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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 (ClNO2) impacts on oxidation, ozone production and 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. (ClNO2) is produced at night by the mechanism. The presence of ClNO2 produces significant reactions of (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 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 , 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 (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 (ClONO ) 2 5 2 nificantly depending on a number of factors such as partic- and (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). After of ClO with NO, which completes a null cycle producing Cl· sunrise, the photolysis of ClNO2 produces Cl· and NO2, and NO2, limits the potential for multiphase Cl2 formation. thereby partially circumventing the removal of NOx through

Atmos. Chem. Phys., 14, 3789–3800, 2014 www.atmos-chem-phys.net/14/3789/2014/ T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts 3791 the formation and loss of 2 HNO3 (Reaction R8a). Here we examine the effects of ClNO2 formation as pre- dicted by a detailed box model that incorporates the master NO2 + O3 → NO3 + O2 (R6) chemical mechanism and is constrained by ground and ship- NO + NO ↔ N O (R7) based ambient measurements taken during the CalNex 2010 3 2 2 5 field study. The aim is not to replicate specific observations γ N2O5 + H2 O −→ 2HNO3 (R8a) or conditions, but rather to use the model to develop concep- γ tual insights into the effect of a morning pulse of chlorine N O + Cl− −→ φ ClNO + 2 − φ  NO− 2 5 ClNO2 2 ClNO2 3 (R8b) atoms in polluted air. We use the model to assess the impact of ClNO on the Cl· budget, RO abundance, NO lifetime The Cl· budget, hydrogen oxide and organic peroxy radical 2 x x and partitioning, and the net ozone production rate. abundance (ROx = OH + HO2 + RO2), NOx lifetime and par- titioning among other forms of reactive nitrogen, and the net ozone production rate are linked through photochemical oxi- dation of VOC. As shown in Reactions (R9)–(R12), the oxi- 2 Measurements and model description dation of a hydrocarbon (RH) is initiated by reaction with OH We chose to constrain a box model using data taken during or Cl· to form the organic peroxy radical (RO ). In polluted 2 the CalNex field study, which occurred in May and June of regions, the dominant fate of RO is to react with NO. The 2 2010 in the southern California region (Ryerson et al., 2013). dominant channel of this reaction eventually leads to a closed The goal of these modeling studies is not to replicate the evo- shell oxygenated hydrocarbon (OVOC – oxygenated volatile lution of specific air masses in the LA Basin, but instead to organic compound), hydroperoxyl radical (HO ) and NO , 2 2 more generally probe the effect of multiphase reactive ni- while the minor channel leads to an alkyl nitrate (RONO ). 2 trogen and reactive halogen chemistry on radical budgets, If the RO is an acyl peroxy radical, then reaction with NO 2 2 ozone production, and the fate of NO in polluted coastal re- produces acyl peroxy (APN) such as acetyl peroxy x gions. There were multiple measurement platforms involved nitrate (PAN). NO also reacts with HO to form NO and 2 2 in CalNex, three of which recorded both ClNO and ex- OH. Through these reactions ozone is produced from the 2 tensive VOC measurements: the Research Vessel Atlantis, a photolysis of NO . 2 ground site located on the California Institute of Technology O campus in Pasadena, CA, and aircraft measurements taken RH + OH −→2 RO + H O (R9) 2 2 on the NOAA WP-3D (Mielke et al., 2013; Riedel et al., O2 2012a; Young et al., 2012). Though the R/V Atlantis sampled RH + Cl· −→ RO2 + HCl (R10) in many locations along the southern California coast, we RO2 + NO → OVOC + HO2 + NO2 (R11a) focus on the measurements made in and around Los Ange- les urban outflow (16–31 May) due to the added constraints RO2 + NO → RONO2 (R11b) provided by the Pasadena ground site measurements. ClNO2 RO2 + NO2 ↔ APN (R12) mixing ratios in the nocturnal outflow from the Los Angeles region were commonly over 500 pptv with maximums on the From the above discussion, we expect that ClNO2 acts simi- order of 2 ppbv (Riedel et al., 2012a; Wagner et al., 2012). larly, though not exactly the same, as an OH source such as Cl· chemistry was incorporated into an existing model 1 that from nitrous acid (HONO) photolysis or O( D) + H2O. framework described in Wolfe and Thornton (2011) which When the radical pool is terminated via cross reactions be- is based on the master chemical mechanism version 3.2 tween ROx and NOx, a higher production rate of HOx or Cl· (MCM) developed at the University of Leeds (more infor- will nearly linearly increase the ozone production rate (Daum mation available at http://mcm.leeds.ac.uk/MCM) (Bloss et et al., 2000; Kleinman, 2005). Moreover, at high NO, pro- al., 2005; Jenkin et al., 1997, 2003; Saunders et al., 2003). duction of an RO2 by Cl· attack directly increases the steady Use of the MCM allows for explicit tracking of approxi- state concentration of OH and HO2 due to the rapid cycling mately 2800 chemical species and about 9000 different re- between OH, HO2 and RO2. However, the increased RO2 due actions with reaction rate constants derived from the Interna- to Cl· arises from a potentially different pool of hydrocarbons tional Union of Pure and Applied Chemistry (IUPAC) kinet- than that from OH, given the large differences in RH abun- ics database (http://www.iupac-kinetic.ch.cam.ac.uk). Simi- dance and relative reactivity towards OH and Cl·. Moreover, lar to Wolfe and Thornton (2011), fast reactions, such as re- ClNO2 photolysis predominantly occurs in the first few hours actions of alkoxy radicals, are treated as instantaneous in or- after sunrise, well before the maximum OH production rate der to reduce model stiffness. 1 from O( D) + H2O and before the maximum in NO/NO2. In total, 44 of the VOC measured at the Pasadena site Thus, the full impact of ClNO2 on ozone production, VOC are used to constrain the model. However, certain VOC, lifetime and NOx abundance and partitioning may not be the for example, ethanol (median value = 8.2 ppbv) and acetone same as simply scaling the daytime average HOx production (median value = 3.8 ppbv), measured at the Pasadena ground rate. site appeared to often be dominated by highly localized www.atmos-chem-phys.net/14/3789/2014/ Atmos. Chem. Phys., 14, 3789–3800, 2014 3792 T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts emissions. To more generally represent an urban air mass We use the final 24 h of a model run as the analysis period. in the model, ground site VOC measurements were scaled All figures and calculations described here are performed on by those measured on the R/V Atlantis. A smaller number the model output from this period. of VOC were measured aboard the R/V Atlantis, so species A number of reactions necessary for evaluating Cl· pro- not represented in the R/V Atlantis data set were scaled by duction and reactivity are not included in the MCM. Ver- species of similar structure (i.e., similar functional groups). sion 3.2 of the MCM only includes Cl· reactions with alka- For example, methanol was measured with median levels of nes. In order to accurately represent the chemistry, mul- approximately 6 and 1 ppbv at the ground site and on the tiple mechanisms were added to the model framework. ship, respectively. Ethanol, however, was only measured at These include the Reactions (R1)–(R3), (R8), (R13)–(R21), the ground site. To estimate ethanol levels in the urban out- and a number of VOC + Cl· reactions such as those for flow and be more representative of what the R/V Atlantis methanol, ethanol, isopropanol, ethene, propene, formalde- might have sampled, the ground site ethanol mixing ratios hyde, ethanal, propanal, acetone, benzene, styrene, o-xylene, were simply scaled down by one-sixth. For a complete list toluene. Several of the added mechanisms are explicitly of the measured VOC used in the model see Table S1 in the shown in the Supplemental information (Figs. S2–S6 in the Supplement. Supplement). A complete list of the added reactions and re- VOC and HCl mixing ratios are held to their ship-scaled action rate constants is given in Table S2 in the Supplement, hourly average diurnal profiles for a 69 h “spin-up” period. and the MATLAB code is freely available for download at The diurnal HCl profile used is shown in Fig. S1 in the Sup- ftp://ftp.atmos.washington.edu/thornton/UWCM/. The reac- plement. NO2,O3, and CO are held to mean values mea- tion rate constants and product branching for these reac- sured at the ground site during this spin-up period. In addi- tions were taken from the IUPAC kinetics database as of tion, we fix methane at a mixing ratio of 1.8 ppmv. Over the 11 May 2012. Chlorinated products not present in the MCM entirety of a model run temperature is held constant at 25 ◦C or available in the IUPAC database were assumed to react and the aerosol surface area concentration is held constant at similarly and with similar rate constants to non-chlorinated 350 µm2 cm−3, which represents some of the largest aerosol species of the same structure already in the MCM (see Ta- surface area concentrations encountered by the R/V Atlantis ble S2 in the Supplement). Additionally, our revised mecha- while sampling Los Angeles outflow. At lower model tem- nism explicitly tracks gas-phase HCl formation that results peratures a larger fraction of NOx will react as N2O5 with from hydrogen atom abstraction reactions by Cl·. ClNO2 higher ClNO2 levels and an increased morning Cl· burden photolysis frequencies were estimated by scaling measured relative to warmer cases. This result suggests that we are pos- NO2 photolysis frequencies down by a factor of 30 (i.e., sibly overestimating the actual yield of ClNO2 per NO2 oxi- jNO2 /30). This approximation produces ClNO2 photolysis dized by ozone at night. The box model does not attempt to frequencies close to observations taken aboard the R/V At- replicate the effects of meteorology and thus the processes lantis (Fig. S7 in the Supplement). Photolysis frequencies of dilution and deposition are not accurately incorporated. for Cl2, ClONO2, and HOCl were calculated using the Tro- To maintain reasonable concentrations of the many modeled pospheric Ultraviolet and Visible (TUV) Radiation Model species which were not constrained by observations, we ap- (available at http://cprm.acd.ucar.edu/Models/TUV) and in- ply a continuous dilution rate of 1.5 % per hour to all species. corporated into the box model. Formaldehyde and nitric acid have an additional deposition rate of 30 % per hour in order to keep mixing ratios at lev- Cl · +O3 → ClO + O2 (R13) els most similar to those sampled during the CalNex study ClO + NO → ClONO (R14) (< 6 ppbv for formaldehyde) (Warneke et al., 2011). The sen- 2 2 sitivity of the results to this additional loss rate is minor ClO + HO2 → HOCl + O2 (R15) (< 20 % adjustments to the Cl· budget), and the need for this additional loss is likely related to our neglect of deposition ClONO2 + hν → Cl · +NO3 (R16) for intermediate organic oxidation products (in the case of ClONO2 + hν → ClO + NO2 (R17) formaldehyde) and of HNO3 itself. Isoprene, alpha-pinene, beta-pinene, and limonene are allowed to freely evolve at HOCl + hν → Cl · +OH (R18) night during the spin-up period to avoid unrealistic condi- + + + − −→γ + tions whereby NO3 reactions with these compounds pro- ClONO2 H Cl Cl2 HNO3 (R19) ceeded indefinitely throughout the night. That is, we assume γ HOCl + H+ + Cl− −→ Cl + H O (R20) that advection of air masses from the land out over the wa- 2 2 ter (either at the surface or aloft of the continental nocturnal ClO + NO → Cl · +NO2 (R21) surface layer) would ultimately limit the source of reactive biogenic VOC. At hour 69, which represents sunset on the For alkenes, the major pathway involves addition of chlo- third model day, all species are released from observational rine to the double bond rather than the typical hydrogen ab- constraints and the chemistry evolves freely for another 27 h. straction pathway (Atkinson et al., 2004). This pathway leads

Atmos. Chem. Phys., 14, 3789–3800, 2014 www.atmos-chem-phys.net/14/3789/2014/ T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts 3793

4 to chlorinated products which might be detectable as tracers x 10 12 1500 of Cl· chemistry in future studies. As we show below, such Cl-atoms w/ ClNO compounds could be another avenue for Cl· recycling. Reac- 2 Cl-atoms w/o ClNO tions of Cl· with isoprene, which also produces unique chlo- 2 ) ClNO

3 2

- rinated products, were not included in the model framework 8 1000 given its modest < 1 % contribution to total Cl· reactivity

(pptv) in the modeled Los Angeles outflow and the large increase 2 in complexity when incorporating the mechanism (Fan and

ClNO Zhang, 2004; Tanaka et al., 2003). Instead, the products of 4 500 Cl· + isoprene were tracked as a single generic species with cm (atoms Cl-atoms no chemical losses. However, in areas where isoprene is a more significant contributor to Cl· reactivity, it would be nec- 0 0 essary to include a more explicit isoprene oxidation mecha- 0 5 10 15 20 Hour of Day nism to accurately capture the effects of Cl·, especially to assess any chlorinated products that might form from these Fig. 1. Model output for the analysis period of a model run show- reactions (Riemer et al., 2008). In such locations the prod- ing ClNO2 mixing ratios (heavy green line, right y axis) and Cl· ucts of chlorine-initiated isoprene oxidation are likely more concentrations for the case including ClNO2 formation (heavy blue pronounced than in the Los Angeles region. line, left y axis) and the case excluding ClNO2 formation (dashed Gas-particle reaction probabilities in the model are set red line, left y axis). to 0.01 for N2O5, ClONO2, and HOCl (Reactions R8, R19, R20). A γ = 0.01 is within the typical range of γ (N2O5) measured on ambient aerosol (< 0.001–0.03) at elevated RH 3 Results and discussion during various field studies (Bertram et al., 2009; Riedel et al., 2012b). Laboratory measurements of ClONO2 and HOCl 3.1 Cl-atom budget uptake under stratospheric and tropospheric conditions on sulfuric acid, sodium chloride, and sodium bromide parti- The model predicts that, integrated over a typical day in cles and pure water droplets generally report γ values < 0.06 the Los Angeles outflow, ClNO2 is the major driver of Cl· for ClONO2 and HOCl (Deiber et al., 2004; Hanson and evolution. Neither HCl + OH nor multiphase chemistry in- Ravishankara, 1994; Hanson et al., 1994). We make the volving ClONO2 and HOCl to produce Cl2 are competi- upper-limit assumption that reactions of ClONO2 and HOCl tive with the ClNO2 source. Moreover, this picture is con- on aerosol particles produce only Cl2 with unit efficiency. sistent with that derived solely from observations in this re- Given that Cl2 production from heterogeneous reactions of gion (Riedel et al., 2012a). Figure 1 shows the Cl· concentra- ClONO2 and HOCl is proportional to the product of γ and tion predicted by the model during the 24 h analysis period the yield, we use γ = 0.01 and a 100 % yield on all particles for both the with-ClNO2 and without-ClNO2 cases. When in the model. ClNO2 formation is included, the Cl· concentration reaches To examine the effects of ClNO2 formation, we vary a maximum at ∼ 07:00 model time (2 h after model sunrise) × 5 −3 φClNO2 between 0 and 50 % in successive model runs, which with a value of 1.08 10 atoms cm . A substantially dif- produce a without-ClNO2 case and a with-ClNO2 case, re- ferent picture results from the without-ClNO2 case where the spectively. A 50 % yield results in ∼ 1.5 ppbv of ClNO2 as maximum Cl· concentration occurs around noon and only shown in Fig. 1, which is similar to levels in the Los Ange- reaches 0.2 × 105 atoms cm−3. The assumptions made about les outflow conditions encountered during CalNex. We also the aerosol reaction probabilities of ClONO2 and HOCl par- performed a series of model runs where HONO was con- tially drive the late afternoon Cl· profile, which, as a result, strained to observations made at the Pasadena ground site. is more uncertain. However, this afternoon Cl· concentra- Its abundance otherwise is determined only by the reaction tion profile is not especially sensitive to the assumed reaction of OH + NO, HONO + OH, and the photolysis of HONO. probabilities. For example, increasing the reaction probabil- Most of our main conclusions reported here are relatively in- ities of ClONO2 and HOCl from 0.01 to 0.1 does not sub- sensitive to HONO. Moreover, the vertical profiles of ClNO2 stantially change the 24 h profile. The maximum in Cl· con- and HONO throughout the nocturnal and evolving daytime centration is increased by ∼ 10 % still occurring in the early boundary layer are likely different (Young et al., 2012), mak- morning hours after sunrise (∼ 07:00 model time), and the ing our primary focus on ClNO2 a reasonable simplification integrated Cl· concentration over the entire day is enhanced for a box model. by only 20 %. Additionally, the choice of 0.01 for a ClONO2 and HOCl reaction probability and a 100 % Cl2 yield is likely more realistic as the formation of Cl2 from these reactions is unlikely to be the sole product (Caloz et al., 1996; Santschi and Rossi, 2005). That said, to fully understand the impact www.atmos-chem-phys.net/14/3789/2014/ Atmos. Chem. Phys., 14, 3789–3800, 2014 3794 T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts

6 of Cl· chemistry in coastal urban areas, the fate of ClONO2 x 10 4 especially needs to be better constrained. ClONO + hv A 2

· ) The evolution of Cl largely follows that of the domi- 1 HOCl + hv - 3 s Cl + hv

3 nant source terms, as shown in Fig. 2. In the absence of - 2 ClNO formation (Fig. 2a), the bulk of Cl· production re- OH + HCl 2 2 sults from the HCl + OH production channel, and the maxi- 6 −3 −1

· × (atoms cm

mum in Cl production rate of 0.5 10 atoms cm s co- l 1

C incides with the maximum production from the HCl + OH P channel. In the with-ClNO case (Fig. 2b), the maximum 2 0 Cl· production rate occurs at 07:00 model time with a value of 3.4 × 106 atoms cm−3 s−1 corresponding to the maximum 6 contribution from ClNO2 photolysis. The ClNO2 production x 10 4 channel represents 56 % of Cl· production over the course of ClNO + hv B 2

) ClONO + hv the entire day, leading to nearly 3.7 times as many Cl· pro- 1 2 - 3 s HOCl + hv

3 duced relative to the without-ClNO case. By noon, ClNO - 2 2 Cl + hv is largely depleted under the clear-sky model conditions, and 2 2 OH + HCl other Cl· production channels such as the reaction of HCl CHOCl + OH

(atoms cm with OH and the photolysis of Cl2 from HOCl and ClONO2 l 1

C heterogeneous chemistry become more dominant. These pro- P duction channels involving multi-phase Cl-recycling to form 0 Cl show significant enhancements when ClNO formation 0 5 10 15 20 2 2 Hour of Day is included. For example, Cl· production from ClONO2 pho- tolysis, HOCl photolysis, and Cl2 photolysis are enhanced Fig. 2. Model calculated Cl· production channels (A) without by factors of 3.3, 2.2, and 3.3, respectively, over the without- ClNO2 formation and (B) with ClNO2 formation. ClNO2 case. To some extent these enhancements should be expected considering the larger Cl· pool available for recy- cling reactions when ClNO2 formation is allowed, but they HOBr to form BrCl are not accounted for at all. Using max- give indication of the degree of indirect coupling between imum HCl levels as a measure of particulate chloride dis- ClNO2 and Cl2 via the increased formation of reactive chlo- placed over Los Angeles and the expected chloride to bro- rine reservoirs like ClONO2 and HOCl. During CalNex, mide ratio in seawater, 2.5 ppbv of chloride corresponds to molecular chlorine was also measured along with ClNO2 ∼ 4 pptv of bromide available for BrCl formation. Incorpo- (Riedel et al., 2012a). Observations of nighttime and early rating this amount of total bromine into the model and as- morning Cl2 were typically in the 5–50 pptv range. Modeled suming γ (BrONO2) and γ (HOBr) = 0.1 with a unit yield of

Cl2 levels are of similar magnitude to these observations, as BrCl, we predict the model could be neglecting a Cl· source well as previous observations of Cl2 in this region (Finley from BrCl on the order of 5 % of the Cl· concentration inte- and Saltzman, 2006, 2008), and show a morning enhance- grated over the model day. Ambient measurements of BrCl in ment with slightly elevated levels throughout the day but only polluted coastal regions would be a particularly useful con- . with the inclusion of ClNO2 (see Fig. S8 in the Supplement). straint on the extent of these Cl recycling reactions and the For additional information on the model concentrations of role of bromide. the Cl· source species see Fig. S9 in the Supplement. The use of a comprehensive chemical mechanism, such as Given the lack of BrCl observations outside of polar re- the MCM, also illustrates a potentially important but hereto- gions, we do not include BrCl formation in the model and fore overlooked source of Cl· in polluted regions. In the therefore do not explicitly account for the potential Cl· with-ClNO2 case, the reaction of OH with formyl chloride source, if any, represented by BrCl. Considering Cl2 repre- (CHOCl), produced from Cl· attack of alkenes, becomes sents 16 % of the integrated Cl· source over the course of a a noticeable Cl· source during the afternoon. Interestingly, model day, the typical seawater ratio of chloride to bromide CHOCl photolysis is predicted to be a Cl· source comparable of ∼ 650 : 1, and assuming that BrCl formation is not sig- in magnitude to that from HOCl photolysis (Fig. 2b). In fact, nificantly faster than 650 times Cl2 formation, we estimate because we possibly overestimate the actual multi-phase re- an upper limit Cl· source from BrCl resulting from ClONO2 cycling of ClONO2 and HOCl to form Cl2, Cl· release from and HOCl reactions that is similar to that predicted from Cl2. such acid may be more important than these mul- That said, BrCl formed from these reactions should not sig- tiphase processes in regions with significant concen- nificantly bias our Cl· estimates considering that we force trations. This result suggests observations of acid chlorides these reactants to produce exclusively Cl2, the photolysis of would be as beneficial as Cl2 in polluted regions. which forms 2 Cl· compared to only 1 Cl· from BrCl pho- HONO has a noticeable impact on the afternoon Cl· bud- tolysis. However, heterogeneous reactions of BrONO2 and get via photolysis to form OH followed by the reaction

Atmos. Chem. Phys., 14, 3789–3800, 2014 www.atmos-chem-phys.net/14/3789/2014/ T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts 3795

45 Cl-atom reactivity 2-chloroPPN 7AM 3PM 40 chloroacetaldehyde 2-chloroPAN methane formyl chloride 35 alkanes 2-chloropropanal alkenes 30 chloroacetone alcohols carbonyls 25 O3O3 (ClO+NO)(ClO+NO)

O3O3 (ClO+NO2)(ClO+NO2) (pptv) 20 O3O3 (ClO+HO2)(ClO+HO2) 15 total reactivity = 44 sec-1 total reactivity = 39 sec-1 10

Fig. 4. Modeled Cl· reactivity at 07:00 model time (left panel) and 5 15:00 model time (right panel) grouped by reactant types. 0 0 5 10 15 20 Hour of Day

order of 5–30 pptv above the background generated during Fig. 3. Mixing ratios of various chlorinated species tracked in the the spin-up period. Chloroacetaldehyde and formyl chloride model for the with-ClNO2 model case. exhibit the largest enhancements. In a polluted region such as the Los Angeles basin and outflow, Cl· will be primarily lost through reactions with of OH + HCl. Constraining the model to the HONO diur- VOC. Cl· reactivity as determined by the model is illus- nal profile measured at the Pasadena ground site leads to trated in Fig. 4, where it is broken into Cl· reactions with a 60 % increase (1.4 × 107 to 2.26 × 107 molecules cm−3) methane, alkanes, alkenes, alcohols, carbonyls (aldehydes in the daily maximum OH concentration and a similar in- and ketones), and ozone. Other Cl· + VOC reactions, such crease in the integrated Cl· formation rate from OH + HCl. as reactions with aromatics, are not shown as they were not Multiphase recycling via ClONO2 and HOCl are also in- a significant sink of Cl· relative to the species listed. In the creased as a result of the larger Cl· concentrations. However, morning (07:00 model time), the bulk of reactivity is due as discussed by Young et al. (2012), afternoon and daytime to reactions with a variety of alkanes, which comprise 42 % HONO concentrations are fairly uncertain, especially when of the 44 s−1 total reactivity. Later in the day (15:00 model considering the extent to which HONO measurements over time), as the VOC are consumed in the model and O3 maxi- land represent HONO concentrations in the marine bound- mizes, the reaction with O3 to form ClO is the dominant Cl· ary layer during morning hours within an air mass that was sink and represents about 60 % of the 39 sec−1 total reac- transported offshore overnight. tivity. However, the majority of ClO (77 % at 07:00 model We also investigate the formation potential of chlori- time and 67 % at 15:00 model time) will react with NO to nated products at potentially detectable quantities result- give NO2 and reform Cl· that can terminally react with VOC. ing from the Cl· oxidation of VOC. These species could Thus, we predict only 23–33 % of the Cl· + O3 reactions pro- represent “tracers” of Cl· chemistry and indicate the ex- duce stable reservoirs like ClONO2 and HOCl; though other tent to which Cl· oxidation reactions are occurring. This is- studies predict even less for the Los Angeles region (Young sue has been investigated previously in the Houston area et al., 2013). Over the course of the day, no single VOC dom- using ground-based measurements of potential products inates the Cl· reactivity (see Fig. S11 in the Supplement). from Cl· oxidation of isoprene (Riemer et al., 2008). As Methane is consistently about 10 % of the total. This implies stated above, chlorinated products of Cl· reactions with that all VOC reactive towards Cl· are relevant when trying to isoprene are not included our model and are likely of estimate the total reactivity and that only using a small sub- minor importance to total Cl· reactivity in the Los An- set may significantly underestimate the total. This behavior geles region. Within our model framework, Cl· addition is different from that of OH, the reactivity of which is often reactions with ethene and propene can produce chlori- dominated by a few VOC or inorganic species such as CO nated products such as 2-chloroperoxypropionyl nitrate (2- and NO2 in highly polluted regions (Kato et al., 2011; Ren et chloroPPN), chloroacetaldehyde, 2-chloroperoxyacetyl ni- al., 2003). trate (2-chloroPAN), formyl chloride, 2-chloropropanal, and chloroacetone – some of which have been previously inves- 3.2 Impact on ROx and NOx tigated in polar regions (Keil and Shepson, 2006). As with most of the previously mentioned effects, these enhance- The ClNO2 formation described above leads to important ments are pronounced in the early part of the day when Cl· and unique impacts on processes relevant to tropospheric air and VOC concentrations are highest in the model. As we quality. Cl· produced by ClNO2 photolysis will react with show in Fig. 3, morning enhancements are typically on the VOC to produce RO2 during morning hours. The RO2 will www.atmos-chem-phys.net/14/3789/2014/ Atmos. Chem. Phys., 14, 3789–3800, 2014 3796 T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts primarily react with NO under the polluted conditions to 2.5

)

1 A form HO2 and closed-shell oxygenated VOC or an alkyl ni- - 2 trate. Alternatively, given the large NO2 / NO ratio during the 1.5 morning, enhanced peroxy nitrate formation is possible via 1 (pptv sec (pptv w/ ClNO

x 2

O

reaction of acyl peroxy radicals with NO . The HO pro- H 2 2 0.5 w/o ClNO duced via RO + NO also reacts with NO to form OH, which P 2 2 0 in turn reacts with VOC to form RO2. The chain length of this cycle tends to be somewhat short in the morning given 2.5

2 higher concentrations of NOx leading to termination via B OH HO OH + NO2 to form HNO3 which is efficiently removed from 2 2 RO the atmosphere through wet and dry deposition. Nonethe- 2

/ w/o ClNO w/o /

2 1.5 less, Cl· will enhance the abundance of morning time ROx APNs (OH + HO2 + RO2) as illustrated in Fig. 5 which will lead to 1 enhanced O3 production rates relative to a model run without ClNO w/ ClNO2.

Figure 5a shows the HOx production rate (PHOx ) for both 15 the with- and without-ClNO2 cases. A factor of 2.2 increase C occurs in the early morning hours around 07:00 model time 10

(w/o) (ppbv) (w/o)

3 when Cl· production from ClNO2 photolysis is the major Cl· O

P  5 source. PHOx remains elevated throughout the day relative to

(w/) - - (w/) the without-ClNO2 case likely due to the larger ozone values 3 O 0

P in the with-ClNO2 case, thus illustrating that the ClNO2 in-  0 5 10 15 fluence persists for more than just the early morning hours. Hour of Day 24 h integrated HOx production for the with- and without- Fig. 5. (A) The HOx production rate with ClNO2 formation (solid ClNO2 cases is 75 and 62 ppbv, respectively. Uncertainties blue line) and without ClNO2 formation (dashed red line). (B) The in modeling HONO have the largest impact on quantifying ratio of the hydroxyl radical (solid black line), hydroperoxyl radi- the perturbation of ClNO2 to PHOx . Constraining modeled cal (dashed green line), sum of organic peroxy radicals (solid pink HONO to the diel average values measured at the ground line), and sum of acyl peroxy nitrates (dotted cyan line) for the with- site results in the same overall pattern of ClNO2 effects on ClNO2 case relative to the without-ClNO2 case. (C) The difference between the integrated ozone production rate with ClNO formation PHOx described above, just at a reduced relative magnitude 2 and the integrated ozone production rate without ClNO formation. – that is, the inclusion of ClNO2 formation increases PHOx 2 by ∼ 35 % in the morning, with moderate enhancements to

PHOx sustained throughout the day resulting in an integrated PHOx of 116 and 105 ppbv for the with- and without-ClNO2 OH are largely uncoupled such that the presence of one does cases, respectively. These two extremes in terms of HONO not largely impact the abundance of the other. This condition are likely both representative – the latter HONO-rich case then allows indirect quantification of Cl· abundance by com- being representative of near surface chemistry while the for- paring VOC which have different reaction rate constants for mer HONO-poor case is more representative of the resid- reaction with OH and Cl· (i.e., Jobson et al., 1994; Platt et ual boundary layer aloft of the surface, which dominates al., 2004; Allan et al., 2007). However, the presence of ad- the column-average radical budget as described by Young et ditional Cl· from ClNO2 in a polluted region has the poten- al. (2012). tial to significantly increase OH via the above mechanism, Figure 5b shows the effects of ClNO2 on OH, HO2, the especially in the morning hours thereby muting this effect. sum of all 717 organic peroxy radicals (RO2), and the sum of Constraining modeled HONO again lowers the magnitude of all 140 acyl peroxy nitrates (APN) predicted by the model. these ClNO2 induced morning perturbations to 25 and 50 % The ratio of the with-ClNO2 case relative to the without- increases in OH and the sum of HO2 and RO2, respectively. ClNO2 case is shown. Inclusion of ClNO2 formation results Again, while even these HONO-rich perturbations are sig- in significant changes in HOx (OH and HO2) with 190 and nificant, these findings, together with the vertically resolved 220 % enhancements during the morning hours in OH and estimates of Young et al. (2012), further imply that strong both HO2 and RO2, respectively. Such enhancements could vertical gradients in HONO will influence the ClNO2 effects be partly related to noted discrepancies between measured on morning oxidant evolution. and modeled morning HOx levels (Dusanter et al., 2009; Mao ClNO2 formation and photolysis has implications for the et al., 2010; Ren et al., 2003). Comparable enhancements in reactive nitrogen budget as well. ClNO2 is relatively unreac- HO2 and RO2 were predicted by Osthoff et al. (2008) using tive at night in these regions, thereby building up and allow- the MCM to assess measurements taken in and around the ing transport of NOx downwind of the urban core with morn- Houston ship channel. In remote low-NOx regions, Cl· and ing photolysis of ClNO2 analogous to thermal decomposition

Atmos. Chem. Phys., 14, 3789–3800, 2014 www.atmos-chem-phys.net/14/3789/2014/ T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts 3797 of acyl peroxy nitrates such as APN. In addition to this direct ClNO2 formation and its daytime impact on oxidants and impact on NOx transport, we find significant perturbations ozone, nighttime N2O5 chemistry does not net destroy Ox to acyl peroxy nitrate formation to occur in the with-ClNO2 but is in fact potentially a net source, or at least a null cycle, case (see Fig. 5b). A 50 % enhancement in total APN oc- for the Los Angeles region conditions we simulate here. curs before noon, and the enhancement remains elevated at While not directly comparable, our results appear gener- around 10 % for the remainder of the day. This ClNO2 in- ally consistent with a recent 3-D CMAQ modeling study duced perturbation to APN formation largely persists even of ClNO2 effects on ozone and particulate nitrate (Sar- when measured HONO values are incorporated. Addition- war et al., 2012). In the Los Angeles region, the CMAQ ally, the sum of alkyl nitrates is also enhanced by 15 % be- modeling showed roughly a 2–4 ppbv increase in daytime fore noon with individual alkyl nitrates enhanced up to 60 % ozone per ppbv ClNO2 photolyzed, with maxima approach- −1 in the with-ClNO2 case. Increasing the partitioning of NOx ing 8 ppbv ppbv . Likely important in setting the actual into reservoirs such as APN implies a greater potential for ozone enhancement caused by ClNO2 is, among other possi- export of NOx to remote regions. In fact, while ClNO2 for- bilities, the extent to which a model mixes background ma- mation extends the lifetime of NOx through the night, our rine air with the polluted core during transport and the model model suggests that faster OH + NO2, APN formation, and predicted vertical distribution of ClNO2. These issues will be alkyl nitrate formation due to increased ROx levels during important to test with observations in order to validate model the subsequent day largely offset this effect, such that NOx is representations of this process. ∼ 6 % lower from sunrise until noon in the with-ClNO2 than in the without-ClNO2 case. 4 Summary and conclusions 3.3 Impact on ozone production rate and odd oxygen These model results suggest that ClNO2 photolysis is likely a major Cl· source, if not the dominant source, under con- We illustrate the influence of ClNO chemistry on ozone pro- 2 ditions similar to those sampled in the Los Angeles region duction in Fig. 5c (also see Fig. S12 in the Supplement). during CalNex 2010. The impact of ClNO on potential Over the entire model day, the difference between the inte- 2 daytime halogen atom recycling is substantial, with signif- grated ozone production rate with 1.5 ppbv ClNO and the 2 icant enhancements predicted on other Cl· reservoirs like integrated ozone production rate without ClNO is about 2 ClONO , HOCl, and Cl . Relative to model runs without 12 ppbv. The majority of this enhancement takes place over 2 2 ClNO formation, the presence of ClNO causes significant the first 5 h after sunrise where at 06:30 model time the ozone 2 2 and non-negligible perturbations in HO , RO , APN, and production rate is enhanced by ∼ 200 %. The pre-noon ozone x 2 ozone production. Relative to a model without ClNO for- mixing ratios relative to the without-ClNO case are in- 2 2 mation and heterogeneous HONO production, incorporat- creased by ∼ 20 % with ∼ 10 % increase over the remainder ing ClNO perturbed the integrated total radical and ozone of the day including peak ozone which occurs at about 17:00 2 production rates by 20 %, with perturbations in RO and model time. Such an influence is potentially large enough to x APN > 100 %. Moreover, we show that, given these effects, affect attainment of air quality standards in polluted coastal the impact of N O reactions on aerosol particles is not a net regions where exceedances are often only tens of ppbv over 2 5 sink of odd oxygen but instead a net source for the polluted the current standard (Parrish et al., 2010; Qin et al., 2004; coastal conditions we model here. The absolute magnitude of US EPA, 2006). The enhancement in ozone production scales the perturbations in these quantities and processes relative to nearly linearly with the ClNO yield for this region, as ex- 2 a model that does not include ClNO will ultimately depend pected, given that the ozone production rate is approximately 2 upon the presence of HONO and the abundance of ClNO linear with the primary radical source in a NO -saturated en- 2 x and HONO vertically as well as seasonally. vironment like Los Angeles. Constraining modeled HONO We conclude by noting that during winter, in locations to the observations results in a slightly smaller 9 ppbv en- such as the northeastern US, the role of ClNO may be sub- hancement in the integrated ozone production rate due to 2 stantially more important to the total radical budget given ClNO chemistry. 2 that O(1D) production and H O vapor concentrations can The above result is also interesting to consider in terms 2 both be factors of 5 lower than presented here, resulting in of the impact of nocturnal nitrogen oxide chemistry on the more than an order of magnitude reduction in primary OH odd-oxygen budget. Defining O as the sum of O and NO , x 3 2 abundances while ClNO approaches similar concentrations our model predicts that N O reactions on aerosol particles 2 2 5 (Kercher et al., 2009). This idea is consistent with the appar- consume 9 ppbv O at night. If we neglect ClNO forma- x 2 ently important role of ClNO at inland locations during win- tion, this 9 ppbv O is permanently lost due to nitrate forma- 2 x tertime as illustrated by recent studies at the Uintah Basin, tion from N O hydrolysis. However, incorporating ClNO 2 5 2 Utah (Edwards et al., 2013). formation, with a yield (branching ratio) of 50 %, results in up to 12 ppbv Ox produced the subsequent day compared to the case where ClNO2 formation is neglected. Thus, due to www.atmos-chem-phys.net/14/3789/2014/ Atmos. Chem. Phys., 14, 3789–3800, 2014 3798 T. P. Riedel et al.: An MCM modeling study of nitryl chloride (ClNO2) impacts

Supplementary material related to this article is Daum, P. H., Kleinman, L., Imre, D. G., Nunnermacker, L. J., available online at http://www.atmos-chem-phys.net/14/ Lee, Y. N., Springston, S. R., Newman, L., and Weinstein-Lloyd, 3789/2014/acp-14-3789-2014-supplement.pdf. J.: Analysis of the processing of Nashville urban emissions on July 3 and July 18, 1995, J. Geophys. Res.-Atmos., 105, 9155– 9164, doi:10.1029/1999jd900997, 2000. Acknowledgements. This work was supported by a grant from the Deiber, G., George, Ch., Le Calvé, S., Schweitzer, F., and Mirabel, National Science Foundation (NSF CAREER ATM-0846183 to Ph.: Uptake study of ClONO2 and BrONO2 by Halide containing J. A. Thornton). T. P. Riedel is grateful for an Earth System Science droplets, Atmos. Chem. Phys., 4, 1291–1299, doi:10.5194/acp-4- graduate fellowship from the National Aeronautics and Space 1291-2004, 2004. Administration (NASA NESSF NNX10AN48H). G. M. Wolfe Dusanter, S., Vimal, D., Stevens, P. S., Volkamer, R., Molina, L. acknowledges support from a NOAA Climate and Global Change T., Baker, A., Meinardi, S., Blake, D., Sheehy, P., Merten, A., Postdoctoral Fellowship administered by the University Corpora- Zhang, R., Zheng, J., Fortner, E. C., Junkermann, W., Dubey, tion for Atmospheric Research. This research was support in part M., Rahn, T., Eichinger, B., Lewandowski, P., Prueger, J., and by the NOAA Health of the Atmosphere Program. We also thank Holder, H.: Measurements of OH and HO2 concentrations dur- the crew of the R/V Atlantis and the Pasadena ground site science ing the MCMA-2006 field campaign – Part 2: Model compar- team for their tireless efforts and continual support throughout the ison and radical budget, Atmos. Chem. Phys., 9, 6655–6675, CalNex study. doi:10.5194/acp-9-6655-2009, 2009. Edwards, P. M., Young, C. J., Aikin, K., deGouw, J., Dubé, W. Edited by: P. Monks P., Geiger, F., Gilman, J., Helmig, D., Holloway, J. 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