Nitrate, non-sea-salt sulfate, and mineral aerosol over the northwestern Indian Ocean

From January 14 to June 1, 1979, 96 bulk aerosol samples were collected aboard three ships in the northwestern Indian Ocean as part of the Global Weather Experiment. These samples were analyzed to determine the atmospheric concentrations of nitrate, non-sea-salt (NSS) sulfate, mineral aerosol, and NSS calcium. On the basis of geographical location, mineral aerosol concentration, and wind regime, the samples were divided into three distinct sets. The highest mean concentrations of nitrate, NSS sulfate, and mineral aerosol (1.0, 2.7, and 7.6 μg/m3, respectively) were observed during dust outbreaks transported from desert regions of the Middle East. The eolian dust comprises a significant fraction of the sediments off the coast of the Arabian peninsula and the Horn of Africa. A portion of the NSS sulfate appears to be derived from the soil material itself. Between dust outbreaks, the mean concentrations of nitrate and NSS sulfate are about 0.4 and 0.9 μg/m3, respectively; a substantial portion of these concentrations are probably transported from the Indian subcontinent. The lowest concentrations were measured in southern hemisphere air, which is transported into the region by the southeast monsoons. Nitrate concentrations in this air, 0.16 μg/m3, are consistent with the estimated input of NOx from lightning and from the stratosphere. Mean NSS sulfate concentrations ranged from 0.3 μg/m3 prior to the onset of upwelling to 0.6 μg/m3 after the onset; estimated dimethyl sulfide emission rates from the ocean are sufficient to account for both of these means. Mineral aerosol concentrations in, the southern hemisphere air averaged 0.27 μg/m3.

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