THE INFLUENCE OF AQUEOUS-PHASE CHEMICAL-REACTIONS ON OZONE FORMATION IN POLLUTED AND NONPOLLUTED CLOUDS

Aqueous-phase reactions among dissolved radicals and trace metals have been incorporated into a comprehensive gas-phase chemical reaction me chanism in order to quantify the influence of heterogeneous chemical p rocesses on ozone (O-3) formation under a wide range of NOx and hydroc arbon concentrations. In-cloud reactions of dissolved HO2 with itself, the reaction of dissolved O-3 and HO2, and when trace metals are pres ent, the reactions of dissolved HO2 and copper dramatically reduce tot al HO2 and other free-radical concentrations in clouds, thereby reduci ng the rate at which O-3 is produced from anthropogenic NOx and hydroc arbon pollutants. Under typical urban or moderately polluted condition s, local ozone formation rates are reduced by 30-90% when aqueous-phas e radical reactions are occurring in the atmosphere. However, when NOx concentrations are less than about 200 ppt, O-3 is slowly destroyed, and in-cloud reactions reducing HO2 concentrations decrease the rate a t which ozone and other reactive NOx and non-methane hydrocarbons (NMH C) are destroyed, resulting in longer atmospheric chemical lifetimes o f O-3, NOx, and NMHC. These results suggest that in-cloud reactions st rongly influence local O-3 production in polluted areas, but longer-te rm impacts of clouds on O-3 formation would be much smaller due to com pensating chemical processes in regions remote from NOx emissions. The effects of heterogeneous chemistry are highly dependent on the concen trations of NOx and hydrocarbons. In polluted clouds, aqueous reaction s of dissolved copper and iron could be the dominant reactions influen cing O-3 formation, suggesting the need for further measurements of tr ace metals in the atmosphere. (C) 1997 Elsevier Science Ltd.