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 th rough natural and anthropogenic activities, and the meteorological conditio ns such as wind system and rainfall pattern due to the Asian monsoon. Altho ugh northeastern China had the largest anthropogenic emission source, preci pitation with relatively high pH value was observed. At islands in the nort hern Pacific far from major anthropogenic emission sources, precipitation w ith relatively low pH value was observed. It was suggested that ammonia and calcium were constantly produced in the continent, and that the higher con centrations of these bases in the atmosphere were incorporated into precipi tation elements and neutralized the acidity due to H2SO4 and HNO3 from foss il fuel combustion. The ratio of [nssCa(2+) + NH4+]/[nssSO(4)(2-) + NO3-] i n precipitation was highest on the continent, moderate in the Korean Penins ula and the Japanese Islands, and lowest in the northern Pacific; in other words, this ratio decreased as the distance from the continent increased. I n the central region of East Asia, which is affected by both continental co ld, dry air masses and subtropical warm, humid air masses, the frequency di stribution of pH of precipitation showed a bimodal pattern in which unimoda l patterns of northern China and that of the Japanese Islands were superimp osed. The wet deposition of nssSO(4)(2-), NO3-, nssCa(2)(+) , and NH4+ show ed 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. (C) 1999 Else vier Science Ltd. All rights reserved.