National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1994

Measurements of chloride depletion and sulfur enrichment in individual sea-salt particles collected from the remote marine boundary layer

McInnes, L.M., D.S. Covert, P.K. Quinn, and M.S. Germani

J. Geophys. Res., 99(D4), 8257–8268, doi: 10.1029/93JD03453 (1994)

Changes in the elemental ratios of Cl/Na and S/Na in sea-salt particles are expected from the atmospheric reactions of sulfuric and nitric acids with these particles. Chloride depletion is expected to occur upon the liberation of HCl to the gas phase, with the particles remaining enriched in sulfate or nitrate. The elemental ratios of Ca/Na, Mg/Na and K/Na should remain constant during this process. Analysis of chloride depletion and sulfur enrichment was obtained for individual sodium-containing particles from the remote marine Pacific atmosphere in both the accumulation mode (0.06 ≤ Dp ≤ 1.0 µm, where Dp is the particle diameter) and the coarse mode (Dp > 1.0 µm) size range. Sodium-containing particles comprised close to 100% of the coarse mode and 11 to 31% of the accumulation mode by number. Aerosols were collected with a low-pressure impactor and examined with a transmission electron microscope (TEM) coupled with an energy-dispersive X ray (EDX) detector. The elemental ratios obtained from the atmospheric particles were determined by comparison with values obtained from laboratory-generated sea-salt, sodium chloride, and sodium sulfate particles of known size and chemical composition, which served as a calibration set. The elemental ratios of Ca/Na, Mg/Na, and K/Na were found to remain fairly constant between individual sea-salt particles of various sizes for more than 85% of the particles examined. Deviations in the ratio of Cl/Na and S/Na from that of reference seawater values were observed most commonly for the submicrometer sea-salt aerosol. The Cl/Na ratio was significantly (Student's t test, 99.9%) lower than that of reference seawater for 89% of the particles examined, while the S/Na ratios were higher for 100% of the particles. The Cl/Na ratio measured in 48% of the coarse sea-salt particles (1.0 < Dp ≤ 2.5 µm) reflected the ratio in bulk seawater, while the remaining particles had statistically lower ratios and qualitatively different morphologies. All but 3% of these coarse particles had enhanced S/Na ratios over that of bulk seawater. Estimates of non-sea-salt (nss) sulfate mass ranged from 216 to 1422 fg for particles of 0.50 µm in diameter to 861 and 5235 fg for particles of 0.80 µm in diameter, corresponding to 74 to 96% of the sea-salt particle mass. These values are compared with the recent measurements of Mouri and Okada [1993] as well as predictions from the atmospheric chemistry models of in-cloud sulfate production of Hegg et al. [1992] and estimations of S(IV) oxidation in sea-salt aerosol water by Chameides and Stelson [1992].

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