Contribution of Asian outflow to atmospheric concentrations of sulfate and trace elements in aerosols during winter in Japan

Abstract

The atmospheric concentrations of nonsea-salt (nss) SO 4 2- and trace elements (As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Se, V, and Zn) were measured for aerosol samples collected concurrently at ten sites across Japan during winter (December to February in 2004-2006), when the contribution of Asian outflow to their concentrations would have been maximized. The sulfur isotope ratio (Δ 34 S) of nss SO 4 2- was also measured and used to identify the sources of nss SO 4 2- in the aerosols. Sites in urban and industrial areas tended to have high concentrations of nss SO 4 2- and trace elements. In other sites, however, the concentrations of nss SO 4 2- and trace elements decreased with increasing longitude. On the basis of the Δ 34 S values of nss SO 4 2- and the results of air trajectory analysis, it is likely that the regional variations in the concentrations of nss SO 4 2- and trace elements are strongly related to their emissions in the regions over which the air mass passed, particularly in the 30-40°N region, which has the highest emissions in China. To evaluate the relative contribution of Asian outflow to the concentrations of nss SO 4 2- and trace elements at each site, for convenience we assumed that the fraction dominated by Asian outflow (referred to as the Asian outflow fraction) corresponds to the concentrations expected from an exponential concentration-longitude relationship among three sites that are far from major emission areas in Japan. For nss SO 4 2- and As, much higher contributions of the Asian outflow fraction (93% and 83% on average, respectively, at 10 sites) were observed, whereas for the other trace elements, only average contributions were observed at all the sites, i.e., within 50-67%. The high contributions of Asian outflow to the concentrations of nss SO 4 2- and As are attributable to a marked difference in their emissions from coal combustion between China and Japan.