Hygroscopicity of Organic Aerosols Linked to Formation Mechanisms

Organic aerosols (OAs) account for a large fraction of tropospheric fine particulate matter, but the hygroscopicity of OA is poorly understood. Here, we show remarkably-enhanced water uptake capacity of OA due to formation of highly-oxidized oxygenated OA on new particle formation (NPF) events in Beijing. While non-nucleation processes also produce oxidized OA, their hygroscopicity exhibits little enhancement. As a result, a correlation between the hygroscopicity and oxidation state is absent for OA on non-NPF days. Further analysis reveals that the highly-oxidized oxygenated OA is 2.5 and 5-fold as hygroscopic as the oxidized primary OA and less-oxidized oxygenated OA, respectively. Our results suggest that nucleation-initiated photooxidation of volatile organic compounds to produce water-soluble organic acids may dominate on NPF days, and the aqueous oligomerization to yield less water-soluble products might occur on non-NPF days. Plain Language Summary While organic aerosols (OAs) account for a large proportion of tropospheric fine particles, the water uptake capacity of OA is poorly understood. Here, we show distinct effects of different atmospheric processes on its hygroscopicity in polluted urban atmosphere. Remarkably-enhanced hygroscopicity is identified for OA formed from nucleation. Our results reveal that it is critical to account for the formation mechanisms in evaluating the impacts of OA on air quality and climate.