Heterogeneous Reactions Affecting Chlorine Activation in the Troposphere

摘要

Recent indirect and direct observations have shown that chlorine and bromine are active initiators of oxidation in the marine and coastal regions. Bromine effectively depletes ozone at Arctic sunrise and reacts with some hydrocarbons in the marine boundary layer (MBL). Chlorine reacts with many natural and anthropogenic pollutants. Morning Cl concentrations were estimated to reach in some cases 10~5 molecule/cm~3 (Spicer et al., 1998; Singh et al., 1996). This can markedly increase the oxidation potential of the coastal areas where polluted air from the continent is purified. Moreover, the back-transport of chlorine-enriched air causes changes in the oxidation potential of the continental troposphere. Sea salt is the largest source of halogens in the lower troposphere. Waves produce small droplets of seawater in the MBL. When the droplets are transported to higher altitudes or inland where the relative humidity is lower, they can crystallize to form solid particles. Transport of the sea salt particles has been observed as far as 900 km away from the ocean (Shaw, 1991). There are other cases when salts were found at high concentrations in particles. For example, the products of oil wells burning in Kuwait contained significant amounts of salt (Lownthal et al, 1993). Aqueous salts are typical of the remote MBL, while solid salts covered with adsorbed water may be more frequent in the coastal area. Thus the chemistry of these regions should differ. In addition the remote MBL chemistry proceeds under NO_X poor conditions whereas liberation of halogens in polluted coastal regions from solid particles can occur in heterogeneous reactions with nitrogen-containing compounds such as NO_2, NO_3 N_2O_5, and ClONO_2.