Natural and manmade processes emit huge quantities of gases. Were it not for oxidation chemistry occurring in the atmosphere, such emissions would build up to harmful concentrations. The emitted gases and their oxidation products affect atmospheric composition, air quality, environmental and public health, and influence climate.  The removal of most trace gases from the atmosphere is initiated by molecules reacting with OH radicals, NO3 radicals, or with ozone [e.g. 2]. Recently however, chlorine atoms (Cl) have been recognised as another important oxidant [3,4,5]. Chlorine atoms are extremely reactive: this means that even small levels of atomic chlorine can enhance atmospheric oxidation rates, and thereby also promote formation of tropospheric ozone and other secondary air pollutants.
Chlorine is activated in a two step process. This starts with the night-time reaction of N2O5 with chloride from particles of sea salt:
N2O5 + NaCl ClNO2 + HNO3
Photolysis of ClNO2 the next morning produces Cl atoms:
ClNO2 + light Cl + NO2
Note how the first step involves the reaction of a natural component of the atmosphere (sea salt aerosol) with N2O5, a species derived from primarily manmade emissions of nitrogen oxides (NOx).
Early studies conducted in the USA focussed on coastal regions, the assumption being that ClNO2 production would be limited by the availability of sea salt. However, later studies observed ClNO2 at inland locations 1000 km or more from the sea [3,4]. Our group has observed ClNO2 at two UK coastal sites and on almost every night when we made measurements in Leicester in central England [5,6]. Thus ClNO2 is widespread in the UK too. However its effects are not well understood. Since we inhabit a (somewhat) polluted island, ClNO2 chemistry could be especially important for the United Kingdom.
 European Environment Agency (2018), “Air quality in Europe – 2018 report”, https://www.eea.europa.eu//publications/air-quality-in-europe-2018
(Accessed: 29 Oct 2018).
 Ball, S. M. (2014), “Atmospheric chemistry at night”, http://www.rsc.org/images/environmental-brief-no-3-2014_tcm18-237724.pdf
(Accessed 29 Oct 2018).
 Thornton, J. A. et al (2010), “A large atomic chlorine source inferred from mid-continental reactive nitrogen chemistry”, Nature, 464, p271.
 Phillips, G. J. et al (2012), “Significant concentrations of nitryl chloride observed in rural continental Europe associated with the influence of sea salt chloride and anthropogenic emissions, Geophys. Res. Lett., 39, L10811, doi:10.1029/2012GL051912.
 Sherwen, T. et al (2017), “Effects of halogens on European air-quality”, Faraday Discussions, 200, p75, doi: 10.1039/C7FD00026J. http://pubs.rsc.org/en/content/articlelanding/2017/fd/c7fd00026j
 Sommariva, R. et al (2018), “Seasonal and geographical variability of nitryl chloride and its precursors in Northern Europe”, Atmospheric Science Letters, 19(8), e844, doi: 10.1002/asl.844 https://rmets.onlinelibrary.wiley.com/doi/10.1002/asl.844
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