Hygroscopicity of organic aerosols linked to formation mechanisms 
Jieyao Liu, Fang Zhang, Weiqi Xu, Yele Sun, Lu Chen, Shangze Li, Jingye Ren, Bo Hu, Hao Wu, Renyi Zhang
Abstract
<p>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.</p>