Spatial variations and trends in acid deposition in Japan within a 100 km x 100 km region from 1989 to 1999

A monitoring network for acid deposition that consisted of five sites in Hyogo, Japan, and covered a region of about 100 km x 100 km revealed the spatial variations and trends in acid deposition in the region. Data from April 1989 to March 1999 were used for comparison and evaluation. Filtering-type bulk samplers were used. Only one rural site showed a significantly (p<0.01) lower pH than the other four sites, although the pAi(-log([nss-SO42-]+[NO3-])) at this site was not lower than those at the other sites. In contrast, the pH at two urban sites was not lower than those at the other three sites (suburban, rural, and remote), although the pAi at these sites was significantly lower than those at the other sites. The concentrations of NO3-, non-seasalt-(nss-)SO42-, and nss- Ca2+ showed similar spatial variations. The concentrations of NO3-, nss- SO42-, and nss- Ca2+ at the urban site were higher than those at the rural and remote sites, whereas the concentration of NH4+ showed a different spatial variation from NO3-, nss- SO42-, and nss- Ca2+; i.e., the level at the remote site was similar to that at the urban site. The mean chemical compositions at the rural and remote sites were very similar. In addition, they had approximately equal concentrations. However, the only difference between the rural and remote sites was observed in the composition of NH4+ and H+; i.e., the concentration of NH4+ (or H+) at the rural site was lower (or higher) than that at the remote site. A large deposition of nss- Ca2+ was observed in the urban area and was attributed to effective neutralization in this area. The large deposition was synchronized with precipitation seasonally in the area where precipitation was high due to a large snowfall. The NO3-/nss-SO42- ratio (N/S ratio) showed a clear trend of increase, and the rate of increase during the investigation ranged from 34% to 59% from April 1989 to March 1999.