Mixture state and size of Asian dust particles collected at southwestern Japan in spring 2000

Abstract

Atmospheric particles were collected at Kumamoto (32°48′N, 130°45′E), a coastal city in southwestern Japan, during three dust storm events in spring 2000. The elemental composition and size of individual dust particles and their mixture state with sea salt, sulfate, and nitrate were analyzed using electron microscopes and an energy dispersive X‐ray spectrometer. About 60 ∼ 85% of dust particles were internally mixed with sea salt. Weather records indicated these particles were most probably formed by the collisions and coagulations of dust particles and sea‐salt particles. The relative weight ratios of mineral components to sea salt in individual particles showed that the mixtures of particles were dominated by mineral, by sea salt, or by both. Size distributions of the particles segregated by the mixture levels of mineral and sea salt in the three dust storm events were similar and all distributions showed a diameter range of 1 ∼ 8 μm with maximum mode around 3 μm. Out of 1 ∼ 8 μm, dust particles were rarely detected. The combination of dust particles with sea salt caused an increase in size of the dust particles. Therefore the decrease of particle concentrations in the range of diameter >3 μm suggests the critical diameter for dust particle dispersion was possibly around 3 μm and a dust particle might be removed rapidly if it became larger than this scale in the marine atmosphere. Detection of sulfate and nitrate revealed that 91% or more dust particles contained sulfate and 27% or less contained nitrate. The comparisons of the relative weight ratios of sodium, sulfur, and chlorine in mixture particles and in sea‐salt particles confirmed previous results that mineral materials could enhance particulate sulfate and nitrate formation and restrain chlorine depletion from the sea‐salt components in mixture particles.