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A one-dimensional chemical transport model has been developed to investigatethe vertical gradients of bromine and iodine compounds in the Antarcticcoastal boundary layer (BL). The model has been applied to interpret recentyear-round observations of iodine and bromine monoxides (IO and BrO) atHalley Station, Antarctica. The model requires an equivalent I atom flux of~1010 molecule cm−2 s−1 from the snowpack in order toaccount for the measured IO levels, which are up to 20 ppt during spring.Using the current knowledge of gas-phase iodine chemistry, the modelpredicts significant gradients in the vertical distribution of iodinespecies. However, recent ground-based and satellite observations of IO implythat the radical is well-mixed in the Antarctic boundary layer, indicating alonger than expected atmospheric lifetime for the radical. This can bemodelled by including photolysis of the higher iodine oxides(I2O2, I2O3, I2O4 and I2O5), andrapid recycling of HOI and INO3 through sea-salt aerosol. The modelalso predicts significant concentrations (up to 25 ppt) of I2O5in the lowest 10 m of the boundary layer. Heterogeneous chemistryinvolving sea-salt aerosol is also necessary to account for the verticalprofile of BrO. Iodine chemistry causes a large increase (typically morethan 3-fold) in the rate of O3 depletion in the BL, compared withbromine chemistry alone. Rapid entrainment of O3 from the freetroposphere appears to be required to account for the observation that onoccasion there is little O3 depletion at the surface in the presence ofhigh concentrations of IO and BrO. The halogens also cause significantchanges to the vertical profiles of OH and HO2 and the NO2/NOratio. The average Hg0 lifetime against oxidation is also predicted tobe about 10 h during springtime. An important result from the model isthat very large fluxes of iodine precursors into the boundary layer arerequired to account for the observed levels of IO. The mechanisms whichcause these emissions are unknown. Overall, our results show that halogensprofoundly influence the oxidizing capacity of the Antarctic troposphere

atmospheric chemistry and physics

On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, HO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;, NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; and the Hg lifetime