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

摘要

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 O_3 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 O_3 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 O_3, 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 O_3 is destroyed. Over a longer time scale, aqueous reactions can reduce or enhance the total O_3 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.