Ubiquity of ClNO₂ in the urban boundary layer of Calgary, Alberta, Canada

The role of nitryl chloride (ClNO₂) as a nocturnal nitrogen oxide reservoir species and chlorine atom precursor is well established for polluted coastal areas, but its role at midcontinental locations is less clear. In this paper, intermittent measurements over the course of several seasons of ClNO₂ mixing ratios by iodide ion chemical ionization mass spectrometry in Calgary, Alberta, Canada, are presented. Mixing ratios were highly variable between nights and seasons and depended on the abundances of precursors and meteorological conditions. The lowest ClNO₂ mixing ratios (nocturnal maximum of 30 parts per trillion by volume (pptv)) were observed in the summer, rationalized by losses of the nitrate radical (NO₃) that were more efficient than in the other months. Higher ClNO₂ mixing ratios (up to 330 pptv) were observed in the winter and spring months but varied between nights. In the fall, ClNO₂ mixing ratios increased from night to night following the application of salt to roads. The ClNO₂ yield relative to the amount of NO₃ produced from oxidation of NO₂ by O₃ ranged from 0.1% to 4.5% (10th and 90th percentiles, median 1.0%). The ClNO₂ yield relative to N₂O₅ consumed by heterogeneous reactions was estimated using a time-integrated box model and ranged from 0.5% to 12.1% (10th and 90th percentiles, median 3.4%). The ubiquity of ClNO₂ implies that the chlorine atom needs to be considered as an oxidant in high-latitude urban environments in winter. (author)