V. Ozone Cycling and CFCs Catalytic Cycles Termination Steps Coupling 3rd Order Kinetics

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 *geochemical cycles – iron deposition *acidification - *Human health – air quality standards based on PM2.5. Small particles can be respired and aren’t caught up in mucus.

Cycles aid understanding, 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 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

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

Catalytic Cycles Interact

Predicted Affect of CFCs on Ozone Year of Kinetics Database used for Prediction

Association Reactions (XO + NO2) [M] = total concentration of gas phase molecules 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

Termolecular.xls Reaction ClO + NO2 ko(300) 1.80E-31 n 3.40E+00 kinf(300) 1.50E-11 m 1.9 h (km) T [M] ko(T)*[M] kinf(T) k(P,T) 10 222 8.50E+18 4.3E-12 2.7E-11 2.7E-12 20 215 2.00E+18 1.1E-12 2.8E-11 9.0E-13 25 218 9.00E+17 4.8E-13 4.2E-13 30 223 3.00E+17 1.5E-13 2.6E-11 1.4E-13 40 240 1.00E+17 3.8E-14 2.3E-11 3.6E-14 50 268 1.70E+16 4.5E-15 1.9E-11 4.3E-15

Fate of ClO 25 km, 218 K k(ClO + NO2) = 4.2 x 10-13 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 ?

Null Cycles ClO + NO2 → ClONO2 ClONO2 + hn → ClO + NO2 Cl + O3 → ClO + O2 ClO + NO → Cl + NO2 NO2 + hn → O + NO O + O2 → O3

Radical Families Compete (at a given altitude)

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)

Decade-Scale Ozone Loss Causes CFCs Global circulation Polar ozone holes October

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