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 radical s) and nitrogen oxide radicals (NOx = NO + NO2). Heterogeneous chemistry in volving reactions in aerosol particles and cloud droplets may affect O-3 co ncentrations in a number of ways including production and loss of HOx and N Ox, direct loss of O-3 and production of halogen radicals. Current knowledg e and hypotheses regarding these processes are reviewed. It is recommended that standard O-3 models include in their chemical mechanisms the following reaction probability parameterizations for reactive uptake of gases by aqu eous aerosols and clouds: gamma(HO2) = 0.2 (range 0.1-1) for HO2 --> 0.5 H2 O2, gamma(NO2) = 10(-4) (10(-6)-10(-3)) for NO2 --> 0.5 HONO + 0.5 HNO3, ga mma(NO3) = 10(-3) (2 X 10(-4)-10(-2)) for NO3 --> HNO3, and gamma(N2O5) = 0 .1 (0.01-1) for N2O5 --> 2 HNO3. Current knowledge does not appear to warra nt a more detailed approach. Hypotheses regarding fast O-3 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 pos sibly be significant in the marine boundary layer but more evidence is need ed. Recommendations for future research are presented. They include among o thers (1) improved characterization of the phase and composition of atmosph eric aerosols, in particular the organic component; (2) aircraft and ship s tudies of marine boundary layer chemistry; (3) measurements of HONO vertica l profiles in urban boundary layers, and of the resulting HOx source at sun rise; (4) laboratory studies of the mechanisms for reactions of peroxy radi cals, NO2 and HNO3 on surfaces representative of atmospheric aerosol; and ( 4) laboratory studies of O-3 reactivity on organic aerosols and mineral dus t. (C) 2000 Elsevier Science Ltd. All rights reserved.