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

The size dependence of the percentage of chloride depletion (%Cl_dep) 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 %Cl_dep generally decreased with increasing particle size, consistent with the literature. In the particles smaller than 1.8μm (fine particles), the mode diameter of %Cl_dep was associated with the mode diameter of sulfate in the droplet mode. When the sulfate peak appeared at 0.32-0.54μm, the %Cl_dep 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 %Cl_dep with increasing particle size. When the sulfate peak appeared at 0.54-1.0μm, the maximum %Cl_dep appeared at 1.0-1.8μm and the %Cl_dep 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 SO₄ ^2- 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 %Cl_dep although direct evidence based on cloud measurements is not available. © 2002 Elsevier Science Ltd. All rights reserved.