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

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 Ig 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 9ppt 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% of the Cl radicals produced on a high radiant-flux day. In contrast, on a low radiant-flux day, this drops to 14%, as both Cl2 production and loss pathways are inhibited by reduced photolysis rates. This results in ClNO₂ making up the dominant fraction (83%) 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% of the Cl radical budget observed here.