Wet and dry deposition fluxes of trace elements in Tokyo Bay

The annual wet and dry deposition fluxes of As, Cd, Cr, Cu, Hg, Mn, Ni, Pb and V were measured on the basis of data obtained from 2 years of observation (December 2003–November 2005) at three sites in the Tokyo Bay area, which is highly affected by anthropogenic sources. Moreover, the concentrations of these trace elements in the air and bay sediments were determined concurrently. Using these data, we evaluated the factors affecting wet and dry deposition fluxes and the contribution of the atmospheric input of each element to the supply of trace elements to the bay. The order of the average ratios of dry/wet deposition fluxes at three sites was Cr, 6.9>As, 5.9>Mn, 4.7>V, 4.5>Cu, 4.4>Ni, 3.7>Cd, 2.9>Pb, 2.0>Hg, 1.0. Thus, the atmospheric deposition of the trace elements except Hg in Tokyo Bay is predominantly dry deposition. For such trace elements, the wet and dry deposition fluxes within the bay were higher than those inland. Except in the case of some elements, the differences in wet and dry deposition fluxes between these sites were primarily due to the differences in scavenging ratio (=concentration in precipitation/concentration in air) and dry deposition velocity (=deposition flux/concentration in air), respectively, rather than in atmospheric concentration. Also, a significant correlation (r2=0.76, P<0.001) was observed between the scavenging ratios and dry deposition velocities of the trace elements. These suggest that large particles from sources in the coastal regions contribute largely to the wet and dry depositions of trace elements to the bay. The contributions of the atmospheric input of the trace elements to the bay ranged from 5% to 26%. Atmospheric deposition seems to have no significant role in the supply of trace elements to Tokyo Bay, suggesting the importance of river and effluent discharges.