Seeder-feeder precipitation scavenging of air pollutants via mist droplets in urban area

摘要

Enhancement of rainfall and concentration of air pollutants with altitude on mountains, where orographic cloud frequently shroud the summit, is known to be seeder-feeder process. This is because the lower-level cloud, which includes greater concentrations, is scavenged by rainfall from higher levels. Here we report that urban mist droplets could also enhance precipitation scavenging of acid substances by seeder-feeder process. In recent years rapid decrease in the number of foggy days has been reported in major cities in Japan like Yokohama since 1960. Urban hygroscopic aerosols, however, can easily form mist droplets at relatively high humidity. We observed 26 % annual misty hours frequency (212 days per one year) and 84 % rain events with mist/fog occurrence on average during six years from 1994 to 1999 in Yokohama, Japan. The lower rainwater pHs during mist/fog occurrence than in the absent of mist/fog indicated the importance of seeder-feeder scavenging process even in urban area, in particular when the rainfall intensity was weak (< 1mm h~(-1)). Precipitation scavenging of acid substances via mist droplets is likely important in urban area with relatively high humidity.