Observation of CINO2 in a Mid-Continental Urban Environment

In the troposphere, nitryl chloride (ClNO2), produced from uptake of dinitrogen pentoxide (N2O5) on chloride containing aerosol, can be an important nocturnal reservoir of NOx (= NO + NO2) and a source of atomic Cl, particularly in polluted coastal environments. Here, we present measurements of ClNO2 mixing ratios by chemical ionization mass spectrometry (CIMS) in Calgary, Alberta, Canada over a 3-day period. The observed ClNO2 mixing ratios exhibited a strong diurnal profile, with nocturnal maxima in the range of 80 to 250 parts-per-trillion by volume (pptv) and minima below the detection limit of 5 pptv in the early afternoon. At night, ClNO2 constituted up to 2% of odd nitrogen, or NOy. The peak mixing ratios of ClNO2 observed were considerably below the modeled integrated heterogeneous losses of N2O5, indicating that ClNO2 production was a minor pathway compared to heterogeneous hydrolysis of N2O5. Back trajectory analysis using HYSPLIT showed that the study region was not influenced by fresh injections of marine aerosol, implying that aerosol chloride was derived from anthropogenic sources. Molecular chlorine (Cl-2), derived from local anthropogenic sources, was observed at mixing ratios of up to 65 pptv and possibly contributed to formation of aerosol chloride and hence the formation of ClNO2.