Observations of Precipitation Scavenging for Water - Soluble Particles

Simultaneous observations of aerosols, gases, and precipitation chemistry were carried out at the cloud base level and the surface during the fall of 1979, the winter of 1979-1980 and the summer of 1980. With due regard to the effects of the evaporation of precipitation particles and the absorption of gases, the relative importance of in-cloud scavenging and below-cloud scavenging, and the collection efficiency for water soluble particles were estimated on the basis of data obtained during the observation periods. For the mass of sulfate particles, the percentage of below-cloud scavenging to total precipitation scavenging was about 20% in the case of rain drops and 40% in the case of snow particles. As for the mass of all the water-soluble particles, it was about 30% in the case of rain drops and 50% in the case of snow particles, respectively. The collection efficiency, which is defined to be the ratio of the actual collection cross section to the geometric cross section, of rain drops was slightly greater than that of snow particles. However, from the viewpoint of the mass of aerosol particles collected by unit mass of precipitation, snow scavenging was several times as efficient as rain scavenging, inversely. The collection efficiencies for sulfate, nitrate, and sea salt particles increased in that order. This was accounted for by the size difference of aerosol particles which contained each chemical constituent.