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V Stratosphere

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Published on February 20, 2008

Author: Calogera

Source: authorstream.com

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V. Ozone Cycling and CFCs Catalytic Cycles Termination Steps Coupling 3rd Order Kinetics:  V. Ozone Cycling and CFCs Catalytic Cycles Termination Steps Coupling 3rd Order Kinetics Slide2:  Catalytic Cycles- Ox Chemistry Chapman Cycle ( © 1930) O2 + hn → O + O O + O2 → O3 O3 + hn → O + O2 O + O3 → O2 + O2 Overpredicts [O3] by x2 Slide3:  Catalytic Cycles- Famous Set X + O3 → XO + O2 XO + O → X + O2 O + O3 → O2 + O2 X, XO H, OH OH, HOO Cl, ClO Br, BrO NO, NO2 Caveats Other types of cycles Cycles aid understanding, not natural units Fate of Atomic Chlorine:  Fate of Atomic Chlorine 25 km, 218 K k(Cl + O3) = 8.8 x 10-12 cm3 molecule-1 s-1 k(Cl + CH4) = 1.8 x 10-14 cm3 molecule-1 s-1 [O3], [CH4] from figures in Section D of Course pack What fraction of Cl reacts with O3 at 25 km ? (Spreadsheet in section F) Catalysis vs. Stoichiometry Slide5:  Radical Families Interact to Create Reservoir Species ClO + NO2 → ClONO2 OH + NO2 → HONO2 Cl + CH4 → HCl + CH3 Reservoir species regenerate radicals… ClONO2 + hn → ClO + NO2 HONO2 + hn → OH + NO2 HCl + OH → Cl + HOH …faster than permanent removal Slide6:  Catalytic Cycles Interact Predicted Affect of CFCs on Ozone:  Predicted Affect of CFCs on Ozone Year of Kinetics Database used for Prediction Association Reactions (XO + NO2) :  Association Reactions (XO + NO2) ko(T) and k∞(T) expressed as power law k(T) = k300 (T/300)-x Or use the Excel Spreadsheet at my web site and Table 2 of the JPL Data Evaluation with F=0.6 [M] = total concentration of gas phase molecules Termolecular.xls :  Termolecular.xls Fate of ClO:  Fate of ClO 25 km, 218 K k(ClO + NO2) = 4.2 x 10-13 cm3 molecule-1 s-1 k(ClO + NO) = 2.4 x 10-11 cm3 molecule-1 s-1 k(ClO + O) = 4.1 x 10-11 cm3 molecule-1 s-1 [O], [NO], [NO2] from figures in Section D of Course pack What fraction of ClO reacts with O at 25 km ? Slide11:  Null Cycles ClO + NO2 → ClONO2 ClONO2 + hn → ClO + NO2 Cl + O3 → ClO + O2 ClO + NO → Cl + NO2 NO2 + hn → O + NO O + O2 → O3 Slide12:  Radical Families Compete (at a given altitude) Slide13:  Ozone-Friendly CFC Substitutes Chlorofluorocarbons (CFCs) – don’t react with anything - don’t absorb light in troposphere Hydro(chloro)fluorocarbons (HFCs and HCFCs) - have hydrogen: react with OH in troposphere CF2ClH + OH → CF2Cl + HOH - HFCs have no Cl (Atomic F is ozone-friendly) Slide14:  Decade-Scale Ozone Loss Causes CFCs Global circulation Polar ozone holes October Key Points:  Key Points Catalytic cycles - Interact and compete XO + O limits slow null cycles CFC substitutes removed in troposphere Kinetics calculations: fate of species steady state concentrations CFCs drive ozone depletion globally

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