Organic aerosol (OA), a large fraction of fine particles, has a large impact on climate radiative forcing and human health, and its impact depends strongly on its size distributions. However, previous studies for characterization of size-resolved OA were either based on the tracer method or multiple linear regression technique, which assume spectral profiles of OA factors for different sizes are constant. Our understanding of the sources and formation processes of size-resolved OA particularly in polluted environment is far from complete.

A severe haze episode in Beijing (Image by ZHU Jiang).

In this work, the research team led by Prof. SUN Yele conducted size-resolved OA measurements using a high-resolution aerosol mass spectrometer at urban and rural sites in the North China Plain (NCP) in summer and winter. The results were recently published in Environmental Science: Atmospheres as inside front cover.

"We found that the size distributions of OA varied very differently between summer and winter, urban and rural sites. For example, OA in summer showed a much higher peak diameter than winter. The peak diameter in summer is approximately ~550 nm, while ~420 nm and 350 nm during wintertime" said Prof. SUN.

The team further applied positive matrix factorization (PMF) to the size-resolved high-resolution mass spectra to investigate the composition and sources of OA at urban and rural sites. "We found that secondary organic aerosol (SOA) with the same nominal name can be largely different across different sizes. Also, the oxidation degree of OA and SOA changed as a function of particle sizes." Prof. SUN said, "Our analysis showed that size-resolved analysis of OA is crucial for a better characterization of sources and properties of OA."

 
The team further demonstrated the importance of size-resoled OA analysis in predicting hygroscopicity parameter and cloud condensation nuclei (CCN). "We found that the hygroscopicity of OA increased significantly with particle sizes, and it was much higher in summer in Beijing than that during wintertime." Concluded Prof. SUN. "Traditional CCN closure analysis by assuming one constant hygroscopicity parameter of OA could introduce large uncertainties, while the size-resolved hygroscopicity parameter of OA would benefit a better prediction of CCN than bulk hygroscopicity parameter in future studies."

This work was supported by National Natural Science Foundation of China (92044301, 41975170).

Citation:
Xu, W., Chen, C., Qiu, Y., Xie, C., Chen, Y., Ma, N., Xu, W., Fu, P., Wang, Z., Pan, X., Zhu, J., Ng, N. L., and Sun, Y.: Size-resolved characterization of organic aerosol in the North China Plain: new insights from high resolution spectral analysis, Environmental Science: Atmospheres, 10.1039/D1EA00025J, 2021.https://pubs.rsc.org/en/content/articlelanding/2021/EA/D1EA00025J