Atmospheric dry deposition of trace elements at a site on Asian-continent side of Japan

Abstract

The sources of dry-deposited trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb, Sb and V) and the factors controlling their dry deposition fluxes were investigated on the basis of two-year observations (April 2004–March 2006) at a site on the Asian-continent side of Japan, which has been strongly affected by air pollutants from the Asian continent. Dry deposition sampling was conducted using a water surface sampler connected to a wet-only precipitation sampler. The dry deposition of As, Cd, Pb and Sb showed a small contribution to atmospheric deposition (0.25–0.44 as ratios of annual dry/wet deposition fluxes). Moreover, the dry deposition fluxes of those elements increased negligibly during the period when their atmospheric particulate matter (PM) concentrations increased owing to transport from the Asian continent. Thus, the dry deposition of As, Cd, Pb and Sb from the Asian continent was not significant, because their overall dry deposition velocities are relatively low (mostly <1 cm s−1). Conversely, the annual dry deposition fluxes of Cr, Cu and Ni exceeded their annual wet deposition fluxes (2.5–12.4 as ratios of annual dry/wet deposition fluxes). Those overall dry deposition velocities were much higher (3.2–9.7 cm s−1), and the crustal enrichment factors (EFs) frequently exceeded ten. These results suggest that the dry deposition of Cr, Cu and Ni is dominated by considerably coarse particles from local anthropogenic sources. For Mn and V, the dry and wet depositions contributed almost equally to the annual deposition fluxes. Their monthly dry deposition fluxes correlated significantly with that of Al (P < 0.001), and the EFs were close to unity, suggesting a large contribution of background soil to their dry deposition. The dry deposition fluxes of all the trace elements were dependent not on their atmospheric PM concentrations but on their overall dry deposition velocities. The particle size distributions of the elements in the atmosphere are likely the most important factor controlling their dry deposition fluxes on the Asian-continent side of Japan and, consequently, the relative contribution of the dry/wet deposition fluxes.