Abstract
The nitrate to non-sea-salt sulfate ratio (NO3-/nssSO42- ) in precipitation is a useful factor in evaluating the relative contributions of H2SO4 and HNO3 to the acidity of precipitation. Based on the wet deposition monitoring data collected in Japan between 1976 and 2011, I analyzed (i) the long-term trends and seasonal variations of the NO3-/nssSO42- ratio in precipitation at Ryori, Komae, and Goto; (ii) changes in the horizontal distribution and temporal variation of the NO3-/nssSO42- ratio in recent years; and (iii) the influence of the east-Asian NOx/SO2 emission ratio on the NO3-/nssSO42- ratio in precipitation over the Japanese Archipelago. The NO3-/nssSO42- ratio in precipitation significantly increased during the 1980s and 1990s, while no significant variation was observed in the 2000s at Ryori, Komae, or Goto. Since the summer of 2000, the eruption of the Miyakejima volcano has impacted the NO3-/nssSO42- ratio at Ryori and Komae. The average annual rate of change of the NO3-/nssSO42- ratio during the 1980s and 1990s at Ryori and Goto was similar to that of the east-Asian NOx/SO2 emission ratio. The average annual rate of change of the NO3-/nssSO42- ratio at Komae, however, was greater than those of Ryori or Goto, and indicated the importance of HNO3 in the metropolitan atmosphere. The NO3-/nssSO42- ratio in precipitation in central Japan, including the Sea of Japan and Pacific Ocean coastal regions, was greater than those in northeastern and southwestern Japan. The NO3-/nssSO42- ratio in precipitation showed no significant variation through the middle of the 2000s, but it has clearly increased since the late 2000s. This tendency was observed across a large area of Japan, particularly on the Sea of Japan side of western Japan. If the increases in the NO3-/nssSO42- ratio in precipitation in western Japan reflect changes in emissions in the region, increases in the NOx emissions and/or decreases in the SO2 emissions would be expected. A recent analysis of the source contributions to sulfate and nitrate in Japan suggests that increases in the SO2 emissions from mainland China are not relevant to the increases in the NO3-/nssSO42- ratio in precipitation of western Japan.
