The Determination of ClNO₂ via Thermal Dissociation-Tunable Infrared Laser Direct Absorption Spectroscopy

Abstract. Nitryl chloride (ClNO₂) is a reservoir species of chlorine atoms and nitrogen oxides, both of which play important roles in atmospheric chemistry. To date, all ambient ClNO₂ observations have been obtained by chemical ionization mass spectrometry (CIMS). In this work, Thermal Dissociation Tunable Infrared Laser Differential Absorption Spectrometer (TD-TILDAS) is shown to be a viable method for quantifying ClNO₂ in laboratory and field settings. This technique relies on the thermal dissociation of ClNO₂ to create chlorine radicals, which undergo fast reactions with hydrocarbons to produce hydrogen chloride (HCl) that is detectable by the TILDAS instrument. Complete quantitative conversion of ClNO₂ to HCl was achieved at temperatures > 400 °C, achieving 1 Hz measurement precision of 11 ± 1 pptv (3σ limits of detection of 34 ± 2 pptv) during laboratory comparisons with other ClNO₂ detection methods. After blank- and line loss-corrections, method accuracy is estimated to be within ± 5 %. Performance metrics of TD-TILDAS during ambient sampling were a 1 Hz precision of 19 ± 1 pptv and 3σ limits of detection of 57 ± 3 pptv), which is directly comparable to previously reported ClNO₂ detection by quadrupole CIMS. Thus, TD-TILDAS can provide an alternative analytical approach for a direct measurement of ClNO₂ that can complement existing datasets and future studies. The quantitative nature of TD-TILDAS also makes it a potentially useful tool for the calibration of CIMS instruments. However, interpretation of ambient data may be potentially complicated by interference from unaccounted-for sources of thermolabile chlorine.