Freezing halide ion solutions and the release of interhalogens to the atmosphere

The effect of freezing on a variety of acidified and neutral, nitrite ion and halide-containing mixtures has been investigated using UV/vis spectroscopy. Several trihalide ions were formed and monitored, including I₂Cl ^-, I₂Br^-, ICI₂^- and IBr₂^-. A mechanism to explain the observations is given in terms of steps involving INO and the nitroacidium ion, [H₂ONO] ⁺. The transformation of sea salt components to specific trihalide ions by freezing represents a potentially important process in a polar atmospheric context. This is because the dichloro- and dibromo-trihalide ions can release chlorine- and bromine-containing gases, which are key intermediates in ozone destruction.