Chemical Composition-Dependent Hygroscopic Behavior of Individual Ambient Aerosol Particles Collected at a Coastal Site

Abstract. This study investigated the hygroscopic behavior of individual ambient aerosol particles collected at a coastal site of Jeju Island, Korea. The particles' size change along with phase transitions during humidification and dehydration processes, and their chemical compositions, were determined by optical microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), respectively. Of the 39 particles analyzed, 24 were aged sea-spray aerosols (SSAs) with diverse mixing ratios of Cl^- and NO₃^-.

The ambient SSAs exhibited multiple deliquescence and efflorescence transitions that were dominantly influenced by NaCl, NaNO₃, MgCl₂, Mg(NO₃)₂ and organic species covering the surface of the aged SSAs. For Cl-rich SSAs with X_{(Na, Mg)Cl} > 0.4, although some particles showed very slow water uptake at low RHs = ~30 %, two major transitions were observed during the humidification process, firstly at RH = ~63.8 %, regardless of their chemical compositions, which is the mutual deliquescence relative humidity (MDRH), and secondly at RH = 67.5–73.5 %, depending on their chemical compositions, which are the final DRHs. During the dehydration process, the Cl-rich SSAs showed single-stage efflorescence at RH = 33.0–50.5 %, due to simultaneous heterogeneous crystallization of inorganic salts. For Cl-depleted SSAs with X_{(Na, Mg)Cl} < 0.4, two prompt deliquescence transitions were observed during the humidification process, firstly at MDRH = 63.8 % and secondly at RH = 65.4–72.9 %. The mutual deliquescence transition was more distinguishable for Cl-depleted SSAs. During the dehydration process, step-wise transitions were observed at efflorescence RHs (ERHs) = 24.6–46.0 % and 17.9–30.5 %, depending on their chemical compositions.

Additionally, aged mineral particles showed partial or complete phase changes with varying RH due to the presence of SSAs and/or NO₃^- species. In contrast, non-reacted mineral and Fe-rich particles maintained their size during the entire hygroscopic process. The mixture particles of organic and ammonium sulfate (AS) exhibited lower deliquescence and efflorescence RHs compared to pure AS salt, highlighting the impact of organic species on the hygroscopic behavior of AS. These findings emphasize the complexity of atmospheric aerosols and the importance of considering their composition and mixing state when modeling their hygroscopic behavior and subsequent atmospheric impacts.