Temporal variations of nitrogen wet deposition across Japan from 1989 to 2008

To evaluate temporal variations in nitrogen wet deposition across Japan during 1989-2008, we analyzed results of a chemical transport model (the Models-3 Community Multiscale Air Quality) and observational data. The model successfully reproduced the general patterns of spatial and temporal variations of observed NO₃^- wet deposition rates. Wet deposition rates of NO₃^- across Japan increased during 1989-2008, with rates of increase of 2-5%/yr. Sensitivity simulations indicated that the increase of NO₃^- wet deposition rates was mostly (61%-94%) explained by the increased emissions of atmospheric pollutants in China. Contributions of China's emissions increased from 29%-35% during 1989-1993 to 43%-61% during 2004-2008, suggesting that transboundary pollution had a large impact on NO₃^- wet deposition in Japan. The contribution of observed NO₃^- to total nitrogen wet deposition (i.e., NO₃^- + NH₄⁺) increased in southwestern Japan, and currently, NO₃^- and NH ₄⁺ make similar contributions to nitrogen wet deposition across Japan. Interannual variation of NO₃^- wet deposition was further evaluated using a meteorological index, area-weighted surface pressure anomaly (ASPA). When ASPA was negative, air masses from the Asian continent were more directly transported to Japan, and NO₃ ^- concentrations across Japan became high. Thus, anomalies of NO ₃^- concentrations were negatively correlated with ASPA. Anomalies of NO₃^- wet deposition rates, however, showed a weak positive correlation with ASPA, reflecting a positive correlation between anomalies in precipitation rates and ASPA. This result strongly suggests that precipitation patterns have a large impact on the interannual variation of NO₃^- wet deposition across Japan.