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

Abstract. 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 cloud 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 with adjacent time (<2 h): cloud droplets (CD), interstitial aerosol (INT), and cloud-free aerosol (CF) in representative cloud episodes. CHO class was the dominant constituent, followed by CHON class. The fraction of CHO was lower in CD 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 CD had more carbon, oxygen, and nitrogen number but lower O/C than INT and CF, which is attributed to accretion reactions in cloud water. We identified aqSOA tracers, including 39 new compounds, which were significantly enriched in CD compared with CF. This study also reveals rapid changes of aqSOA composition, which highlight the necessity for high time resolution measurement to capture the processing of aqSOA in cloud. Overall, this study provides clear information of processing of aqSOA in cloud and highlights the importance of accretion reactions, which has implications on the composition and physicochemical properties of SOA.