, in troposphere: • no photons < 240 nm • no oxygen photolysis; • neglible O atom conc. • gno XO + O loss

  10. UNTIL ~1990, PREVAILING VIEW WAS THAT TROPOSPHERIC OZONE ORIGINATED MAINLY FROM STRATOSPHERE…but that cannot work. • Estimate ozone flux FO3across 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 FO3 = 3x1013 moles yr-1 SCH4 = 3x1013 moles yr-1 SCO = 9.7x1013moles yr-1 SCO+ SCH4 > 2FO3 e OH would be titrated! We need a much larger source of tropospheric ozone

  11. CONSTRAINT ON CROSS-TROPOPAUSE OZONE FLUXFROM OBSERVED OZONE-NOy CORRELATION IN LOWER STRATOSPHERE NOy chemical family Oxidation products (HNO3, etc.) FN2O = EN2O tropopause EN2O = 13 Tg N yr-1 (±17%)

  12. OZONE LOSS IN TROPOSPHERE tropopause Chemical loss: deposition Ozone chemical loss is driven by photolysis frequency J(O3gO(1D)) at 300-320 nm: Closing the tropospheric ozone budget requires a tropospheric chemical source >> FO3 dJ/dl, 10-6 s-1 nm-1

  13. Photochemical oxidation of CO and volatile organic compounds (VOCs) catalyzed by hydrogen oxide radicals (HOx) and nitrogen oxide radicals (NOx) OZONE PRODUCTION IN TROPOSPHERE HOx = H + OH + HO2 + RO + RO2 NOx = NO + NO2 OH can also add to double bonds of unsaturated VOCs Oxidation of VOC: Oxidation of CO: 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)

  14. GLOBAL BUDGET OF TROPOSPHERIC OZONE (Tg O3 yr-1) IPCC (2007) average of 12 models O2 hn O3 Ozone lifetime: 24 ± 4 days STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, VOC

  15. OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSBox model calculation NOx-limited regime Ridge NOx- saturated regime

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