Synergistic O₃ + OH oxidation pathway to extremely low-volatility dimers revealed in β-pinene secondary organic aerosol
Dimeric compounds contribute significantly to the formation and growth of atmospheric secondary organic aerosol (SOA) derived from monoterpene oxidation. However, the mechanisms of dimer production, in particular the relevance of gas- vs. particle-phase chemistry, remain unclear. Here, through a combination of mass spectrometric, chromatographic, and synthetic techniques, we identify a suite of dimeric compounds (C_(15–19)H_(24–32)O_(5–11)) formed from concerted O₃ and OH oxidation of β-pinene (i.e., accretion of O₃- and OH-derived products/intermediates). These dimers account for an appreciable fraction (5.9–25.4%) of the β-pinene SOA mass and are designated as extremely low-volatility organic compounds. Certain dimers, characterized as covalent dimer esters, are conclusively shown to form through heterogeneous chemistry, while evidence of dimer production via gas-phase reactions is also presented. The formation of dimers through synergistic O₃ + OH oxidation represents a potentially significant, heretofore-unidentified source of low-volatility monoterpene SOA. This reactivity also suggests that the current treatment of SOA formation as a sum of products originating from the isolated oxidation of individual precursors fails to accurately reflect the complexity of oxidation pathways at play in the real atmosphere. Accounting for the role of synergistic oxidation in ambient SOA formation could help to resolve the discrepancy between the measured atmospheric burden of SOA and that predicted by regional air quality and global climate models.
Additional Information© 2018 National Academy of Sciences. Published under the PNAS license. Edited by Joost A. de Gouw, University of Colorado Boulder, Boulder, CO, and accepted by Editorial Board Member A. R. Ravishankara July 2, 2018 (received for review March 16, 2018). Published ahead of print August 3, 2018. We thank Xuan Zhang, John Crounse, and Paul Wennberg for useful discussions. UPLC/(−)ESI-Q-TOF-MS was performed in the Caltech Environmental Analysis Center. This work was supported by National Science Foundation Grants AGS-1523500 and CHE-1508526. R.Z. acknowledges support from a Natural Science and Engineering Research Council of Canada Postdoctoral Fellowship. J.C.H. acknowledges support from the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry. Author contributions: C.M.K. designed research; C.M.K., Y.H., and R.Z. performed research; J.C.H. and B.M.S. contributed new reagents/analytic tools; C.M.K., Y.H., R.Z., and N.F.D. analyzed data; and C.M.K. and J.H.S. wrote the paper. This article is a PNAS Direct Submission. J.A.d.G. is a guest editor invited by the Editorial Board. The authors declare no conflict of interest. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1804671115/-/DCSupplemental.
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