Open Access Atmos. Chem. Phys., 14, 3789–3800, 2014 Atmospheric www.atmos-chem-phys.net/14/3789/2014/ doi:10.5194/acp-14-3789-2014 Chemistry © Author(s) 2014. CC Attribution 3.0 License. and Physics An MCM modeling study of nitryl chloride (ClNO2) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow T. P. Riedel1,2,*, G. M. Wolfe3,4, K. T. Danas2, J. B. Gilman5,6, W. C. Kuster5,6, D. M. Bon5,6, A. Vlasenko7, S.-M. Li7, E. J. Williams5,6, B. M. Lerner5,6, P. R. Veres5,6, J. M. Roberts5, J. S. Holloway5, B. Lefer8, S. S. Brown5, and J. A. Thornton2 1Department of Chemistry, University of Washington, Seattle, Washington, USA 2Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA 3Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland, USA 4Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 5NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, Colorado, USA 6Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA 7Air Quality Research Division, Science and Technology Branch, Environment Canada, Toronto, Canada 8Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA *Present address: Department of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA Correspondence to: J. A. Thornton ([email protected]) Received: 13 October 2013 – Published in Atmos. Chem. Phys. Discuss.: 6 November 2013 Revised: 17 February 2014 – Accepted: 19 February 2014 – Published: 16 April 2014 Abstract. Nitryl chloride (ClNO2) is produced at night by the mechanism. The presence of ClNO2 produces significant reactions of dinitrogen pentoxide (N2O5) on chloride con- changes to oxidants, ozone, and nitrogen oxide partitioning, taining surfaces. ClNO2 is photolyzed during the morning relative to model runs excluding ClNO2 formation. From a hours after sunrise to liberate highly reactive chlorine atoms nighttime maximum of 1.5 ppbv ClNO2, the daytime maxi- (Cl·). This chemistry takes place primarily in polluted envi- mum Cl· concentration reaches 1 × 105 atoms cm−3 at 07:00 ronments where the concentrations of N2O5 precursors (ni- model time, reacting mostly with a large suite of volatile or- trogen oxide radicals and ozone) are high, though it likely oc- ganic compounds (VOC) to produce 2.2 times more organic curs in remote regions at lower intensities. Recent field mea- peroxy radicals in the morning than in the absence of ClNO2. surements have illustrated the potential importance of ClNO2 In the presence of several ppbv of nitrogen oxide radicals as a daytime Cl· source and a nighttime NOx reservoir. How- (NOx = NO + NO2), these perturbations lead to similar en- ever, the fate of the Cl· and the overall impact of ClNO2 on hancements in hydrogen oxide radicals (HOx = OH + HO2). regional photochemistry remain poorly constrained by mea- Neglecting contributions from HONO, the total integrated surements and models. To this end, we have incorporated daytime radical source is 17 % larger when including ClNO2, ClNO2 production, photolysis, and subsequent Cl· reactions which leads to a similar enhancement in integrated ozone into an existing master chemical mechanism (MCM version production of 15 %. Detectable levels (tens of pptv) of chlo- 3.2) box model framework using observational constraints rine containing organic compounds are predicted to form as from the CalNex 2010 field study. Cl· reactions with a set of a result of Cl· addition to alkenes, which may be useful in alkenes and alcohols, and the simplified multiphase chem- identifying times of active Cl· chemistry. istry of N2O5, ClNO2, HOCl, ClONO2, and Cl2, none of which are currently part of the MCM, have been added to Published by Copernicus Publications on behalf of the European Geosciences Union. 3790 T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts 1 Introduction BrCl photolysis to form Cl· and atomic bromine is also thought to be an important Cl· source, especially in remote Chlorine atoms (Cl·) are highly reactive, often having rate regions. In polar regions, BrCl mixing ratios on the order of constants for reactions with volatile organic compounds tens of pptv have been measured (Buys et al., 2013; Foster et (VOC) that are factors of 10 to 200 larger than the hydroxyl al., 2001; Spicer et al., 2002). To our knowledge there have radical, OH, which is considered the atmosphere’s primary been no reported observations of BrCl in ambient air outside initiator of oxidation. As a result, the presence of Cl· can of polar regions (Finley and Saltzman, 2008). BrCl can form lead to shorter lifetimes for VOC and an enhanced radical through heterogeneous reactions of BrONO2 and HOBr on pool which can potentially enhance the production of ozone acidic, chloride-containing particles in an analogous manner · in polluted areas. The global tropospheric Cl budget re- to the Cl2 formation reactions described above or through re- ∼ mains uncertain, with a large range in recent studies ( 15– actions of ClONO2 and HOCl on acidic, bromide-containing − 40 Tg Cl yr 1) developed from indirect means (Allan et al., particles. 2007; Platt et al., 2004) as tropospheric Cl· concentrations Nitrosyl chloride (ClNO) has also been proposed as a po- are not presently measurable by existing methods. There are tential Cl· source (Raff et al., 2009). These theoretical and a number of potential Cl· sources in the troposphere, the ma- laboratory studies have yet to be confirmed by field measure- jor sources are outlined in Reactions (R1)–(R5). ments of ClNO in ambient air. Using a regional 3-D chemical transport model, Raff et al. (2009) predict that ClNO mixing + → · + HCl OH Cl H2O (R1) ratios in polluted marine areas could reach ppbv values. That said, the hydrolysis of ClNO at moderate and high relative Cl + hν → 2Cl· (R2) 2 humidity (RH > 20 %) will likely be sufficiently rapid to pre- BrCl + hν → Cl · +Br· (R3) vent the buildup of appreciable atmospheric concentrations of ClNO (Karlsson and Ljungström, 1996; Rubasinghege and ClNO + hν → Cl · +NO (R4) Grassian, 2012; Scheer et al., 1997). Since its proposed atmospheric formation by Finlayson- ClNO2 + hν → Cl · +NO2 (R5) Pitts et al. (1989) and first observation in ambient air by Osthoff et al. (2008), nitryl chloride (ClNO ) has been ob- The reaction of hydrochloric acid (HCl) with the hydroxyl 2 served during a number of different field studies worldwide radical (OH) is a daytime source of Cl·. Typical HCl mixing with nighttime maximum mixing ratios ranging from tens ratios in the troposphere vary from 100–5000 pptv with the of pptv to over 2 ppbv (Kercher et al., 2009; Mielke et al., highest found in polluted regions with direct HCl emissions 2011, 2013; Osthoff et al., 2008; Phillips et al., 2012; Riedel from industrial processes and acid displacement of aque- et al., 2012a; Thornton et al., 2010; Young et al., 2012). ous chloride by HNO and H SO (Keene et al., 2007). Cl· 3 2 4 These observations have occurred in both continental and formed by HCl + OH tend to peak around midday with the marine locations illustrating the importance of ClNO as a peak in OH formed from O(1D) + H O. Additionally, the ox- 2 2 Cl· source in a variety of different environments. ClNO rep- idation of many VOC by Cl· proceeds via a hydrogen abstrac- 2 resents a Cl· source with clear anthropogenic origins as it tion to form HCl, thus recycling this Cl· source. is formed at night by reactions involving NO (NO + NO), Photolysis of molecular chlorine (Cl ) produces two Cl· x 2 2 ozone, and chloride containing aerosols. Anthropogenic ac- and has been the focus of many Cl· investigations since it tivities associated with power generation, motor vehicle use, was first measured at elevated concentrations in ambient air and agriculture now dominate the global NO source (Jaegle (Finley and Saltzman, 2006, 2008; Lawler et al., 2011; Riedel x et al., 2005). Natural sources of NO , such as microbial ac- et al., 2012a; Spicer et al., 1998). Cl mixing ratios were of- x 2 tivity, lightning, and wildfires, are also significant globally, ten on the order of tens of pptv with maximum reported mix- but the impact of these NO sources on ClNO formation ing ratios near 100–200 pptv. Direct Cl emissions are related x 2 2 remain unknown. At night, a fraction of NO is converted to power generation, water treatment, and oil refineries (Sar- x into ClNO through Reactions (R6)–(R8). The branching war and Bhave, 2007). Recently, a low pH Cl production 2 2 ratio between Reactions R8a and R8b, commonly referred channel that may be atmospherically relevant has been iden- to as the ClNO yield (φ ), is determined by the ef- tified in the reaction of N O with chloride containing sub- 2 ClNO2 2 5 ficiency of ClNO formation from heterogeneous reactions strates, which involved ClNO as an intermediate (Roberts 2 2 of N O . The φ and the N O -particle reaction proba- et al., 2008). In addition, Cl can be formed in situ through 2 5 ClNO2 2 5 2 bility, γ (N O ), are uncertain quantities that can vary sig- multiphase chemistry involving chlorine nitrate (ClONO ) 2 5 2 nificantly depending on a number of factors such as partic- and hypochlorous acid (HOCl). These species, in turn, can ulate water, chloride, nitrate, and organic content (Badger photolyze to reform Cl· or ClO or react on acidic, chloride- et al., 2006; Bertram and Thornton, 2009; McNeill et al., containing particles to form Cl . In polluted air, the reaction 2 2006; Mentel et al., 1999; Thornton et al., 2003).