Reactive Chlorine in the Polluted Marine Boundary Layer during the Halifax Fog and Air Quality Study (HaliFAQS)

Abstract

Chlorine atom (Cl) initiated oxidation of volatile organic compounds (VOCs) yields hydrogen chloride (HCl) that can be detected using a high time-resolution cavity ringdown spectrometer (CRDS). We present continuous HCl measurements in the polluted marine boundary layer during the Halifax Fog and Air Quality Study (HaliFAQS) and demonstrate the efficacy of high time resolution measurements as capable of tracing photochemical Cl formation. Throughout the campaign HCl accumulated at a faster rate alongside high irradiance, particularly in the early morning. Bimodal HCl features were observed on the high irradiance days, consistent with two known photochemical processes: (1) morning photolysis of Cl precursors, and (2) midday photochemical nitric acid (HNO₃) displacing HCl from chloride (Cl⁻) containing aerosols. Morning HCl rates of increase and irradiance were used to estimate nitryl chloride (ClNO₂) present at sunrise. We modeled HCl for eight additional, individual days from the campaign based on this estimated ClNO₂. The modelled HCl agrees well for six high irradiance days. It captures the features of the morning mode within the combined uncertainty of the estimated ClNO₂ and a range of dry deposition rates from the literature. Production of Cl from ClNO2₂ photolysis (P(Cl)_ClNO2) to the production of hydroxyl radical (OH) from the photolysis of measured ozone (P(HOx)_O3) was compared and P(Cl)_ClNO2 was found to be 13% of P(HOx)_O3 when considering the full day, and 37% in the morning. Highly time resolved HCl observations can therefore be used to estimate nighttime ClNO₂ and its influence on Cl morning tropospheric oxidation chemistry in polluted coastal atmospheres.