Aircraft observations of atmospheric pollutants were made over the seas between Japan and the Asian continent in five missions since 1991. Continuous SO
2 measurements were made in Nov. 1992, March 1994, Dec. 1994, and Nov. 1995. The average concentrations of SO
2 were 0.93 and 0.41 ppb at altitudes of less than and greater than 2,000 m, respectively. The vertical distributions of SO
2 reflected characteristics intermediate between those of the continental polluted atmosphere and the marine background atmosphere. It was also found that air masses transported from the Asian continent to Japan can be divided into 3 types: (a) China ∼ Yellow Sea ∼ Kyushu: low SO
2 and high SO
42-/SO
2 ratio, (b) Korea ∼ western Japan: high SO
2 and low SO
42-/SO
2 ratio, and (c) North Asia ∼ eastern Japan: low SO
2 and low SO
42-.
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