Abstract: Relative humidity (RH) has a significant and complex effect on aerosols because of the aqueous phase process and gas-particle partition. The mass concentration and size distribution of organic aerosols, sulfate, nitrate, ammonium, and chloride were measured using high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS). These measurements were recorded from Aug. 5 to Sept. 23, 2016 in Binjiang District, Hangzhou, China, during which period more than 78% of the readings showed an RH over 60%, while the average temperature was 26 °C. Correlation analysis was applied to inorganic aerosol measurements while positive matrix factorization (PMF) was applied for source apportionment of organic aerosols (OA). The pattern of fixation of ammonium in aerosols changed as the RH increased, suggesting that RH enhances nitrate participation in particles, while sulfate is scavenged by droplets. All species of non-refractory submicron particles (NR-PM₁) showed an increase in their peak size as the RH increased. Primary OA (POA) continuously accumulated as the RH increased. When RH<60%, oxygenated OA (OOA) increased with increasing RH because of oxidation; semi-volatile OOA (SV-OOA) had a higher mass concentration during the daytime than at nighttime, indicating that the aqueous phase process and photochemistry synergistically affect the formation of oxygenated SV-OOA. When RH>60%, there was a relatively slow decrease in OOA, dominated by the wet removal effect rather than oxidation. The degree of oxidation of OA decreased as RH increased; this can be explained by most of the OOA with higher hygroscopicity being removed as droplets.

Key words: Relative humidity (RH); Aerosol composition; Size distribution; Wet removal; Aqueous-phase

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