Sulfur dioxide distribution over the Pacific Ocean 1991-1996

In this study we combined the sulfur dioxide (SO2) data from the NASA Pacif ic Exploratory Missions (PEM) and the First Aerosol Characterization Experi ment(ACE 1) to create a data set containing 4679 observations of SO2 in the troposphere of the Pacific Ocean during the period 1991-1996, These data h ave exceptionally high precision due to the use of isotopically labeled SO2 as an internal standard in each sample. The lower limit of detection was l ess than 2 pptv. The spatial extent of the data ranged from 60 degrees N to 72 degrees S, 110 degrees E to 80 degrees W, and from 50 m to 12 km above the ocean surface, A significant zonal gradient was observed between the no rthern and southern hemispheres. The western North Pacific was particularly well characterized during the NASA PEM-West A and B missions that focused on that region. Our data show that anthropogenic sources in eastern Asia do minated the sulfur chemistry in the lower troposphere of the western North Pacific eastward from the Asian continent for more than 1500 km and substan tially farther in the mid and upper troposphere, The impact of Asian source s far from the continent was due primarily to transported SO2 with a substa ntially smaller impact from transported sulfate. Dimethyl sulfide was a sig nificant source of SO2 only in the tropical boundary layer. In the southern hemisphere, anthropogenic sources had much less impact with very little SO 2 detected in biomass burning plumes. Sulfur dioxide in the middle and uppe r troposphere of both hemispheres was strongly influenced by volcanic sourc es. Sulfur dioxide from the eruption of Mount Pinatubo dominated the SO2 di stribution in the upper troposphere in the northern hemisphere in the secon d half of 1991, A significant fraction of the SO2 in the upper free troposp here in the northern hemisphere was attributed to SO2 transported from the stratosphere to the upper troposphere. Evidence for the transport of SO2 fr om the stratosphere to troposphere existed as far south as 30 degrees N, bu t it was most intense at high latitudes. In the absence of major volcanic a ctivity, such as the cataclysmic eruption of Mount Pinatubo, volcanic sourc es in East Asia contribute significant amounts of SO2 in the mid and upper troposphere of the northern hemisphere. In the southern hemisphere where an thropogenic sources are much weaker, volcanoes may contribute most of the S O2 found in the mid and upper troposphere, Deep convection by tropical and extratropical storms appeared to be a significant process contributing to l ong-range transport of volcanic SO2 for the southern hemisphere.