Photochemical chlorine and bromine activation from artificial saline snow

The activation of reactive halogen species - particularly Cl-2 - from sea ice and snow surfaces is not well understood. In this study, we used a photochemical snow reactor coupled to a chemical ionization mass spectrometer to investigate the production of Br-2, BrCl and Cl-2 from NaCl/NaBrdoped artificial snow samples. At temperatures above the NaCl-water eutectic, illumination of samples (lambda > 310 nm) in the presence of gas phase O-3 led to the accelerated release of Br-2, BrCl and the release of Cl-2 in a process that was significantly enhanced by acidity, high surface area and additional gas phase Br-2. Cl-2 production was only observed when both light and ozone were present. The total halogen release depended on [ozone] and pre-freezing [NaCl]. Our observations support a halogen explosion mechanism occurring within the snowpack, which is initiated by heterogeneous oxidation and propagated by Br-2 or BrCl photolysis and by recycling of HOBr and HOCl into the snowpack. Our study implicates this important role of active chemistry occurring within the interstitial air of aged (i.e. acidic) snow for halogen activation at polar sunrise.