* Author to whom correspondence should be addressed; Tel.:

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1 Supplementary Material OPEN ACCESS atmosphere ISSN - Inland Concentrations of Cl and ClNO in Southeast Texas Suggest Chlorine Chemistry Significantly Contributes to Atmospheric Reactivity. Atmosphere 5,, -5, doi:./atmos Cameron B. Faxon, Jeffrey K. Bean and Lea Hildebrandt Ruiz * Center for Energy and Environmental Resources, The University of Texas at Austin, Austin, TX 5, USA; s: cfaxon@gmail.com (C.B.F.); jbean5@gmail.com (J.K.B.) * Author to whom correspondence should be addressed; lhr@che.utexas.edu; Tel.: Academic Editor: Armin Sorooshian S. Calibration of the HR-ToF-CIMS Calibrations were performed for each species and indicated a linear response across the concentrations observed during the campaign. Calibration curves were generated as linear least squares fits to the correlation between known measured concentrations and normalized analyte signals. Analyte signals were normalized by the sum of I and IHO ion signals. Instrument response to Cl was calibrated directly from sequential dilutions of standards of known concentration (Airgas). The resulting Cl calibration curve is shown in Figure S. Calibration of the ClNO signal response was performed by passing a known concentration of Cl over wet NaNO salt to generate ClNO by the reaction Cl + NaNO NaCl + ClNO, as cited by previous work [ 5]. The wet salt bed was placed in the inlet (.5 length.5 inner diameter) of a glass bulb, and the residence time for reaction of Cl was calculated to be approximately. s. The exact technique for this calibration in previous work has varied with some authors using NaNO salt [,] and others using a combination of NaCl and NaNO [,5]. Thaler et al. () suggest that a marginal increase in ClNO yield is obtained from the addition of NaCl. Furthermore, one study, [] citing previous work [,], estimates a ±5% uncertainty in the measured sensitivity of ClNO using this calibration method, which appears much larger than differences in the yield using pure NaNO and a mixture of NaNO and NaCl. We therefore estimate the yield of ClNO from the heterogeneous reaction of Cl with NaNO salt is % with an uncertainty of 5%. Similarly, NO5 was calibrated for by passing NO5 over a wetted NaCl bed to produce ClNO. The decrease in NO5 from the reaction with

2 Atmosphere 5, S NaCl was assumed to be equal to the concentration of ClNO produced (i.e., a % yield) [,]. The calibration curves for ClNO and NO5 are shown in Figures S and S, respectively. x - Normalized Cl I - Signal 5 r =. [Cl ] (ppt) Figure S. Calibration Curve for Cl as quantified by normalized ClI signal in the HR- ToF-CIMS. Reagent ion signal during the calibration was approximately. ions s. x - Normalized ClNO I - Signal r =. 5 [ClNO ] (ppt) Figure S. Calibration Curve for ClNO as quantified by normalized ClNOI signal in the HR-ToF-CIMS. The reagent signal during calibration was approximately. ions s.

3 Atmosphere 5, S x - Normalized N O 5 I - Signal 5 r = [N O 5 ] (ppt) Figure S. Calibration Curve for NO5 as quantified by normalized NO5I signal in the HR- ToF-CIMS. Reagent ion signal during the calibration was approximately ions s. Detection limits were determined for each species using the formula shown in Equation (S) []. / = / + (S) In this equation, Cf (Hz ppt ) is the calibration coefficient for the species and B (Hz) is the underlying background count rate. S/N is the signal to noise ratio, which was set to to be consistent with the statistical definition of a detection limit. The detection limit of the measured species, [X], is determined for an integration period, t (s). For Cl, ClNO and NO5, the calculated detection limits were.,. and. pptv over a sampling period of s. S. Modified Version of the Carbon Bond Photochemical Mechanism The Carbon Bond mechanism was modified to include basic gas phase chlorine reactions, as well as ClNO photolysis. The full list of reactions that were added is shown in Table S.

4 Atmosphere 5, S Table S. List of chlorine reactions added to the Carbon Bond (CBr) mechanism for the simulations discussed in this work. Species are written in terms of their CBr identities. Reaction # Reactants Products CL + hv CL HOCl + hv OH + CL CL + O CLO CLO + CLO. C +. CL 5 CLO + NO CL + NO CLO + HO HOCL CLO + NO NCL NCL + hv CLO + NO NCL + hv CL + NO CL + NCL CL + NO OH + HCL CL OH + FMCL CL + CO FMCL + hv CL + CO + HO CL + H HCL + HO 5 CL + CH HCL + MEO CL + PAR HCL +. XO +. XON +. HO +. ALD. PAR +. ROR +.5 ALDX CL + ETHA HCL +. ALD +. XO +. XON + HO CL + ETH FMCL + XO + HO + FORM CL + OLE. HCL +. FMCL +.5 ALD +. ALDX +. OLE +. FORM +. XO + HO - PAR CL + IOLE. HCL +. FMCL +. ALD +.5 ALDX +. OLE +. FORM +. PAR +. XO + HO CL + ISOP.5 HCL +.5 FMCL +. ISPD +. XO +. XON +. HO CL + TERP. HCL +. FMCL +.5 ALDX +. PAR +. XO +.5 XON +.5 HO CL + TOL HCL +. XO +. XON +. +HO CL + XYL HCL +. XO +. XON +. HO 5 CL + FORM HCL + HO + CO CL + ALD HCL + CO CL + ALDX HCL + CXO CL + MEOH HCL + HO + FORM CL + ETOH HCL + HO + ALD ClNO + hv CL + NO S. Derivation of the OH Production Rate Used in Box Modeling Simulations Equation in the main text was used to calculate the rate of OH production, and was derived from the rate equation for the reaction of primary reactions of the radical species, O( D) (Reaction (SR)), with HO, N and O (Reactions (SR) and (SR)). The rate of O( D) formation was calculated from the simulated rate of O photolysis. These three primary reactions of O( D) were combined in a rate expression to determine the concentration of O( D). Using Reaction (SR) as the primary source of OH

5 Atmosphere 5, S5 production, Equations (S) and (S) were combined to calculate the rate of OH production as shown in Equation (S). O + hv O( D) + O (SR) = O( D) + HO OH (SR) O( D) + M O + M (SR) / = (S) = / = (S) = / + + (S) References of Supplementary Material. Lopez-Hilfiker, F.D.; Constantin, K.; Kercher, J.P.; Thornton, J.A. Temperature dependent halogen activation by NO5 reactions on halide-doped ice surfaces. Atmos. Chem. Phys.,, Mielke, L.H.; Furgeson, A.; Osthoff, H.D. Observation of ClNO in a mid-continental urban environment. Environ. Sci. Technol., 5,.. Phillips, G.J.; Tang, M.J.; Thieser, J.; Brickwedde, B.; Schuster, G.; Bohn, B.; Lelieveld, J.; Crowley, J.N. Significant concentrations of nitryl chloride observed in rural continental Europe associated with the influence of sea salt chloride and anthropogenic emissions. Geophys. Res. Lett.,. doi:./gl5.. Riedel, T.P.; Bertram, T.H.; Crisp, T.A.; Williams, E.J.; Lerner, B.M.; Vlasenko, A.; Li, S.M.; Gilman, J.; de Gouw, J.; Bon, D.M.; et al. Nitryl chloride and molecular chlorine in the coastal marine boundary layer. Environ. Sci. Technol.,,. 5. Thaler, R.D.; Mielke, L.H.; Osthoff, H.D. Quantification of nitryl chloride at part per trillion mixing ratios by thermal dissociation cavity ring-down spectroscopy. Anal. Chem.,,.. Thornton, J.A.; Kercher, J.P.; Riedel, T.P.; Wagner, N.L.; Cozic, J.; Holloway, J.S.; Wolfe, G.M.; Quinn, P.K.; Middlebrook, A.M.; Alexander, B.; et al. A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry. Nature,,.. Kercher, J.P.; Riedel, T.P.; Thornton, J.A. Chlorine activation by NO5: simultaneous, in situ detection of ClNO and NO5 by chemical ionization mass spectrometry. Atmos. Measu. Tech.,,.. Thornton, J.A.; Kercher, J.P.; Riedel, T.P.; Wagner, N.L.; Cozic, J.; Holloway, J.S.; Dubé, W.P.; Wolfe, G.M.; Quinn, P.K.; Middlebrook, A.M.; Alexander, B.; Brown, S.S. A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry. Nature,,.. Behnke, W.; George, C.; Scheer, V.; Zetzsch, C. Production and decay of ClNO from the reaction of gaseous NO5 with NaCl solution: Bulk and aerosol experiments. J. Geophys. Res.,, 5. 5 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (