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The role of aqueous-phase chemistry in the formation of secondary organic aerosols (SOA) is still poorly constrained.Here we present observation results of the degree of oxygenation of organic aerosols (OA) based on high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) measurements made at a coastal site in Hong Kong from late April to the end of May,2011 for 37 days.Two foggy periods and one hazy period were chosen for detailed analysis to compare the changes in degree of oxygenation of OA due to different processes.The Extended Aerosol Inorganic Model (E-AIM) predicted fine particle liquid water content (LWCfp) was up to 85 μg/m3 during the foggy days.Particle concentration as measured by the HR-ToF-AMS was up to 60 μg/m3 during the hazy days.The degree of oxygenation of OA,as indicated by several parameters including fraction of m/z 44 in organic mass spectra (f44),oxygen to carbon elemental ratio (O:C),and carbon oxidation state ( ),was evaluated against odd oxygen (Ox) concentration,LWCfp,ionic strength (IS),and in-situ pH (pHis).Results suggested that for the hazy period,the high concentration of OA (on average 11 μg/m3) and the high degree of oxygenation (f44 = 0.15,O:C = 0.51,and = ?0.31) were mainly due to gas-phase oxidation.During the foggy periods with low photochemical activities,the degree of oxygenation of OA was almost as high as that in the hazy days and significantly higher than that during non-fog/non-haze days.However,the evolution of OA behaved quite differently in these two foggy periods.The first foggy period in late April and early May had larger LWCfp and lower Ox concentration and the OA had more semi-volatile oxygenated organic aerosols (SVOOA) as resolved by positive matrix factorization (PMF).The second foggy period in mid-May had higher Ox concentration and lower LWCfp,and produced more low-volatility oxygenated organic aerosols (LVOOA).Examination into the particle-phase constituents suggests that partitioning may be the dominating process that incorporates oxygenated species into the particle phase for the first foggy period,while oxidation may be the dominating process for the second foggy period.Both physical and chemical processes are important in multi-phase mechanisms for oxygenated organic aerosol formation.