Observations of organic and inorganic chlorinated compounds and their contribution to chlorine radical concentrations in an urban environment in northern Europe during the wintertime

• Main Authors: M. Priestley, M. le Breton, T. J. Bannan, S. D. Worrall, A. Bacak, A. R. D. Smedley, E. Reyes-Villegas, A. Mehra, J. Allan, A. R. Webb, D. E. Shallcross, H. Coe, C. J. Percival
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/13481/2018/acp-18-13481-2018.pdf
• ISSN: 1680-7316; 1680-7324

<p>A number of inorganic (nitryl chloride, ClNO₂; chlorine, Cl₂; and hypochlorous acid, HOCl) and chlorinated, oxygenated volatile organic compounds (ClOVOCs) have been measured in Manchester, UK during October and November 2014 using time-of-flight chemical ionisation mass spectrometry (ToF-CIMS) with the I⁻ reagent ion. ClOVOCs appear to be mostly photochemical in origin, although direct emission from vehicles is also suggested. Peak concentrations of ClNO₂, Cl₂ and HOCl reach 506, 16 and 9&thinsp;ppt respectively. The concentrations of ClNO₂ are comparable to measurements made in London, but measurements of ClOVOCs, Cl₂ and HOCl by this method are the first reported in the UK. Maximum HOCl and Cl₂ concentrations are found during the day and ClNO₂ concentrations remain elevated into the afternoon if photolysis rates are low. Cl₂ exhibits a strong dependency on shortwave radiation, further adding to the growing body of evidence that it is a product of secondary chemistry. However, night-time emission is also observed. The contribution of ClNO₂, Cl₂ and ClOVOCs to the chlorine radical budget suggests that Cl₂ can be a greater source of Cl than ClNO₂, contributing 74&thinsp;% of the Cl radicals produced on a high radiant-flux day. In contrast, on a low radiant-flux day, this drops to 14&thinsp;%, as both Cl₂ production and loss pathways are inhibited by reduced photolysis rates. This results in ClNO₂ making up the dominant fraction (83&thinsp;%) on low radiant-flux days, as its concentrations are still high. As most ClOVOCs appear to be formed photochemically, they exhibit a similar dependence on photolysis, contributing 3&thinsp;% of the Cl radical budget observed here.</p>