Heterogeneous Chemistry and Tropospheric Ozone

Ozone is produced in the troposphere by gas-phase oxidation of hydrocarbons and CO catalyzed by hydrogen oxide radicals (HOx ≡ OH + H + peroxy radicals) and nitrogen oxide radicals (NOx ≡ NO+NO2). Heterogeneous chemistry involving reactions in aerosol particles and cloud droplets may affect O3 concentrations in a number of ways including production and loss of HOx and NOx, direct loss of O3, and production of halogen radicals. Current knowledge and hypotheses regarding these processes are reviewed. It is recommended that standard O3 models include in their chemical mechanisms the following reaction probability parameterizations for reactive uptake of gases by aqueous aerosols and clouds: γHO2 = 0.2 (range 0.1-1) for HO2 → 0.5 H2O2, γNO2 = 10-4 (10-6-10-3) for NO2 → 0.5 HONO + 0.5 HNO3, γNO3 = 10-3 (2x10-4-10-2) for NO3 → HNO3, and γN2O5 = 0.1 (0.01-1) for N2O5 → 2 HNO3. Current knowledge does not appear to warrant a more detailed approach. Hypotheses regarding fast O3 loss on soot or in clouds, fast reduction of HNO3 to NOx in aerosols, or heterogeneous loss of CH2O are not supported by evidence. Halogen radical chemistry could possibly be significant in the marine boundary layer but more evidence is needed. Recommendations for future research are presented. They include among others (1) improved characterization of the phase and composition of atmospheric aerosols, in particular the organic component; (2) aircraft and ship studies of marine boundary layer chemistry; (3) measurements of HONO vertical profiles in urban boundary layers, and of the resulting HOx source at sunrise; (4) laboratory studies of the mechanisms for reactions of peroxy radicals, NO2, and HNO3 on surfaces representative of atmospheric aerosol; and (4) laboratory studies of O3 reactivity on organic aerosols and mineral dust.