Bromine Chloride in the Coastal Arctic: Diel Patterns and Production Mechanisms

Bromine and chlorine chemistry in the Arctic atmospheric boundary layer has significant impacts on tropospheric ozone depletion and the fates of atmospheric pollutants, including mercury and hydrocarbons. Bromine chloride (BrCl) produces bromine and chlorine radicals upon photolysis and links these two halogen cycles. However, because of the limited number of BrCl measurements, the relative importance of its production and removal pathways are uncertain. Here we report BrCl observations near Utqiaġvik, AK, during March-May 2016 using chemical ionization mass spectrometry as part of the Photochemical Halogen and Ozone Experiment: Mass Exchange in the Lower Troposphere (PHOXMELT). Two distinct BrCl diel regimes were identified, with daytime BrCl primarily observed in March and nighttime BrCl observed in April and May, demonstrating a dependence on photochemistry. The dominant BrCl production mechanisms for these regimes were explored using a zero-dimensional numerical model constrained to a suite of halogen measurements. Multiphase reactions on the snowpack surface, mainly via Cl₂ + Br^-(aq) and HOBr + Cl^-(aq), are predicted to be the largest contributors to near-surface BrCl production. Average net snowpack fluxes of 1.9 × 10⁸ and 2.2 × 10⁸ BrCl molecules cm^-2 s^-1 for two case periods in March and May are needed to explain the observations. The findings in this work highlight coupled bromine and chlorine chemistry and important halogen activation pathways in the springtime Arctic boundary layer.