Pollutant wet deposition mechanisms in precipitation and fog water

Wet deposition of acid-related substances takes place by two processes: precipitation scavenging and fog water impaction/sedimentation on natural surfaces. The relative importance of each deposition pathway depends on the frequency of occurrence of precipitation or fog, the magnitude of the event and the efficiency of pollutant removal by each mechanism. The latter, in turn, is governed by the type of cloud or fog, complex precipitation formation mechanisms and cloud-surface interactions. These factors are examined in the light of our current knowledge. Particular emphasis is placed on how cloud micro-physical as well as air and precipitation measurements, made aloft by aircraft and at the ground, have been used to further our knowledge of wet deposition mechanisms. Future research is needed to quantify the importance of the fog-water deposition pathway in eastern North America to better understand the interaction of gaseous pollutants with cloud and fog-water and to improve our knowledge of pollutant scavenging processes in mesoscale and synoptic weather systems.