CHAPTER 10: STRATOSPHERIC CHEMISTRY

1. CHAPTER 10: STRATOSPHERIC CHEMISTRY

2. 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

3. STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979 Method: UV solar backscatter Last Saturdays’s ozone layer… Notice the Antarctic ozone hole l1 l2 Ozone layer Scattering by Earth surface and atmosphere Ozone absorption spectrum l1 l2

4. CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930) Odd oxygen family [Ox] = [O3] + [O] slow R2 R1 O2 O O3 fast R3 R4 slow

5. Lifetime of O atoms: STEADY-STATE ANALYSIS OF CHAPMAN MECHANISM …is sufficiently short to assume steady state for O: …so the budget of O3 is controlled by the budget of Ox. Lifetime of Ox: Ox Steady state for Ox:

6. SOLAR SPECTRUM AND ABSORPTION X-SECTIONS O3+hv O2+hv

7. PHOTOLYSIS RATE CONSTANTS: VERTICAL DEPENDENCE quantum yield absorption X-section photon flux

8. shape determined by j1nO2 CHAPMAN MECHANISM vs. OBSERVATION -3 Chapman mechanism reproduces shape, but is too high by factor 2-3 e missing sink!

9. Chapman got it almost right…CATALYTIC CYCLES FOR OZONE LOSS: General Idea O3 + X  XO + O2 O + XO  X + O2 Net: O3 + O  2 O2 X is a catalyst The catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations

10. H2O mixing ratio WATER VAPOR IN STRATOSPHERE Source: transport from troposphere, oxidation of methane (CH4)

11. HOx-CATALYZED OZONE LOSS HOx H + OH + HO2 hydrogen oxide radical family Initiation: Propagation: Termination: slow H2O OH HO2 fast HOx radical family slow

12. H2O mixing ratio NITROUS OXIDE IN THE STRATOSPHERE

13. NOx-CATALYZED OZONE LOSS (NOx NO + NO2) Also emitted Propagation NO + O3  NO2 + O2 NO + O3  NO2 + O2 NO2 + h NO + O NO2 + O  NO + O2 O + O2 + M  O3 + M Null cycleNet O3 + O  2O2 Initiation N2O + O(1D) 2NO O3 loss rate: • TerminationRecycling • NO2 + OH + M  HNO3 + M HNO3 + h NO2 + OH • NO2 + O3 NO3 + O2 HNO3 + OH NO3 + H2O • NO3 + NO2 + M  N2O5 + M NO3 + h NO2 + O • N2O5 + H2O  2HNO3 N2O5 + hNO2 + NO3 Day Night NOy NOx + reservoirs (HNO3, N2O5, ..)

14. ATMOSPHERIC CYCLING OF NOx AND NOy

15. STRATOSPHERIC OZONE BUDGET FOR MIDLATITUDES CONSTRAINED FROM 1980s SPACE SHUTTLE OBSERVATIONS Approximate closure! Source of Ox Gas-phase chemistry only Paul Crutzen shared 1995 Nobel Prize for his work on the NOx catalyzed destruction of ozone

16. STRATOSPHERIC DISTRIBUTION OF CFC-12

17. ClOx-CATALYZED OZONE LOSS (ClOxCl + ClO) Initiation: Cl radical generation from non-radical precursors (e.g., CFC-12) CF2Cl2 + hn CF2Cl + Cl Propagation: Cl + O3  ClO + O2 ClO + O Cl + O2 Net: O3 + O  2O2 O3 loss rate: Termination: Recycling: Cl + CH4  HCl + CH3 HCl + OH  Cl + H2O ClO + NO2 + M  ClNO3 + M ClNO3 + hv Cl + NO3 Cly ClOx + reservoirs (HCl, ClNO3) http://www.atmos.washington.edu/2004Q4/211/09_OzoneDep.swf

18. ATMOSPHERIC CYCLING OF ClOx AND Cly Molina and Rowland shared 1995 Nobel Prize for their work on the ClOx catalyzed destruction of ozone