Development, Validation, and Application of Methods for High Time-Response Measurement of Gaseous Atmospheric Chlorinated Species

Halogenated compounds that participate in catalytic cycles in the atmosphere can influence the fate of chemicals, including ozone, methane, and volatile organic compounds (VOCs). These halogen radicals, in particular atomic chlorine (Cl), can deplete ozone and will react rapidly with VOCs. Reliable, sensitive, and widely available hydrogen chloride (HCl) measurements are important for understanding Cl initiated oxidation in many regions of the troposphere. We configured a commercial HCl cavity ring-down spectrometer (CRDS) for sampling HCl in the ambient atmosphere and developed validation techniques to characterize the measurement uncertainties. The HCl analyzer was used to make continuous HCl measurements in the polluted marine boundary layer during the Halifax Fog and Air Quality Study (HaliFAQS). Bimodal HCl features in the high irradiance days indicated two photochemical processes; (1) morning time photolysis of Cl precursors, and (2) midday formation of nitric acid followed by acid displacement onto chloride (Cl) containing aerosols. A box model used measured HCl to estimate nitryl chloride mixing ratios at sunrise and assessed the contribution of photolabile Cl precursors to radical formation. Total gaseous chlorine (TClg) measurements can illuminate unknown sources of Cl to the atmosphere. Techniques for measuring TClg have been limited to offline analysis of extracted filters and do not provide suitable temporal information on fast atmospheric process. The utility of this novel TClg measurement technique will be crucial to future estimates and assessments of chlorinated compounds and their impact on air quality, climate, and health.