Assessment of nitryl chloride as a missing oxidant in the UK atmosphere

Nitryl chloride (ClNO2) is potentially an important contributor to the troposphere’s oxidising capacity in the UK and European context. A lack of observational evidence means the role of ClNO2 is poorly understood, however, with only two prior publications of European ClNO2 observations – one in the UK and one in Germany. This thesis greatly extends the UK dataset with five sets of ClNO2 measurements made on Leicester University’s campus and two datasets from coastal locations (Weybourne in North Norfolk and Penlee Point near Plymouth). ClNO2 forms at night from the heterogeneous reaction of dinitrogen pentoxide (N2O5), derived from man-made NOx emissions, with chloride-containing aerosol particles (sea spray). ClNO2 photolysis subsequently releases highly reactive chlorine atoms which oxidise volatile organic compounds, enhancing tropospheric ozone production and its associated effects on air quality and climate. A chemical ionisation mass spectrometer operating with iodide reagent ions was used for work in this thesis. This instrument was calibrated for ClNO2 via broadband cavity-enhanced absorption spectroscopy and for Cl2 via reference standard. Typical sensitivities of 6-13 Hz/pptv were achieved for both ClNO2 and Cl2. The highest ambient ClNO2 concentration observed in this work was 1104 pptv at the Weybourne coastal site, attributed to air from the London outflow. For all sites, air mass back-trajectories showed that high ClNO2 concentrations tended to correlate with chemically-aged air masses that had spent considerable time over land. Indeed ClNO2 was observed almost every night at Leicester, and sea salt was found to be the chloride source even at this inland location. N2O5 availability limited ClNO2 production at both coastal sites. Nitryl chloride displayed strong seasonality. Repeat measurements at Leicester showed ClNO2 was highest in winter and lowest in summer. ClNO2 was also observed to persist throughout the day on several days in December 2014 and February 2016.