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Abstract: We present surface layer measurements made over Hudson Bay sea ice during February/March 2008 from the COBRA (Impact of combined iodine and bromine release on the Arctic atmosphere) experiment which formed part of the International OASIS (Ocean-Atmosphere-Sea Ice-Snowpack) IPY programme. All components of the local surface energy balance were measured and it was defined by net radiative cooling throughout most of the day, mainly balanced by the conductive heat flux from the warmer sea water to the cooler sea ice at the surface, and a small net radiative warming for a few hours after midday. Unique ground-level ozone fluxes were measured by eddy covariance and deposition velocities ranged from +0.5 mm s−1 (deposition) to −1.5 mm s−1 (emission). Ozone profile measurements suggested ozone flux divergence within the surface layer. The observed bi-directional fluxes and flux divergence with height reveal the complexity of surface ozone fluxes in the Arctic spring time surface layer, and show that ozone exchange with the sea ice surface is best probed using the eddy covariance method alongside frequent or continuous profile measurements. In this study, the local in-situ ozone-halogen photochemistry was identified as weakly controlling the measured ozone flux, whereas horizontal advection and vertical mixing were considered more important in influencing fluxes. Under these conditions, several measurement sites would be desirable in order to quantify the contribution of advection to the local surface exchange. [Copyright &y& Elsevier]