The size dependence of chloride depletion in fine and coarse sea-salt particles

The size dependence of the percentage of chloride depletion (%Cldep) in sea-salt aerosols was investigated using 16 sets of 48-96h size distribution measurements of atmospheric aerosols, collected from June 2000 to May 2001 in Hong Kong. Chloride and sodium in HK are predominantly from sea-salt aerosols because the prevailing wind is from the sea. In the particles larger than 1.8μm (coarse particles), the %Cldep generally decreased with increasing particle size, consistent with the literature. In the particles smaller than 1.8μm (fine particles), the mode diameter of %Cldep was associated with the mode diameter of sulfate in the droplet mode. When the sulfate peak appeared at 0.32-0.54μm, the %Cldep peaked at 0.54-1.0μm. Heterogeneous reactions between sea-salt particles and acidic gases are the major routes of the sulfate formation and chloride depletion, leading to a decrease in the %Cldep with increasing particle size. When the sulfate peak appeared at 0.54-1.0μm, the maximum %Cldep appeared at 1.0-1.8μm and the %Cldep increased with increasing particle size in the fine mode. Both heterogeneous reactions and in-cloud processes occurred to form sulfate and to deplete chloride. However, heterogeneous reactions, sampling artifacts, and anthropogenic emissions of chloride cannot explain this observed size dependence. Cloud processing including the activation of sea-salt particles with subsequent SO42- formation, the neutralization by NH3 and the evaporation of HCl in conjunction with NH3 during water evaporation from cloud droplets can cause chloride depletion in the droplet mode. The smaller amount of evaporation of NH3 and HCl from cloud droplets in forming the 0.54-1.0μm particles than the 1.0-1.8μm particles can account for the observed size dependence of %Cldep although direct evidence based on cloud measurements is not available. © 2002 Elsevier Science Ltd. All rights reserved.