Single-particle analysis of aerosols at Cheju Island, Korea, using low-Z electron probe X-ray microanalysis: A direct proof of nitrate formation fromsea salts

A recently developed electron probe X-ray microanalysis (EPMA), called low- Z EPMA, employing an ultrathin window energy-dispersive X-ray detector, was applied to characterize aerosol particles collected at two sampling sites, namely, Kosan and 1100 Hill of Cheju Island, Korea, on a summer day in 199 9. Since low-ZEPMA can provide quantitative information on the chemical com position of aerosol particles, the collected aerosol particles were classif ied and analyzed based on their chemical species. Many different particle t ypes were identified, such as marine-originated, carbonaceous, soil-derived , and anthropogenic particles. Marine-originated particles, such as NaNO3 a nd Na2SO4-containing particles, are very frequently encountered in the two samples. In this study, it was directly proven that the observed nitrate pa rticles were from sea salts. In addition, two types of nitrate particles fr om sea salts were observed, with and without Mg. The sodium nitrate particl es without Mg were believed to be collected as crystalline form, either wit h the sodium nitrate particles being fractionally recrystallized within eva porating seawater drops or with recrystallized sodium chloride particles ha ving reacted with gaseous nitrogen species in the air to form the crystalli ne sodium nitrate particles. The other seemed to be collected as seawater d rops, where the atmospheric reaction had occurred in the droplets, and thus sodium as well as magnesium nitrates were observed. Carbonaceous particles are the most abundant in the samples at both sites. From this study, it wa s found that about three-quarters of the carbonaceous particles in the samp les were biogenic, which partially explains a previously reported observati on of a large concentration of organic carbon particles as compared to elem ental carbon. Various soil-derived particles were also observed. In additio n to aluminosilicate- and iron oxide-containing particles, which are ubiqui tous components in soil-derived particles, CaCO3-, Al2O3- and Cr-containing particles were also frequently encountered.