Understanding the underlying mechanisms of secondary organic aerosols (SOA) formation upon volatile organic compounds (VOCs) oxidation is key to an accurate interpretation of aerosol climate effects and sustainable improvement of China’s air quality. Low-volatility organic vapours are critical intermediates connecting the oxidation of volatile organic compounds to secondary organic aerosol formation. However, the complexity and extremely low concentrations ( ppt or even ppq level) of oxygenated organic molecules (OOMs) have made it difficult to measure them effectively by conventional means and have resulted in most previous studies ignoring or oversimplifying the intermediate steps of SOA formation. To date, the fundamental mechanisms of SOA formation from VOCs oxidation remain unexplored.
Fig. 1: Distribution of OOMs in Beijing, Nanjing, Shanghai and Hong Kong.
In view of this, Aijun Ding’s team has conducted coordinated comprehensive measurements for VOCs, OOMs and organic aerosols in three most urbanized regions in China: Beijing in the North China Plain (NCP), Nanjing and Shanghai in the Yangtze River Delta (YRD) and Hong Kong in the Pearl River Delta (PRD) in collaboration with several teams including Beijing University of Chemical Technology, University of Helsinki, Shanghai Academy of Environmental Sciences, Hong Kong University of Science and Technology and Tsinghua University. A newly developed method of mass-spectral binning combined with positive matrix factorization (binPMF) was used in the identification of OOMs. With the help of binPMF, the authors identify ~1,500 molecular formulae, accounting for more than 80% of the spectral signals. A framework analysis technique has been developed to identify the precursors of OOMs and further assess the quantitative contribution to SOA formation, and thus enables a direct link between SOA formation and the oxidation of various precursors, laying foundation for improving SOA formation modelling.
The results show a strong homogeneity in the formation of OOMs and SOA in different megacities of China. The oxidation of anthropogenic VOCs dominates OOMs formation, with an approximately 40% contribution from aromatics and a 40% contribution from aliphatic hydrocarbons (predominantly alkanes), a previously under-accounted class of volatile organic compounds. It is found that multi-step oxidation and autoxidation processes are the key chemical mechanisms for OOMs formation in urbanized regions, and that nitrogen oxides are deeply involved in the formation of OOMs and dominate the peroxyl radical termination process. The irreversible condensation of these anthropogenic OOMs contributes 38-71% of SOA and increases significantly in highly polluted conditions, accounting for a major fraction of the production of SOA.
Fig. 2: Contribution of OOMs to SOA
On April 9, 2022, Nature Geoscience published online a paper titled “Secondary organic aerosol formed by condensing anthropogenic vapors over China's megacities”. Associate Professor Wei Nie and associate professor Chao Yan from the school of Atmospheric Sciences (SAS) of Nanjing University, researcher Dandan Huang from Shanghai Academy of Environmental Sciences, and Assistant Professor Zhe Wang from Hong Kong University of Science and Technology are the co-first authors of this paper. Professor Aijun Ding of SAS and Jingkun Jiang of Tsinghua University are corresponding authors. This work was supported by the National Natural Science Foundation of China (NSFC) project (92044301, 41875175, 42075101, 21806108, 91744204 and 22188102), the Jiangsu Provincial Collaborative Innovation Center of Climate Change, Samsung PM2.5 SRP, the Research Grants Council of Hong Kong Special Administrative Region (grants nos. T24/504/17-N and 15265516), the Shanghai Rising-Star Program (19QB1402900) and the US National Science Foundation (AGS1801897).