Distributions of atmospheric non-sea-salt sulfate and methanesulfonic acid over the Pacific Ocean between 48°N and 55°S during summer

Abstract

Atmospheric concentrations of non-sea-salt sulfate (nss-SO42-) and methanesulfonic acid (MSA) were measured over the Pacific Ocean between 48°N and 55°S during the KH-08-2 and MR08-06 cruises in summers of 2008 and 2009, in order to investigate spatial distributions of each species and MSA/nss-SO42- ratio. In the subarctic western North Pacific, mean concentrations of nss-SO42- and MSA in bulk (fine + coarse) aerosols were 1.1 μg m-3 and 0.061 μg m-3, whereas those in the South Pacific were 0.25 μg m-3 and 0.043 μg m-3, respectively. In the subtropical western North Pacific, it was observed that nss-SO42- concentration sharply increased from 0.45 μg m-3 up to 4.2 μg m-3 under the dominant influence of the Kilauea volcano, while that of MSA remained low. Mean MSA/nss-SO42- ratio observed in the South Pacific was approximately 3.7 times higher than that in the subarctic western North Pacific, although the mean MSA concentration in the subarctic western North Pacific was a factor of 1.4 higher than that in the South Pacific. The distributions of nss-SO42-, MSA, and MSA/nss-SO42- ratio suggested that aerosol nss-SO42- plays a key role in the latitudinal variation in MSA/nss-SO42- ratio over the North and South Pacific during summer periods, and that high MSA concentrations in the subarctic western North Pacific and the South Pacific were related to high biological productivity and low air temperature. During the cruises, an inverse relationship (r = -0.72, p < 0.01) was observed between satellite-derived chlorophyll a concentration and air temperature, showing that high biological productivity occurred at high latitudes, where air temperature were relatively low, in both hemispheres during the summer periods. Although both MSA concentration and MSA/nss-SO42- ratio showed inverse and positive relationships with air temperature and chlorophyll a concentration, respectively, the correlations between these variables were weak, suggesting that the distributions of MSA concentration and MSA/nss-SO42- ratio over the North and South Pacific during the summer periods were influenced by more complex factors. Estimates using the MSA/nss-SO42- ratios measured in different latitude regions in the Pacific Ocean indicated that the contributions from biogenic sources accounted for 9.6-58% of the total nss-SO42- in aerosols collected in the subarctic western North Pacific, 15-85% in the subtropical western North Pacific, 10-70% in the central North Pacific, and 12-97% in the South Pacific, showing strong influence of anthropogenic nss-SO42- in the subarctic western North Pacific despite the higher mean concentration of MSA in the subarctic western North Pacific than in the South Pacific.