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Aerosol particles play critical roles in climate and human health. However, aerosol composition and evolution, particularly secondary organic aerosol (SOA), and aerosol interactions with clouds in high-altitude background areas in China remain less understood. Here, we conducted real-time measurements of submicron aerosols (PM 1) using aerosol mass spectrometers at a forested mountain site (1128 m a.s.l.) in southeastern China in November 2022. The average (±1σ) PM 1 mass concentration was 4.3±4.8 µ g m -3 , which was ubiquitously lower than that at other mountain sites in China. Organic aerosol (OA) constituted the largest fraction of PM 1 (42.9 %) and was predominantly secondary, as indicated by the high oxygen-to-carbon (O / C) ratio (0.85–0.96) and carbon oxidation state (0.21–0.49). Notably, the remarkably enhanced PM 1 concentrations observed during the daytime on cloudless days were identified to be likely to be produced from cloud evaporation. While more oxidized oxygenated OA was scavenged efficiently during cloud events, cloud evaporation was found to release a significant amount of less oxidized oxygenated OA from air masses transported from polluted regions. The distinct decrease in OA / Δ CO with the increase in O / C during the cloud evaporative period further demonstrates that OA remaining in cloud droplets is generally in a moderate oxidation state. Moreover, organic nitrates were also estimated and showed a higher contribution to the total nitrate during the cloudy period (27 %) than during the evaporative period (3 %). Overall, our results demonstrate the importance of SOA and the influences of cloud processes in regional mountain areas in southeastern China. [ABSTRACT FROM AUTHOR]

Aerosol particles play crucial roles in both climate dynamics and human health. However, there remains a significant gap in our understanding of aerosol composition and evolution, particularly regarding secondary organic aerosols (SOA), and their interaction with clouds in high-altitude background areas in China. Here we conducted real-time measurements of submicron aerosols (PM1) using aerosol mass spectrometers at a forested mountain site (1128 m a.s.l.) in southeastern China in November 2022. Our results revealed that organic aerosol (OA) constituted a substantial portion of PM1 (41.1 %), with the OA being primarily of secondary origin, as evidenced by a high oxygen-to-carbon (O/C) ratio (0.85–0.96). Positive matrix factorization resolved two distinct SOA factors: less oxidized oxygenated OA (LO-OOA) and more oxidized OOA (MO-OOA). Interestingly, MO-OOA was scavenged efficiently during cloud events, while cloud evaporation contributed significantly to LO-OOA. The ratio of OA/ΔCO increased with a decrease in the O/C ratio, suggesting that OA remaining in cloud droplets generally maintained a moderate oxidation state. Furthermore, our results indicated a higher contribution of organic nitrates to total nitrate during cloudy periods (27 %) compared to evaporative periods (3 %). Notably, a substantial contribution of nitrate in PM1 (20.9 %) was observed, particularly during high PM periods, implying that nitrate formed in polluted areas interacted with clouds and significantly impacted the regional background site. Overall, our study underscores the importance of understanding the dynamics of secondary organic aerosols and the impacts of cloud processing in regional mountainous areas in southeastern China. [ABSTRACT FROM AUTHOR]

Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health. Here a single particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier was deployed during the Beijing 2022 Olympic Winter Games (OWG) to investigate the impacts of emission controls on particle mixing state and density. Nearly 760,000 particles were detected, which were classified into seven major classes. Our results showed the dominance of carbonaceous particles comprising mainly Total-EC (13.4 %), Total-OC (10.5 %) and Total-ECOC (47.1 %). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG although those from primary emissions decreased. The composition of carbonaceous particles also changed significantly which was characterized by the decreases in EC-NS, KEC-N, and amine-containing particles, and increase in ECOC-NS. This result indicates that emission controls during OWG reduced the mixing of elemental carbon (EC) with inorganic aerosol species and amines, yet increased the mixing of EC with organic aerosol. The average effective density (ρeff) of aerosol particles (150–300 nm) was 1.20 g cm−3 with higher values during OWG (1.26 g cm−3) due to the increase in secondary particle contribution. The two types of fresh particles, i.e., Total-EC and high molecular weight organic matter presented the lowest ρeff (0.97 g cm−3 and 0.87 g cm−3, respectively). In addition, the ρeff of most particles increased as the increases in pollution levels and relative humidity, yet varied differently for different types of particles, highlighting the impacts of aging and formation processes on the changes of particle density and mixing state. [ABSTRACT FROM AUTHOR]