Comment on acp-2022-575

Abstract

To investigate the aging impact on sources, volatility, and viscosity of organic aerosols (OA) in the Chinese outflows, a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS) coupled with a thermodenuder (TD) was deployed in the spring of 2018 in Dongying, which is a regional receptor site of metropolitan emissions in North China Plain (NCP). The average mass concentration of PM1 was 31.5 ± 22.7 μg m–3, which was mainly composed of nitrate (33 %) and OA (25 %). The source apportionment results show the OA was mainly contributed by oxygenated OA (OOA) from secondary sources, including background-OOA (33 %) representing a background concentration of OA (2.6 μg m–3) in the NCP area, and transported-OOA (33 %) oxidizing from urban emissions. The other two factors include aged hydrocarbon-liked OA (aged-HOA, 28 %) from transported vehicle emissions and biomass burning OA (BBOA, 5 %) from local open burnings. The volatility of total OA (average C* = 3.2×10–4 µg m–3) in this study is generally lower than those in previous field studies, which is mainly due to the high OA oxidation level resulting from aging processes during transport. The volatilities of OA factors follow the order of background-OOA (average C* = 2.7×10–5 μg m–3) < transported-OOA (3.7×10–4 μg m–3) < aged-HOA (8.1×10–4 μg m–3) < BBOA (0.012 μg m–3), indicating the probable existence of oligomers. The viscosity estimation suggests that the majority of ambient OA in this study behaves as semi-solid (60 %), liquifies at higher RH (21 %), and solidifies (19 %) during noon time when the RH is low and the oxidation level is high. Finally, the estimated mixing time of OA varies dramatically from minutes at night to years in the afternoon, emphasizing the necessity to consider its dynamic kinetic limits when modeling OA. In general, the overall results of this study improve the understanding of the aging impact on OA volatility and viscosity.