Reassessing the atmospheric oxidation mechanism of toluene

Yuemeng Ji; Jun Zhao; Hajime Terazono; Kentaro Misawa; Nicholas P. Levitt; Yixin Li; Yun Lin; Jianfei Peng; Yuan Wang; Lian Duan; Bowen Pan; Fang Zhang; Xidan Feng; Taicheng An; Wilmarie Marrero-Ortiz; Jeremiah Secrest; Annie L. Zhang; Kazuhiko Shibuya; Mario J. Molina; Renyi Zhang

doi:10.1073/pnas.1705463114

Contributed by Mario J. Molina, June 8, 2017 (sent for review April 3, 2017; reviewed by Sasha Madronich and Fangqun Yu)

Significance

Aromatic hydrocarbons account for 20 to 30% of volatile organic compounds and contribute importantly to ozone and secondary organic aerosol (SOA) formation in urban environments. The oxidation of toluene, the most abundant aromatic compound, is believed to occur mainly via OH addition, primary organic peroxy radical (RO₂) formation, and ring cleavage, leading to ozone and SOA. From combined experimental and theoretical studies, we show that cresol formation is dominant, while primary RO₂ production is negligible. Our work reveals that the formation and subsequent reactions of cresols regulate the atmospheric impacts of toluene oxidation, suggesting that its representation in current atmospheric models should be reassessed for accurate determination of ozone and SOA formation. The results from our study provide important constraints and guidance for future modeling studies.

Abstract

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O₂ to form the cresol proceeds with a smaller barrier than O₂ addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Footnotes

↵¹Y.J. and J.Z. contributed equally to this work.
↵²Present address: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Tokyo 192-0364, Japan.
↵³Present address: The Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706.
↵⁴To whom correspondence may be addressed. Email: renyi-zhang{at}tamu.edu, antc99{at}gdut.edu.cn, or mjmolina{at}ucsd.edu.

Author contributions: Y.J. and R.Z. designed research; Y.J., J.Z., H.T., K.M., N.P.L., Y. Li, Y. Lin, J.P., Y.W., L.D., B.P., F.Z., X.F., T.A., W.M.-O., J.S., A.L.Z., K.S., and R.Z. performed research; M.J.M. and R.Z. contributed new reagents/analytic tools; Y.J., M.J.M., and R.Z. analyzed data; and Y.J., J.Z., and R.Z. wrote the paper.
Reviewers: S.M., National Center for Atmospheric Research; and F.Y., State University of New York at Albany.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1705463114/-/DCSupplemental.

Freely available online through the PNAS open access option.

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