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 mechanism in order to quantify the influence of heterogeneous chemical processes on ozone (O3) formation under a wide range of NOx and hydrocarbon concentrations. In-cloud reactions of dissolved HO2 with itself, the reaction of dissolved O3 and HO2, and when trace metals are present, the reactions of dissolved HO2 and copper dramatically reduce total HO2 and other free-radical concentrations in clouds, thereby reducing the rate at which O3 is produced from anthropogenic NOx and hydrocarbon pollutants. Under typical urban or moderately polluted conditions, local ozone formation rates are reduced by 30–90% when aqueous-phase radical reactions are occurring in the atmosphere. However, when NOx concentrations are less than about 200 ppt, O3 is slowly destroyed, and in-cloud reactions reducing HO2 concentrations decrease the rate at which ozone and other reactive NOx and non-methane hydrocarbons (NMHC) are destroyed, resulting in longer atmospheric chemical lifetimes of O3, NOx, and NMHC. These results suggest that in-cloud reactions strongly influence local O3 production in polluted areas, but longer-term impacts of clouds on O3 formation would be much smaller due to compensating chemical processes in regions remote from NOx emissions. The effects of heterogeneous chemistry are highly dependent on the concentrations of NOx and hydrocarbons. In polluted clouds, aqueous reactions of dissolved copper and iron could be the dominant reactions influencing O3 formation, suggesting the need for further measurements of trace metals in the atmosphere.