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QUESTIONS

QUESTIONS. If CFC concentrations were to double, how much faster would ozone loss in Antarctica proceed? The ozone hole is of limited vertical extent because PSCs form only in the lowest part of the polar stratosphere. Why don’t they form at higher altitudes too?.

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QUESTIONS

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  1. QUESTIONS • If CFC concentrations were to double, how much faster would ozone loss in Antarctica proceed? • The ozone hole is of limited vertical extent because PSCs form only in the lowest part of the polar stratosphere. Why don’t they form at higher altitudes too?

  2. CHAPTER 11: TROPOSPHERIC CHEMISTRY

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

  4. 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

  5. WHY WAS TROPOSPHERIC OH SO DIFFICULT TO FIGURE OUT?Production of O(1D) in troposphere takes place in narrow band [290-320 nm] Primary source: O3 + hn g O2 + O(1D) (1) O(1D) + M g O + M (2) O(1D) + H2O g 2OH(3) solar flux I ozone absorption cross-section s Sink: oxidation of reduced species fsI CO + OH g CO2 + H CH4 + OH g CH3 + H2O HCFC + OH Major OH sinks O(1D) quantum yield f g H2O + … GLOBAL MEAN [OH] = 1.0x106 molecules cm-3

  6. 10 ppmv ~tropopause 40 ppbv TYPICAL OZONE PROFILE: ~10% OF OZONE COLUMN GLOBALLY IS IN THE TROPOSPHERE

  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 O3 col = 5x1013 moles • 10% of that is in troposphere • Res. time of air in strat = 2 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 = 2x1013 moles yr-1 SCH4 = 3x1013 moles yr-1 SCO = 10x1013moles yr-1 SCO+ SCH4 > 2FO3 e OH would be titrated! Recycling of OH involving NOx is critical, and this recycling drives tropospheric ozone production

  8. CHAIN MECHANISM FOR O3 PRODUCTION: CO OXIDATION • Initiation: source of HOx (OH production) • Propogation: • CO + OH  CO2 + H • H+ O2 + M HO2 + M • HO2 + NO  OH + NO2 • NO2 + hv (+O2)  NO + O3 • NET: CO + 2O2  CO2 + O3 • Termination: by loss of HOx (self reaction of HO2) • Propagation efficiency of the chain determined by the abundance of NOx NOTE: HOx and NOx catalyze O3 production in the troposphere, and O3 destruction in the stratosphere! The key difference is that [O3] and [O] are much lower in the troposphere, thus NO2 does not react with O, and OH is far more likely to react with CO, HC, etc. than with O3

  9. RADICAL CYCLE CONTROLLING TROPOSPHERIC OH AND OZONE CONCENTRATIONS O2 hn O3 STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, CH4 SURFACE

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

  11. SATELLITE OBSERVATION OF CARBON MONOXIDE MOPITT CO (2000)

  12. SATELLITE OBSERVATIONS OF BIOMASS FIRES (1997)

  13. GLOBAL DISTRIBUTION OF CONOAA/GMD surface air measurements

  14. SPACE-BASED METHANE COLUMN OBSERVATIONS by solar backscatter at 2360-2385 nm

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

  16. HISTORICAL TRENDS IN METHANE The last 30 years The last 1000 years

  17. CHAIN MECHANISM FOR O3 PRODUCTION: CH4 OXIDATION • Initiation: source of HOx (OH production) • Propogation: • CH4 + OH CH3 + H2O • CH3 + O2 + M  CH3O2 + M • CH3O2 + HO2  CH3OOH + O2 • CH3O2+ NO  CH3O + NO2 • CH3OOH + OH  CH2O + OH + H2O • CH3OOH + OH  CH3O2 + H2O • CH3OOH + hv  CH3O + OH • CH3O + O2  CH2O + HO2 • CH2O + OH  CHO + H2O • CH2O + hv + O2  CHO + HO2 • CH2O + hv  CO + H2 • CHO + O2  CO + HO2 • (…then CO oxidation…) * * • Oxidation from C(-IV) in CH4 through to C(+IV) in CO2 * * * Ozone production from NO2 photolysis following peroxy+NO rxns (where peroxy radicals generated by reactions above) High NOx: CH3O2 and HO2 react only with NO, and CH2O removed only by photolysis CH4 + 10O2  CO2 + H2O + 5O3 + 2OH Low NOx: CH3O2 reacts with HO2, CH3OOH reacts with OH and CH2O reacts with OH CH4 + 3OH + 2O2  CO2 + 3H2O + HO2

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