Heterogeneous chemistry and tropospheric ozone

Ozone is produced in the troposphere by gas-phase oxidation of hydrocarbons and CO catalyzed by hydrogen oxide radicals (HO _x≡OH+H+peroxy radicals) and nitrogen oxide radicals (NO _x≡NO+NO ₂). Heterogeneous chemistry involving reactions in aerosol particles and cloud droplets may affect O ₃ concentrations in a number of ways including production and loss of HO _x and NO _x, direct loss of O ₃, and production of halogen radicals. Current knowledge 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 aqueous aerosols and clouds: γ_{HO 2}=0.2 (range 0.1-1) for HO ₂→0.5 H ₂O ₂, γ_{NO 2}=10 ^-4 (10 ^-6-10 ^-3) for NO ₂→ 0.5 HONO+0.5 HNO ₃, γ_{NO 3}=10 ^-3 (2×10 ^-4-10 ^-2) for NO ₃→HNO ₃, and γ_{N 2O 5}=0.1 (0.01-1) for N ₂O ₅→2 HNO ₃. Current knowledge does not appear to warrant a more detailed approach. Hypotheses regarding fast O ₃ loss on soot or in clouds, fast reduction of HNO ₃ to NO _x in aerosols, or heterogeneous loss of CH ₂O 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 HO _x source at sunrise; (4) laboratory studies of the mechanisms for reactions of peroxy radicals, NO ₂, and HNO ₃ on surfaces representative of atmospheric aerosol; and (4) laboratory studies of O ₃ reactivity on organic aerosols and mineral dust.