MEASUREMENTS OF CHLORIDE DEPLETION AND SULFUR ENRICHMENT IN INDIVIDUAL SEA-SALT PARTICLES COLLECTED FROM THE REMOTE MARINE BOUNDARY-LAYER

Changes in the elemental ratios of Cl/Na and S/Na in sea-salt particle s are expected from the atmospheric reactions of sulfuric and nitric a cids with these particles. Chloride depletion is expected to occur upo n the liberation of HCI 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 chlor ide depletion and sulfur enrichment was obtained for individual sodium -containing particles from the remote marine Pacific atmosphere in bot h the accumulation mode (0.06 less-than-or-equal-to Dp less-than-or-eq ual-to 1.0 mum, where Dp is the particle diameter) and the coarse mode (Dp > 1.0 mum) size range. Sodium-containing particles comprised clos e 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 exam ined with a transmission electron microscope (TEM) coupled with an ene rgy-dispersive X ray (EDX) detector. The elemental ratios obtained fro m the atmospheric particles were determined by comparison with values obtained from laboratory-generated sea-salt, sodium chloride, and sodi um sulfate particles of known size and chemical composition, which ser ved 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 pa rticles 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 seawa ter values were observed most commonly for the submicrometer sea-salt aerosol. The CI/Na ratio was significantly (Student's t test, 99.9%) l ower 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 le ss-than-or-equal-to 2.5 mum) reflected the ratio in bulk seawater, whi le the remaining particles had statistically lower ratios and qualitat ively 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 mum in diameter to 861 and 5235 fg for particles of 0.80 mum in di ameter, corresponding to 74 to 96% of the sea-salt particle mass. Thes e 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., [19921 and estimations of S(IV) oxidation in sea-salt aerosol water by Chameides and Stelson [1 992].