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Published June 16, 2020 | Supplemental Material + Published
Journal Article Open

Carbenium ion-mediated oligomerization of methylglyoxal for secondary organic aerosol formation

Abstract

Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.

Additional Information

© 2020 the Author(s). Published by PNAS. This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). Contributed by Mario J. Molina, April 10, 2020 (sent for review July 16, 2019; reviewed by Yiqin Gao and Keith Kuwata). PNAS first published June 3, 2020. This work was supported by National Natural Science Foundation of China Grants (41731279 and 41675122); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01Z032); Natural Science Foundation of Guangdong Province, China (2019B151502064); Science and Technology Program of Guangzhou City (201707010188); a collaborative research program between Texas A&M University and the National Natural Science Foundation of China; Innovation Team Project of Guangdong Provincial Department of Education (2017KCXTD012); Science and Technology Key Project of Guangdong Province, China (2019B110206002); and the Robert A. Welch Foundation Grant (A-1417). Additional support for this research was provided by the Texas A&M University Supercomputing Facilities. We acknowledge the use of the Laboratory for Molecular Simulations at Texas A&M University. A.L.Z. was supported by a fellowship from the Robert A. Welch Foundation. Data Availability: All data relevant to this research are available in the main text and SI Appendix. Y.J. and Y.L. contributed equally to this work. Author contributions: Y.J. and R.Z. designed research; Y.J., Q.S., Y.L., T.A., Y.W., J. Zhao, M.J.M., and R.Z. performed research; Y.J., Y.L., T.A., J. Zheng, J.P., Y.G., J.C., G.L., F.Z., J. Zhao, M.J.M., and R.Z. analyzed data; and Y.J., A.L.Z., and R.Z. wrote the paper. Reviewers: Y.G., Peking University; and K.K., Macalester College. The authors declare no competing interest. This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1912235117/-/DCSupplemental.

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Additional details

Created:
August 22, 2023
Modified:
October 20, 2023