A long-range transport model for East Asia to estimate sulfur deposition in Japan

A long-range transport model for East Asia was developed to estimate the sulfur deposition in Japan. The model is a hybrid type that combines a trajectory model for distant sources with a Eulerian model for nearby sources. The processes of transport and diffusion, chemical reactions, cloud scavenging, transfer of sulfate from cloud water to rainwater, and dry and wet depositions are considered. The emission distribution of sulfur dioxides in East Asia, with approximately 80 km × 80 km resolution, was updated. The emission distribution in Japan on a grid system with resolution in both the horizontal and vertical directions three times higher than that of the grid system for East Asia was also estimated for the Eulerian part of the hybrid model. The important assumption used in the hybrid model is that substances emitted from distant sources are well mixed during the long-range transport and are uniformly distributed in the vertical direction. This assumption was confirmed from the result of airplane measurements in the area of the sea northwest of Kyushu Island in Japan. The results obtained using the hybrid model were evaluated through comparison with observed data of acidic deposition. Observations were conducted at 21 stations throughout Japan during one year. The calculated amount of total sulfur deposition in Japan was 0.43 Tgy^-1 (Tgy^-1 = 10¹² g per year) in sulfur equivalents, while the observed amount was 053 Tgy^-1. The long-range transport model can predict more than 80% of observed sulfur deposition. The tendency of underprediction could be improved by changing the treatment of mass transfer to liquid phases. The sources contributing to the total sulfur deposition in Japan were estimated using the hybrid model. The contributions of Japanese anthropogenic sources, volcanic eruptions, and Asian continental sources were 40%, 20%, and 40%, respectively.