Sulfur dioxide distribution over the Pacific Ocean 1991–1996

Abstract

In this study we combined the sulfur dioxide (SO2) data from the NASA Pacific Exploratory Missions (PEM) and the First Aerosol Characterization Experiment (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 have exceptionally high precision due to the use of isotopically labeled SO2 as an internal standard in each sample. The lower limit of detection was less than 2 pptv. The spatial extent of the data ranged from 60°N to 72°S, 110°E to 80°W, and from 50 m to 12 km above the ocean surface. A significant zonal gradient was observed between the northern 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 dominated the sulfur chemistry in the lower troposphere of the western North Pacific eastward from the Asian continent for more than 1500 km and substantially farther in the mid and upper troposphere. The impact of Asian sources far from the continent was due primarily to transported SO2 with a substantially smaller impact from transported sulfate. Dimethyl sulfide was a significant source of SO2 only in the tropical boundary layer. In the southern hemisphere, anthropogenic sources had much less impact with very little SO2 detected in biomass burning plumes. Sulfur dioxide in the middle and upper troposphere of both hemispheres was strongly influenced by volcanic sources. Sulfur dioxide from the eruption of Mount Pinatubo dominated the SO2 distribution in the upper troposphere in the northern hemisphere in the second half of 1991. A significant fraction of the SO2 in the upper free troposphere in the northern hemisphere was attributed to SO2 transported from the stratosphere to the upper troposphere. Evidence for the transport of SO2 from the stratosphere to troposphere existed as far south as 30°N, but it was most intense at high latitudes. In the absence of major volcanic activity, such as the cataclysmic eruption of Mount Pinatubo, volcanic sources in East Asia contribute significant amounts of SO2 in the mid and upper troposphere of the northern hemisphere. In the southern hemisphere where anthropogenic sources are much weaker, volcanoes may contribute most of the SO2 found in the mid and upper troposphere. Deep convection by tropical and extratropical storms appeared to be a significant process contributing to long‐range transport of volcanic SO2 for the southern hemisphere.