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Daytime atmospheric oxidation chemistry is conventionally considered to be driven primarily by the OH radical, formed via photolytic sources. In this paper we examine how, during winter when photolytic processes are slow, chlorine chemistry can have a significant impact on oxidative processes in the urban boundary layer. Photolysis of nitryl chloride (ClNO 2 ) provides a significant source of chlorine atoms, which enhances the oxidation of volatile organic compounds (VOCs) and the production of atmospheric pollutants. We present a set of observations of ClNO 2 and HONO made at urban locations in central England in December 2014 and February 2016. While direct emissions and in-situ chemical formation of HONO continue throughout the day, ClNO 2 is only formed at night and is usually completely photolyzed by midday. Our data show that, during winter, ClNO 2 often persists through the daylight hours at mixing ratios above 10-20 ppt (on average). In addition, relatively high mixing ratios of daytime HONO (>65 ppt) provide a strong source of OH radicals throughout the day. The combined effects of ClNO 2 and HONO result in sustained sources of Cl and OH radicals from sunrise to sunset, which form additional ozone, PAN, oxygenated VOCs, and secondary organic aerosol. We show that radical sources such as ClNO 2 and HONO can lead to a surprisingly photoactive urban atmosphere during winter and should therefore be included in atmospheric chemical models., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)