Nitrate and sulfate in individual Asian dust-storm particles in Beijing, China in Spring of 1995 and 1996

Energy dispersive X-ray analysis of elements and reagent thin-film test of particulate nitrate and sulfate were carried out to examine individual dust particles collected in Beijing during five dust-storm events occurring in spring of 1995 and 1996. Dominant particles were electron-opaque and had irregular shapes during the dust-storm periods, and their size was frequently in the range larger than 1 μm (diameter). Besides, some mineral particles that showed regular cubic shapes were found in the range from 0.1 to 2 μm. Their X-ray spectrums indicated calcium was abundant and little or no other elements with atomic number larger than 11 existed in such particles. They were supposed to be emitted initially from construction sites, and then formed through crystallization in the atmosphere. Their most possible composition was CaO or Ca(OH)2. It was estimated that 93% of the collected electron-opaque particles are dust particles and the cubic particles in term of number frequency. On reagent films, few dust particles reacted apparently with barium chloride suggesting there was no water-soluble sulfate on the surface of dust particles although X-ray spectrums of about 14.6% of dust particles showed peaks of sulfur. The frequency of nitrate-containing particles in dust particles was 10.8%, which was much smaller than that in mineral particles collected in non-dust-storm periods. These results suggest that almost no sulfate is formed and nitrate is hardly formed on the surface of dust particles during their transport from source areas to Beijing.