Abstract
Elevated levels of reactive bromine and chlorine species in the springtime Arctic boundary layer contribute to ozone depletion and mercury oxidation, as well as reactions with volatile organic compounds. Recent laboratory and field studies have revealed that snowpack photochemistry leads to Br-2 and Cl-2 production, the mechanisms of which remain poorly understood. In this work, we use a photochemical box model, with a simplified snow module, to examine the halogen chemistry occurring during the March 2012 Bromine, Ozone, and Mercury Experiment (BROMEX) near Utqiagvik (Barrow), Alaska. Elevated daytime Br-2 levels (e.g., 6-30 parts per trillion (ppt) at around local noon) reported in previous studies and in this work may be explained by Br + BrNO2/BrONO2 reactions under conditions of depleted O-3 (<similar to 10 ppb) and background NO2 (10-100 ppt). Even at low background NOx levels at Utqiagvik, ClONO2 is predicted to be important in the production of Cl-2 via multiphase reaction with Cl-. In the late afternoon, photolysis alone cannot explain the rapid decrease of Cl-2 observed in the Arctic boundary layer. Heterogeneous reactions of Cl-2 on aerosol particles and surface snowpack are suggested to play a key role in atmospheric Cl-2 removal and possible BrCl production. Given the importance of the snowpack in the multiphase chemistry of the Arctic boundary layer, future measurements should focus on vertically resolved measurements of NOx and reactive halogens, as well as simultaneous particulate and snow halide measurements, to further evaluate and isolate the halogen production and vertical propagation mechanisms through one-dimensional modeling.