Transport of sulfur dioxide from the Asian Pacific Rim to the North Pacific troposphere

The NASA Pacific Exploratory Mission over the Western Pacific Ocean (PEM‐West B) field experiment provided an opportunity to study sulfur dioxide (SO 2 ) in the troposphere over the western Pacific Ocean from the tropics to 60°N during February–March 1993. The large suite of chemical and physical measurements yielded a complex matrix in which to understand the distribution of sulfur dioxide over the western Pacific region. In contrast to the late summer period of Pacific Exploratory Mission‐West A (PEM‐West A) (1991) over this same area, SO 2 showed little increase with altitude, and concentrations were much lower in the free troposphere than during the PEM‐West B period. Volcanic impacts on the upper troposphere were again found as a result of deep convection in the tropics. Extensive emission of SO 2 from the Pacific Rim land masses were primarily observed in the lower well‐mixed part of the boundary layer but also in the upper part of the boundary layer. Analyses of the SO 2 data with aerosol sulfate, beryllium‐7, and lead‐210 indicated that SO 2 contributed to half or more of the observed total oxidized sulfur (SO 2 plus aerosol sulfate) in free tropospheric air. The combined data set suggests that SO 2 above 8.5 km is transported from the surface but with aerosol sulfate being removed more effectively than SO 2 . Cloud processing and rain appeared to be responsible for lower SO 2 levels between 3 and 8.5 km than above or below this region.