Hygroscopic Behavior of Individual Ambient Aerosol Particles Collected at Gosan, Jeju Island, South Korea with Special Emphasis on Reacted Sea Salt Aerosols

초록

The hygroscopic behavior of ambient aerosols collected at Gosan (33°29'N, 126°16'E), Jeju island, Korea were investigated on a single particle level. The changes in size and phase transitions of individual aerosol particles during either humidification or dehydration process were observed using optical microscopy. The morphology and chemical compositions of overall 39 particles were determined by offline scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). 61 % of the particles studied are reacted (aged) sea spray/salt aerosols (SSAs), supported by the observation of different degrees of Cl-depletion. The ambient SSAs, which this study focused on, showed multiple hygroscopic (deliquescence and efflorescence) phase transitions. During humidification, although some particles (mostly Cl-rich) showed very slow water uptake at low RHs, mostly two prompt deliquescence transitions occurred, firstly the mutual deliquescence transitions at RH = 63.8 (± 1.5) %, which were more clearly observed in Cl-depleted than Cl-rich particles, and secondly the final transitions at RHs ranging in 65.4?73.5 % were observed. During dehydration, these particles showed mostly single-stage efflorescence at RH= 39.6?50.5 %, due to simultaneous crystallization of salts. On the other hand, Cl-depleted particles showed clear first and second transitions at RH = 63.8 % and 65.4?72.9 %, respectively, during humidification. During dehydration, Cl-depleted particles showed multiple efflorescence transitions at RH = 36.2?46.0 % and 17.9?33.0 %, respectively. The hygroscopic growth factors seem to be affected by the presence of organic species. The phase diagrams obtained by plotting the deliquescence and efflorescence RHs (DRHs and ERHs) as a function of the mole fractions [X(Na, Mg)Cl] show that the hygroscopic behavior of the reacted SSAs are dominated by NaCl and NaNO3 [1], whereas contribution due to SO4- seems to be minor.