Observations of Organic and Inorganic Chlorinated Compounds and Their Contribution to Chlorine Radical Concentrations in an Urba
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Atmos. Chem. Phys., 18, 13481–13493, 2018 https://doi.org/10.5194/acp-18-13481-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Observations of organic and inorganic chlorinated compounds and their contribution to chlorine radical concentrations in an urban environment in northern Europe during the wintertime Michael Priestley1, Michael le Breton1,a, Thomas J. Bannan1, Stephen D. Worrall1,b, Asan Bacak1, Andrew R. D. Smedley1,c, Ernesto Reyes-Villegas1, Archit Mehra1, James Allan1,2, Ann R. Webb1, Dudley E. Shallcross3, Hugh Coe1, and Carl J. Percival1,d 1Centre for Atmospheric Science, School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK 2National Centre for Atmospheric Science, University of Manchester, Manchester, M13 9PL, UK 3School of Chemistry, The University of Bristol, Cantock’s Close BS8 1TS, UK anow at: Department of Chemistry and Molecular Biology, University of Gothenburg, 412 96 Gothenburg, Sweden bnow at: School of Materials, University of Manchester, Manchester, M13 9PL, UK cnow at: School of Mathematics, University of Manchester, Manchester, M13 9PL, UK dnow at: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Correspondence: Carl Percival ([email protected]) Received: 6 March 2018 – Discussion started: 7 March 2018 Revised: 27 July 2018 – Accepted: 28 August 2018 – Published: 21 September 2018 Abstract. A number of inorganic (nitryl chloride, ClNO2; tion and loss pathways are inhibited by reduced photolysis chlorine, Cl2; and hypochlorous acid, HOCl) and chlori- rates. This results in ClNO2 making up the dominant frac- nated, oxygenated volatile organic compounds (ClOVOCs) tion (83 %) on low radiant-flux days, as its concentrations have been measured in Manchester, UK during October are still high. As most ClOVOCs appear to be formed photo- and November 2014 using time-of-flight chemical ionisa- chemically, they exhibit a similar dependence on photolysis, tion mass spectrometry (ToF-CIMS) with the I− reagent ion. contributing 3 % of the Cl radical budget observed here. ClOVOCs appear to be mostly photochemical in origin, al- though direct emission from vehicles is also suggested. Peak concentrations of ClNO2, Cl2 and HOCl reach 506, 16 and 9 ppt respectively. The concentrations of ClNO2 are compa- rable to measurements made in London, but measurements 1 Introduction of ClOVOCs, Cl2 and HOCl by this method are the first re- ported in the UK. Maximum HOCl and Cl2 concentrations Oxidation controls the fate of many atmospheric trace gases. are found during the day and ClNO2 concentrations remain For example, increasing the oxidation state of a given species elevated into the afternoon if photolysis rates are low. Cl2 may increase its deposition velocity (Nguyen et al., 2015) exhibits a strong dependency on shortwave radiation, further or solubility (Carlton et al., 2006) and reduce its volatility adding to the growing body of evidence that it is a product of (Carlton et al., 2006), all of which act to reduce the atmo- secondary chemistry. However, night-time emission is also spheric lifetime of that species and can lead to the forma- observed. The contribution of ClNO2, Cl2 and ClOVOCs to tion of secondary material such as secondary organic aerosol the chlorine radical budget suggests that Cl2 can be a greater (SOA) or ozone (O3). As the identity of the chemical species source of Cl than ClNO2, contributing 74 % of the Cl rad- change with oxidation, intrinsic and diverse properties of icals produced on a high radiant-flux day. In contrast, on a the chemical species are altered, influencing their toxicity low radiant-flux day, this drops to 14 %, as both Cl2 produc- (Borduas et al., 2015) and their impact on the environment, Published by Copernicus Publications on behalf of the European Geosciences Union. 13482 M. Priestley et al.: Observations of organic and inorganic chlorinated compounds e.g. cloud-particle nucleating efficiency (Ma et al., 2013) or Bannan et al., 2017). These processes may be represented by global warming potential (Boucher et al., 2009). the following equations; The hydroxyl radical (OH) is considered the most impor- tant daytime atmospheric oxidant due to its ubiquity and NO3 C NO −! 2NO2 ; (3) high reactivity, with an average tropospheric concentration C −! −! of 106 molecules cm−3 (Heal et al., 1995). However, rate co- NO3 NO2 N2O5.g/ N2O5.aq/ ; (4) − C −! C − efficients for the reaction of the chlorine radical (Cl) can Cl N2O5 ClNO2 NO3 ; (5) be 2 orders of magnitude larger than those for OH (Spicer JClNO2 et al., 1998), indicating that lower Cl concentrations of ClNO2.aq/ −! ClNO2.g/ −! Cl C NO2: (6) 1 × 104 atoms cm−3 that are estimated to exist in urban areas (e.g. Bannan et al., 2015) can be just as significant in their The anthropogenic emission of molecular chlorine is identi- contribution to oxidation. fied as another inland source of Cl− in the US (e.g. Thornton Cl-initiated oxidation of volatile organic compounds et al., 2010; Riedel et al., 2012) and in China (e.g. Wang et (VOCs) forms chlorinated analogues of the OH-initiated oxi- al., 2017; Liu et al., 2017), where some of the highest concen- dation products, via addition Eq. (1) or hydrogen abstraction trations 3.0–4.7 ppb have been recorded. As well as industrial Eq. (2), forming HCl that may react with OH to regenerate processes, the suspension of road salt used to melt ice on Cl. Subsequent peroxy radicals formed through Cl oxidation roads during the winter has been suggested as a large source − can take part in the HOx cycle and contribute to the enhanced of anthropogenic Cl Mielke et al., 2016). This wintertime- formation of O3 and SOA (Wang and Ruiz, 2017). This is only source, combined with reduced nitrate radical photoly- represented by the following; sis, is expected to yield greater ClNO2 concentrations at this time of the year (Mielke et al., 2016). O R C X −!2 R.X/OO·; (1) The photolysis of molecular chlorine (Cl2) is another po- tential source of Cl. Numerous heterogeneous formation O2 − RH C X −! ROO ·CHX; (2) mechanisms leading to Cl2 from particles containing Cl are known. These include the reaction of Cl− and OH (Vogt et where X is OH or Cl. al., 1996), which may originate from the photolysis of O3.aq/ Nitryl chloride (ClNO2) is a major reservoir of Cl that is (Oum, 1998) or from the reactive uptake of ClNO2 (Leu et produced by aqueous reactions between particulate chloride al., 1995), ClONO2 (Deiber et al., 2004) or HOCl (Eigen and − − (Cl ) and nitrogen pentoxide (N2O5), as seen in Eq. (4) and Kustin, 1962) to acidic Cl containing particles. Thornton et Eq. (6). Gaseous ClNO2 is produced throughout the night al. (2010) also suggest that inorganic Cl reservoirs such as and is typically photolysed at dawn before OH concentra- HOCl and ClONO2 may also enhance the Cl concentration, tions reach their peak, as in Eq. (6). This early morning re- potentially accounting for the shortfall in the global burden −1 −1 lease of Cl induces oxidation earlier in the day and has been (8–22 Tg yr source from ClNO2 and 25–35 Tg yr as cal- shown to increase maximum 8 h mean O3 concentrations by culated from methane isotopes). This may be direct through up to 7 ppb under moderately elevated NOx levels (Sarwar photolysis or indirect through heterogenous reactions with − et al., 2014). Typical ClNO2 concentrations measured in ur- Cl on acidic aerosol. ban regions range from 10 s of ppt to 1000 s of ppt. Mielke et Globally, Cl2 concentrations are highly variable. In the al. (2013) measured a maximum of 3.6 ppb (0.04 Hz) during marine atmosphere, concentrations of up to 35 ppt have been the summertime in Los Angeles, with maximum sunrise con- recorded (Lawler et al., 2011), whereas at urban costal sites centrations of 800 ppt. Bannan et al. (2015) measured a maxi- in the US, concentrations on the order of 100s ppt have been mum concentration of 724 ppt (1 Hz) at an urban background measured (Keene et al., 1993; Spicer et al., 1998). Sampling site in London during summer. They state that in some in- urban outflow, Riedel et al. (2012) measure a maximum of stances, ClNO2 concentrations increase after sunrise and at- 200 ppt Cl2 from plumes and mean concentrations of 10 ppt tribute this to the influx of air masses with higher ClNO2 on a ship in the LA basin. Maximum mixing ratios of up to concentrations by either advection or from the collapse of 65 ppt have also been observed in the continental US (Mielke the residual mixing layer. In urban environments where NOx et al., 2011). − emission and subsequent N2O5 production is likely, Cl may More interestingly, these studies (Keene et al., 1993; be the limiting reagent in the formation of ClNO2 if excess Lawler et al., 2011; Mielke et al., 2011; Spicer et al., 1998) NO does not reduce NO3 as seen in Eq. (3) before N2O5 is report maximum Cl2 concentrations at night and minima produced (e.g. Bannan et al., 2015). Whilst the distance from during the day. However, there is a growing body of evi- − a marine source of Cl may explain low, inland concentra- dence suggesting that daytime Cl2 may also be observed. Al- tions Faxon et al., 2015), the long-range transport of marine though the primary emission may be one source of daytime air can elevate inland ClNO2 concentrations (Phillips et al., Cl2 (Mielke et al., 2011), others demonstrate that the diurnal 2012) and the long-range transport of polluted plumes to a characteristics of the Cl2 time series have a broader signal marine location can also elevate ClNO2 concentrations (e.g.