Spectroscopic studies of the heterogeneous reaction between O ₃(g) and halides at the surface of frozen salt solutions

Springtime ozone depletion events have been linked to elevated levels of bromine in the polar boundary layer. It has also been suggested that iodine may play an important role in boundary layer chemistry. However, the mechanism(s) for initial halogen activation is not well understood. We report results of experiments using glancing-angle laser-induced fluorescence (LIF) and glancing-angle Raman spectroscopy to investigate the heterogeneous reaction of gas-phase ozone with bromide and iodide ions at the surface of frozen salt solutions. The results suggest that halides that have been excluded to the ice surface during the freezing process react in the dark and under non-acidified conditions to produce molecular halogen species. At environmentally relevant bromide concentrations, the reaction proceeds ∼60 times more rapidly at ice surfaces than at the surface of aqueous solutions. At an ozone concentration of ∼8 × 10¹⁵ molec cm^-3, and sea water concentrations of bromide and iodide, we estimate uptake coefficients γ(Br^-) = (1.3 ± 0.5) × 10^-8 and γ(I^-) = (1.6 ± 0.5) × 10^-9. We estimate that at atmospheric ozone concentrations, the uptake coefficients will be approximately a factor of ten greater.