The influence of aqueous-phase chemical reactions on ozone formation in polluted and nonpolluted clouds

Abstract

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 (O 3) formation under a wide range of NO x and hydrocarbon concentrations. In-cloud reactions of dissolved HO 2 with itself, the reaction of dissolved O 3 and HO 2, and when trace metals are present, the reactions of dissolved HO 2 and copper dramatically reduce total HO 2 and other free-radical concentrations in clouds, thereby reducing the rate at which O 3 is produced from anthropogenic NO x 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 NO x concentrations are less than about 200 ppt, O 3 is slowly destroyed, and in-cloud reactions reducing HO 2 concentrations decrease the rate at which ozone and other reactive NO x and non-methane hydrocarbons (NMHC) are destroyed, resulting in longer atmospheric chemical lifetimes of O 3, NO x , and NMHC. These results suggest that in-cloud reactions strongly influence local O 3 production in polluted areas, but longer-term impacts of clouds on O 3 formation would be much smaller due to compensating chemical processes in regions remote from NO x emissions. The effects of heterogeneous chemistry are highly dependent on the concentrations of NO x and hydrocarbons. In polluted clouds, aqueous reactions of dissolved copper and iron could be the dominant reactions influencing O 3 formation, suggesting the need for further measurements of trace metals in the atmosphere.