The influence of in-cloud chemical reactions on ozone formation in polluted areas

Abstract

Aqueous reactions among dissolved radicals and trace metals have been incorporated into a comprehensive gas-phase chemical reaction mechanism in order to quantify the chemical influences of clouds on ozone (O 3 ) formation in the lower troposphere. In-cloud reactions of dissolved HO 2 with itself, the reaction of dissolved 03 and HO 2 , and reactions of dissolved HO 2 with copper dramatically reduce total HO 2 and other free-radical concentrations in clouds, thereby reducing the rate at which 03 is produced from anthropogenic NO X and hydrocarbons. Under typical urban or moderately polluted conditions, local ozone formation rates are reduced by 30-90% when aqueous reactions are occurring. However, the rate at which NO X , Non-Methane HydroCarbons (NMHC), and O 3 are oxidized is also reduced, resulting in longer atmospheric chemical lifetimes of 03, NO X , and NMHC. When NO X concentrations are less than about 200 ppt, in-cloud reactions reducing HO 2 concentrations decrease the rate at which 03 is destroyed. Over a longer time scale, aqueous reactions can reduce or enhance the total 03 produced per molecule of NO X emitted, depending on the concentration conditions under which NO X is emitted into the atmosphere. Longer-term impacts of clouds on O 3 formation are much smaller than their local impacts, in the range of a few 10's of percent increase or decrease, due to compensating chemical processes under high and low NO X conditions. The effects of cloud chemistry on O 3 formation are highly dependent on the concentrations of NO X and hydrocarbons.