Influences of sulfate and nitrate on the hygroscopic behaviour of coarse dust particles

Asian dust particles of diameter >1 μm collected in southwestern Japan were analyzed with environmental scanning electron microscopy. The hygroscopic behaviour of sea salt-free dust particles were investigated with respect to their contents of sulfate and nitrate. Sulfate- and nitrate-free dust particles (not containing sulfur and nitrogen), as well as sulfate-containing ones, did not change in their crystalline morphology as relative humidity (RH) was increased from 15% to 90%. In contrast, calcium nitrate-containing dust particles appeared in aqueous phase even at 15% RH. These results indicate that nitrate formation on coarse dust particles could substantially enhance the hydrophilicity of the particles and possibly vice versa, whereas sulfate formation could not.