Molecular composition and processing of aqueous secondary organic aerosol in clouds at a mountain site in southeastern China

Aqueous secondary organic aerosol (aqSOA) contributes substantially to organic aerosol (OA), affecting air quality, human health, and climate. However, the molecular composition and processing of aqSOA in clouds remain unclear due to limited online field measurements. We measured molecular composition of OA online (time resolution 20 s) and tracked its processing at a mountain site in southeastern China, using an Extractive ElectroSpray Ionization inlet coupled with a Time-of-Flight Mass Spectrometer (EESI-ToF-MS). We identified 2084 molecular formulas and compared OA composition from three sample types: cloud droplet residuals (CDR), interstitial aerosol particles (INT), and cloud-free aerosol particles (CF) in representative cloud episodes. CHO class was the dominant constituent, followed by CHON class. In most cloud episodes, the fraction of CHO was lower in CDR than that in INT and CF, while the fraction of CHON was higher, which may result from the uptake of organonitrates or nitration in cloud water. Compounds in CDR had more carbon number and higher molecular weight than CF, which is attributed to accretion reactions in cloud water. We identified 39 significantly enriched compounds in CDR compared with CF, which could be potentially used as aqSOA tracers formed via cloud processing. This study also reveals rapid changes in aqSOA composition, which highlight the necessity for high time resolution measurements to capture the processing of aqSOA in clouds. Overall, this study provides clear information on processing of aqSOA in clouds and highlights the importance of accretion reactions, which have implications on the composition and physicochemical properties of SOA.