On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O 3 , HO x , NO x and the Hg lifetime

• Main Authors: Saiz-Lopez, A., Plane, J. M. C., Mahajan, A. S., Anderson, P. S., Bauguitte, S. J.-B., Jones, A. E., Roscoe, H. K., Salmon, R. A., Bloss, W. J., Lee, J. D., Heard, D. E.
• Other Authors: School of Chemistry, Jet Propulsion Laboratory (JPL), NASA-California Institute of Technology (CALTECH), British Antarctic Survey (BAS), Natural Environment Research Council (NERC)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2007
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; Antarctic; Halley Station; The Antarctic; Antarc*; Antarctica
• Online Access: https://hal.science/hal-00302928; https://hal.science/hal-00302928/document; https://hal.science/hal-00302928/file/acpd-7-9385-2007.pdf

International audience 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. 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 ~10 9 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 boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I 2 O 2 , I 2 O 3 , I 2 O 4 and I 2 O 5 ), and rapid recycling of HOI and INO 3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I 2 O 5 in the lowest 10 m of the boundary layer, which could lead to the formation of ultrafine iodine oxide aerosols. 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 O 3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O 3 from the free troposphere is required to account for the observation that on occasion there is little O 3 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 HO and HO 2 and the NO 2 /NO ratio. The average Hg 0 lifetime against oxidation is also predicted to be about 10 h during springtime. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.