Chemical Composition-Dependent Hygroscopic Behavior of Individual Ambient Aerosol 1 Particles Collected at a Coastal Site

16 This study investigated the hygroscopic behavior of individual ambient aerosol particles 17 collected at a coastal site of Jeju Island, Korea. The particles' size change along with phase transitions 18 during humidification and dehydration processes, and their chemical compositions, were determined 19 by optical microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-20 EDX), respectively. Of the 39 particles analyzed, 24 were aged sea-spray aerosols (SSAs) with diverse 21 mixing ratios of Cl - and NO 3. 22 The ambient SSAs exhibited multiple deliquescence and efflorescence transitions that were 23 dominantly influenced by NaCl, NaNO 3 , MgCl 2 , Mg(NO 3 ) 2 and organic species covering the surface 24 of the aged SSAs. For Cl-rich SSAs with X (Na, Mg)Cl > 0.4, although some particles showed very slow 25 water uptake at low RHs = ~30%, two major transitions were observed during the humidification 26 process, firstly at RH = ~63.8%, regardless of their chemical compositions, which is the mutual 27 deliquescence relative humidity (MDRH), and secondly at RH = 67.5 − 73.5%, depending on their 28 chemical compositions, which are the final DRHs. During the