ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-14-3427-2014 Chlorine as a primary radical: evaluation of methods to understand its role in initiation of oxidative cycles Young C. J. https://orcid.org/0000-0002-6908-5829 1 2 10 Washenfelder R. A. 1 2 Edwards P. M. https://orcid.org/0000-0002-1076-6793 1 2 Parrish D. D. 2 Gilman J. B. 1 2 Kuster W. C. https://orcid.org/0000-0002-8788-8588 2 Mielke L. H. 3 11 Osthoff H. D. https://orcid.org/0000-0001-7155-6493 3 Tsai C. 4 Pikelnaya O. 4 Stutz J. https://orcid.org/0000-0001-6368-7629 4 Veres P. R. https://orcid.org/0000-0001-7539-353X 1 2 Roberts J. M. https://orcid.org/0000-0002-8485-8172 2 Griffith S. 5 Dusanter S. 5 6 Stevens P. S. https://orcid.org/0000-0001-9899-4215 5 Flynn J. 7 Grossberg N. 7 Lefer B. https://orcid.org/0000-0001-9520-5495 7 Holloway J. S. 1 2 Peischl J. https://orcid.org/0000-0002-9320-7101 1 2 Ryerson T. B. 2 Atlas E. L. https://orcid.org/0000-0003-3847-5346 8 Blake D. R. 9 Brown S. S. 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA Department of Chemistry, University of Calgary, Calgary, AB, Canada Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA School of Public and Environmental Affairs, Indiana University, Bloomington, IN, USA Mines Douai, CE, 59508, Douai, France Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA Department of Chemistry, University of California Irvine, Irvine, CA, USA now at: Department of Chemistry, Memorial University, St. John's, NL, Canada now at: School of Public and Environmental Affairs, Indiana University, Bloomington, IN, USA 07 04 2014 14 7 3427 3440 Copyright: © 2014 C. J. Young et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/14/3427/2014/acp-14-3427-2014.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/14/3427/2014/acp-14-3427-2014.pdf

The role of chlorine atoms (Cl) in atmospheric oxidation has been traditionally thought to be limited to the marine boundary layer, where they are produced through heterogeneous reactions involving sea salt. However, recent observation of photolytic Cl precursors (ClNO<sub>2</sub> and Cl<sub>2</sub>) formed from anthropogenic pollution has expanded the potential importance of Cl to include coastal and continental urban areas. Measurements of ClNO<sub>2</sub> in Los Angeles during CalNex (California Nexus – Research at the Nexus of Air Quality and Climate Change) showed it to be an important primary (first generation) radical source. Evolution of ratios of volatile organic compounds (VOCs) has been proposed as a method to quantify Cl oxidation, but we find no evidence from this approach for a significant role of Cl oxidation in Los Angeles. We use a box model with the Master Chemical Mechanism (MCM v3.2) chemistry scheme, constrained by observations in Los Angeles, to examine the Cl sensitivity of commonly used VOC ratios as a function of NO<sub>x</sub> and secondary radical production. Model results indicate VOC tracer ratios could not detect the influence of Cl unless the ratio of [OH] to [Cl] was less than 200 for at least a day. However, the model results also show that secondary (second generation) OH production resulting from Cl oxidation of VOCs is strongly influenced by NO<sub>x</sub>, and that this effect obscures the importance of Cl as a primary oxidant. Calculated concentrations of Cl showed a maximum in mid-morning due to a photolytic source from ClNO<sub>2</sub> and loss primarily to reactions with VOCs. The [OH] to [Cl] ratio was below 200 for approximately 3 h in the morning, but Cl oxidation was not evident from the measured ratios of VOCs. Instead, model simulations show that secondary OH production causes VOC ratio evolution to follow that expected for OH oxidation, despite the significant input of primary Cl from ClNO<sub>2</sub> photolysis in the morning. Even though OH is by far the dominant oxidant in Los Angeles, Cl atoms do play an important role in photochemistry there, constituting 9% of the primary radical source. Furthermore, Cl–VOC reactivity differs from that of OH, being more than an order of magnitude larger and dominated by VOCs, such as alkanes, that are less reactive toward OH. Primary Cl is also slightly more effective as a radical source than primary OH due to its greater propensity to initiate radical propagation chains via VOC reactions relative to chain termination via reaction with nitrogen oxides.

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