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



[1] 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 NH4+ 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 NO3 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.