Hygroscopic mineral dust particles as influenced by chlorine chemistry in the marine atmosphere

Morphology and elemental compositions of individual dust particles were investigated with the use of Asian dust samples collected at a site along the Sea of Japan side of the archipelago. Our results indicate the preferential formation of chloride in Ca-containing dust particles (0.1 < Cl/Ca < 0.65) in cases when the particles contain little or no sulfate. Most of them are in an amorphous state and nearly spherical even under high vacuum. A likely explanation for the results is that highly soluble salts such as calcium chloride (CaCl₂) are formed as a result of heterogeneous reactions of dust particles with chloride precursor gases (mostly, HCl) in the marine atmosphere. The chloride formation is expected to play an important role in enhancing the hygroscopicity of dust particles.