Precipitation chemistry in East Asia

The chemical composition of precipitation in East Asia was analyzed on the basis of the results of cooperative monitoring performed during the period from 1992 to 1993. The wet deposition of soluble ions was characterized by two major factors; the geographical distribution of precursor production through natural and anthropogenic activities, and the meteorological conditions such as wind system and rainfall pattern due to the Asian monsoon. Although northeastern China had the largest anthropogenic emission source, precipitation with relatively high pH value was observed. At islands in the northern Pacific far from major anthropogenic emission sources, precipitation with relatively low pH value was observed. It was suggested that ammonia and calcium were constantly produced in the continent, and that the higher concentrations of these bases in the atmosphere were incorporated into precipitation elements and neutralized the acidity due to H2SO4 and HNO3 from fossil fuel combustion. The ratio of [nssCa2++NH4+]/[nssSO42−+NO3−] in precipitation was highest on the continent, moderate in the Korean Peninsula and the Japanese Islands, and lowest in the northern Pacific; in other words, this ratio decreased as the distance from the continent increased. In the central region of East Asia, which is affected by both continental cold, dry air masses and subtropical warm, humid air masses, the frequency distribution of pH of precipitation showed a bimodal pattern in which unimodal patterns of northern China and that of the Japanese Islands were superimposed. The wet deposition of nssSO42−, NO3−, nssCa2+, and NH4+ showed relatively small differences between the sites, while that of Na+ and Cl−, relatively large differences between the sites, reflecting the field of ionic concentration and amount of precipitation in East Asia.