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Non-sea-salt sulfate and nitrate in trade wind aerosols at Barbados: Evidence for long-range transport

  • Author(s): Savoie, Dennis L
  • Prospero, Joseph M
  • Saltzman, Eric S
  • et al.
Abstract

From mid-May 1984 through December 1987, more than 1100 daily high-volume bulk aerosol samples were collected during onshore trade winds at Barbados, West Indies. All of these have been analyzed to determine the concentrations of particulate non-sea-salt (nss) sulfate, nitrate, and Saharan dust; 91 of the samples were also analyzed for methanesulfonate (MSA). The mean concentrations (in μg m−3) during the period were nitrate, 0.509 (s = 0.389); nss sulfate, 0.751 (s = 0.602); mineral dust, 16.0 (s = 21.1); and MSA, 0.0207 (s = 0.0093). The concentrations of both nitrate and nss sulfate are significantly correlated with those of Saharan dust, indicating that substantial fractions of both are transported across the tropical North Atlantic in association with the dust. This transport accounts for about 60% of the mean total concentration of each of the anions at Barbados. Our data, combined with those from previous studies, indicate that these dust-related fractions are probably not derived from the Sahara soil material; they are more likely derived from anthropogenic sources. The nitrate-to-nss sulfate ratio in this dust-related fraction changes markedly from the summer to the winter. During the summer the general meteorology and nitrate-to-nss sulfate mass ratio (0.36) are consistent with Europe being the major source region; during the winter the ratio increases by a factor of 4 to 1.44, which may be consistent with a source region over southern West Africa. The ratio of the mean MSA concentration to that of the nss sulfate that is not related to dust transport is 0.066, a value similar to the ratio of MSA to total nss sulfate at relatively pristine stations in the Pacific: Fanning Island and American Samoa. The similarity suggests that this fraction of the nss sulfate at Barbados may be derived predominantly from oxidation of reduced sulfur compounds emitted from the ocean. The mean concentration of nitrate that is not related to dust transport is 0.22 μg m−3, about double the total mean concentration over the tropical South Pacific (0.11 μg m−3) and 40% higher than that over the equatorial Pacific (0.16 μg m−3). The major source (or sources) of this nondust-related nitrate is still very uncertain.

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