Chloride and bromide depletions in sea-salt particles over the northeastern Pacific Ocean

[1] We have characterized halide enrichment factors (EF) in aqueous extracts of size-resolved marine aerosol particles collected from the northeastern Pacific Ocean at a coastal site ( June through October, 1997 - 1999) and during two ship cruises ( January and June, 2000). Our investigation focused on the chemistry of marine particles with diameters of 1.6 - 4.8 and 4.8 - 15 mu m, which we define as "sea-salt particulate matter'' (SS PM). The ratios of Mg2+, Ca2+, and K+ to Na+ in our SS PM samples matched well with those of bulk seawater, suggesting there were minor contributions from other particle types such as soil dust. Anthropogenic nitrate and sulfate concentrations in our SS PM samples were relatively low, with average (+/- 1 sigma) molar ratios of NO3-/ Na+ = 0.05 +/- 0.03 and nonsea-salt SO42-/ Na+ = 0.005 +/- 0.008. The amounts of chloride missing from the sea-salt particles were generally small and occasional enrichments in chloride were observed: the average (+/- 1 sigma) EF was 0.97 +/- 0.10. In general, the missing Cl- could be accounted for by displacement by nitric acid ( with smaller contributions from nonsea-salt sulfate (nss-SO42-)), but this was not the case in the samples with the greatest chloride depletions. The fraction of bromide lost in the SS PM was always greater than that for chloride in a given sample, with an average enrichment factor of 0.57 +/- 0.14. In general, the smaller, slightly more acidic SS PM (1.6 - 4.8 mu m) had larger Br- depletions compared to the larger, slightly less acidic ( 4.8 - 15 mm) particles. Although acid displacement cannot account for these bromide depletions, the depletions were correlated with particle acidity ( based on NO3- and nss-SO42- concentrations), suggesting that acid-dependent oxidation reactions formed photoactive bromine species that volatilized from the particles.