On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O₃, HO_x, NO_x and the Hg lifetime

Abstract. A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer (BL). The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of ~10¹⁰ molecule cm⁻² s⁻¹ from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the Antarctic boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I₂O₂, I₂O₃, I₂O₄ and I₂O₅), and rapid recycling of HOI and INO₃ through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I₂O₅ in the lowest 10 m of the boundary layer. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O₃ depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O₃ from the free troposphere appears to be required to account for the observation that on occasion there is little O₃ depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of OH and HO₂ and the NO₂/NO ratio. The average Hg⁰ lifetime against oxidation is also predicted to be about 10 h during springtime. An important result from the model is that very large fluxes of iodine precursors into the boundary layer are required to account for the observed levels of IO. The mechanisms which cause these emissions are unknown. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.