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