Evaluating the degree of oxygenation of organic aerosol during foggy and hazy days in Hong Kong using high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS)

Abstract. The chemical characteristics of organic aerosol (OA) are still poorly constrained. Here we present observation results of the degree of oxygenation of OA based on high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) measurements made at a coastal site in Hong Kong from late April to the end of May in 2011. Two foggy periods and one hazy period were chosen for detailed analysis to compare the changes in the degree of oxygenation of OA due to different processes. Using HR-ToF-AMS measured inorganic species as input, the Extended Aerosol Inorganic Model (E-AIM) predicted a fine-particle liquid water content (LWC_fp) up to 85 μg m⁻³ during the foggy days. Particle concentration as measured by HR-ToF-AMS was up to 60 μg m⁻³ during the hazy days and up to 30 μg m⁻³ during the foggy days. The degree of oxygenation of OA, as indicated by several parameters including the fraction of m/z 44 in organic mass spectra (f₄₄), the elemental ratio of oxygen to carbon (O : C), and the carbon oxidation state (OS_c), was evaluated against the odd oxygen (O_x) concentration, LWC_fp, ionic strength (IS), and in situ pH (pH_is). Observations suggest that the high concentration of OA (on average 11 μg m⁻³) and the high degree of oxygenation (f₄₄ = 0.15, O : C = 0.51, and OS_c = −0.31) during the hazy period 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 on the hazy days, and significantly higher than that during non-foggy/non-hazy days. However, the OA evolved quite differently in the two foggy periods. The first foggy period in late April saw a larger LWC_fp and a lower O_x concentration and the OA were made up of ~ 20 % semi-volatile oxygenated organic aerosol (SVOOA) as resolved by positive matrix factorization (PMF). In the second foggy period in mid-May, higher O_x concentration and lower LWC_fp were observed, and the OA were found to contain > 50 % low-volatility oxygenated organic aerosols (LVOOA). An examination of the particle characteristics (pH_is, IS, and LWC_fp) suggests that partitioning may have been the dominating process through which oxygenated species were incorporated into the particle phase during the first foggy period, while oxidation in the aqueous phase dominated over gas-phase processes during the second foggy period.