Abstract. The activation of reactive halogen species – particularly Cl₂ – 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 Br₂, BrCl and Cl₂ from NaCl/NaBr-doped artificial snow samples. At temperatures above the NaCl-water eutectic, illumination of samples (λ > 310 nm) in the presence of gas phase O₃ led to the accelerated release of Br₂, BrCl and the release of Cl₂ in a process that was significantly enhanced by acidity, high surface area and additional gas phase Br₂. Cl₂ 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 Br₂ 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.