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

  3. 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]

  4. 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

  5. 10 ppmv ~tropopause 40 ppbv TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE

  6. O2+hv O3+hv Stratospheric ozone mechanism doesn’t apply to troposphere In stratosphere: • By contrast, in troposphere: • no photons < 240 nm • no oxygen photolysis; • neglible O atom conc. • gno XO + O loss

  7. 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 O3col = 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!

  8. Photochemical oxidation of CO and volatile organic compounds (VOCs) catalyzed by HOx and 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)

  9. GLOBAL BUDGET OF TROPOSPHERIC OZONE (MODEL) Present-day Preindustrial O2 hn O3 STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, VOC

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

  11. SATELLITE OBSERVATIONS OF TROPOSPHERIC NO2 SCIAMACHY data. May-Oct 2004 (R.V. Martin, Dalhousie U.) detection limit

  12. 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

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

  14. TES thermal IR satellite observations for 2006, seasonal means at 500 hPa • Maximum values at northern mid-latitudes in spring-summer due to anthropogenic pollution; • High values in tropical regions affected by seasonal biomass burning; • Minimum values over tropical oceans due to chemical loss GLOBAL DISTRIBUTION OF TROPOSPHERIC OZONE Zhang et al. [2010]

  15. LONDON FOG Aerosols a.k.a.particulatematter (PM) from domestic+industrialcoal combustion “Killer fog” of December 1952 resulted in 10,000 excess deaths Altitude inversion < 1km sulfate organic carbon black carbon particles Coal combustion Temperature

  16. Respiratory problems, vegetation damage due to high surface ozone LOS ANGELES SMOG altitude produced by photolysis of oxygen (O2) stratosphere 8-18 km troposphere temperature ozone inversion UV radiation Nitrogen oxides (NOx≡ NO + NO2) Volatile organic compounds (VOCs) Ozone (O3) vehicles, industry, vegetation

  17. AIR POLLUTION IN THE US TODAY:Ozone and fine particulate matter (PM2.5) are the two main pollutants 75 ppb (8-h average) 15 mg m-3 (1-y av.) http://epa.gov/airtrends/2010/ PM2.5 Ozone

  18. 2008 REVISION TO OZONE STANDARD FROM 84 to 75 PPBCAUSED MORE U.S. AREAS TO BE OUT OF COMPLIANCE

  19. …AND INCREASED THE IMPORTANCE OF THE OZONE BACKGROUND Canadian AQS (8-h avg.) Europe AQS (8-h avg.) Mexican AQS (1-h avg.) Europe AQS (seasonal) U.S. AQS (8-h avg.) U.S. AQS (1-h avg.) 2014? 2008 0 20 40 60 80 100 120 ppb Preindustrial ozone background Present-day ozone background at northern mid-latitudes

  20. Currently proposed 60-70 ppb standard would have extensive non-compliance

  21. OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed NOx-limited Ridge NOx- saturated

  22. LARGE SUPPLY OF BIOGENIC VOCs – unrecognized until the 1990s Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime! recognized in Revised Clean Air Act of 1999 Anthropogenic VOCs Isoprene (biogenic VOC) Jacob et al., J. Geophys. Res. [1993]

  23. MAPPING OF VOC EMISSIONS FROM SPACEusing satellite measurements of formaldehyde confirms dominance of biogenic over anthropogenic VOCs Millet et al. [2008]

  24. 1970-2003 TREND OF U.S. EMISSIONS Focus until past decade was on VOC emission controls

  25. DECREASE OF POWER PLANT NOx EMISSIONSOVER THE PAST DECADE Decreasing US NOx emissions from power plants

  26. Growth Trends Gross State Product Vehicle Miles Traveled Population

  27. Emission Trends CO2 ROG NOx SOx CO

  28. PM10Trends South Coast San Joaquin Valley State Standard

  29. 0.5 0.4 South Coast 0.3 Max 1-hr Ozone (ppm) San Joaquin Valley 0.2 State Standard 0.1 0.0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 Ozone Trends

  30. Stage II > 350 ppb O3 Stage I > 200 ppb O3 Historical Ozone Levels Number of Days

  31. 0.6 0.5 0.4 (ppm) 0.3 3 1971 – CARB adopts 1st auto NOx standards Max O 1970 – EPA created, CAA Ammend. 0.2 1976 – Volvo introduces first car to 1984 – Smog Check Program EPA promulgates NAAQS 1963 – Clean Air Act (CAA) 1952 – Prof. Haagen-Smit NAAQS discovers smog formation 1994 – Smog Check II use 3-way catalyst 0.1 1990 – CAAA 0.0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Years SOUTH COAST O3 HISTORY

  32. Improvement Over Past 20 Years 0% Nitrogen Sulfur Carbon Ozone PM10 Air Toxics Dioxide Dioxide Monoxide (Cancer Risk) -20% -40% Percent Change Approaching Standards -60% Attained Standards -80%

  33. T H A N K S ! !

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