Direct observation of nitrate and sulfate formations from mineral dust and sea-salts using low-Z particle electron probe X-ray microanalysis

In the present work, it is demonstrated that a single particle analytical technique, named low-Z particle electron probe X-ray microanalysis, is a practically useful tool for the study of heterogeneous reactions of mineral dust and sea-salts when this analytical technique was applied to a sample collected during an Asian Dust storm event. The technique does not require a special treatment of sample to identify particles reacted in the air. Also, quantitative chemical speciation of reacted particles can provide concrete information on what chemical reaction, if any, occurred for individual particles. Among overall 178 analyzed particles, the number of reacted particles is 81 and heterogeneous chemical reactions mostly occurred on CaCO3 mineral dust (54 particles) and sea-salts (26 particles). Several observations made for the Asian Dust sample in the present work are: (1) CaCO3 species almost completely reacted to produce mostly Ca(NO3)2 species, and CaSO4 to a much lesser extent. (2) When reacted particles contain CaSO4, almost all of them are internally mixed with nitrate. (3) Reacted CaCO3 particles seem to contain moisture when they were collected. (4) Some reacted CaCO3 particles have unreacted mineral species, such as aluminosilicates, iron oxide, SiO2, etc., in the core region. (5) All sea-salt particles are observed to have reacted in the air. Some of them were recrystallized in the air before being collected and they are observed as crystalline NaNO3 particles. (6) Many sea-salts were collected as water drops, and some of them were fractionally recrystallized on Ag collecting substrate. When sea-salts were not recrystallized on the substrate, they are found as particles internally mixed with NaNO3 and Mg(NO3)2, and in some cases SO4 and Cl species as additional anions.