We found a significant geographic gradient (longitudinal and latitudinal) in the sulfate (SO42-) concentrations measured at multiple sites over the East Asian Pacific Rim region. Furthermore, the observed gradient was well reproduced by a regional chemical transport model. The observed and modeled SO42- concentrations were higher at the sites closer to the Asian continent. The concentrations of SO42- from China as calculated by the model also showed the fundamental features of the longitudinal/latitudinal gradient. The proportional contribution of Chinese SO42- to the total in Japan throughout the year was above 50-70% in the control case, using data for Chinese sulfur dioxide (SO2) emission from the Regional Emission Inventory in Asia (40-60% in the low Chinese emissions case, using Chinese SO2 emissions data from the State Environmental Protection Administration of China), with a winter maximum of approximately 65-80%, although the actual concentrations of SO42- from China were highest in summer. The multiple-site measurements and the model analysis strongly suggest that the SO42- concentrations in Japan were influenced by the outflow from the Asian continent, and this influence was greatest in the areas closer to the Asian continent. In contrast, we found no longitudinal/latitudinal gradient in SO2 concentrations; instead SO2 concentrations were significantly correlated with local SO2 emissions. Our results show that large amounts of particulate sulfate are transported over long distances from the East Asian Pacific Rim region, and consequently the SO42- concentrations in Japan are controlled by the transboundary outflow from the Asian continent. (C) 2009 Elsevier Ltd. All rights reserved.