Observations of highly oxidized molecules and particle nucleation in the atmosphere of Beijing

Brean, James; Harrison, Roy M.; Shi, Zongbo; Beddows, David C. S.; Acton, W. Joe F.; Hewitt, C. Nicholas; Squires, Freya A.; Lee, James

Particle nucleation is one of the main sources of atmospheric particulate matter by number, with new particles having great relevance for human health and climate. Highly oxidized multifunctional organic molecules (HOMs) have been recently identified as key constituents in the growth and, sometimes, in initial formation of new particles. While there have been many studies of HOMs in atmospheric chambers, flow tubes, and clean environments, analyses of data from polluted environments are scarce. Here, measurements of HOMs and particle size distributions down to small molecular clusters are presented alongside volatile organic compounds (VOCs) and trace-gas data from a campaign in June 2017, in Beijing. Many gas-phase HOMs have been characterized and their temporal trends and behaviours analysed in the context of new particle formation. The HOMs identified have a degree of oxidation comparable to that seen in other, cleaner, environments, likely due to an interplay between the higher temperatures facilitating rapid hydrogen abstractions and the higher concentrations of inline-formulaNOx and other inline-formula M2inlinescrollmathml chem normal RO normal 2 normalRadical ⚫ 23pt13ptsvg-formulamathimg2a31e5f20ffb542287eded706ce09aed acp-19-14933-2019-ie00001.svg23pt13ptacp-19-14933-2019-ie00001.png terminators ending the autoxidation sequence more rapidly. Our data indicate that alkylbenzenes, monoterpenes, and isoprene are important precursor VOCs for HOMs in Beijing. Many of the inline-formulaC5 and inline-formulaC10 compounds derived from isoprene and monoterpenes have a slightly greater degree of average oxidation state of carbon compared to those from other precursors. Most HOMs except for large dimers have daytime peak concentrations, indicating the importance of inline-formulaOHRadical⚫ chemistry in the formation of HOMs, as inline-formulaO3 tends to be lower on days with higher HOM concentrations; similarly, VOC concentrations are lower on the days with higher HOM concentrations. The daytime peaks of HOMs coincide with the growth of freshly formed new particles, and their initial formation coincides with the peak in sulfuric acid vapours, suggesting that the nucleation process is sulfuric-acid-dependent, with HOMs contributing to subsequent particle growth.

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Brean, James / Harrison, Roy M. / Shi, Zongbo / et al: Observations of highly oxidized molecules and particle nucleation in the atmosphere of Beijing. 2019. Copernicus Publications.

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