Annual and seasonal trends of wet deposition in Japan

In order to discuss temporal trends of the deposition of major ions, nonlinear least-squares regression analysis was applied to nation-wide wet deposition measurements in Japan. From the wet deposition database produced by Japan Environment Agency since 1983, the monitoring results at 17 sites over Japan from April 1989 through March 1998 were selected for the present study because of the data quality and current availability. These sites were further classified into four area groups in terms of the major sea of each area: the Pacific Ocean (PO), the Japan Sea (JS), the Seto Inland Sea (SI), and the East China Sea (EC) areas. The regression analysis elucidated temporal trends. The non-sea salt sulfate (nss-SO42−) deposition decreased on a national scale with a mean change rate of −3.5% a−1. The NO3− and NH4+ deposition for the JS area showed increasing mean change rates of 3.4% a−1 and 3.7% a−1, respectively; most of the sites, however, had no significant (p>0.05) annual trends in deposition of the two ions. Pronounced decreases in non-sea salt calcium (nss-Ca2+) deposition were recognized in northern Japan and some industrialized areas. For H+ deposition, negative change rates (−4 to −6% a−1) were estimated for the PO, EC, and SI areas. Annual trends of precipitation amount, however, made an only negligible contribution toward those of ionic deposition. Annual trends of the deposition were generally comparable with those of their domestic emissions and ionic concentrations, while the seasonality was attributed to the seasonal variation of precipitation amount. The decrease of the deposition sum of nss-SO42− and NO3−, and the increase of the NH4+ and nss-Ca2+ deposition sum will be responsible for the decreased deposition of H+ for some sites in the PO and SI areas. In consideration of microbial nitrification of ammonium in the soil, effective hydrogen ion deposition defined as the sum of H+ deposition and two times of NH4+ deposition for each site ranged from 50 to 119 meq m−2 a−1, which would be nearly equal to or less than their critical load estimates.