Thermal evolution of diffusive transport of atmospheric halocarbons through artificial sea–ice

Abstract

Diffusion through brine channels in sea–ice is a potential pathway for trace gases produced under and within sea–ice to exchange with the overlying atmosphere. The effectiveness of this transport pathway is highly dependent on temperature and sea–ice thickness, both of which are changing in favour of increased gas diffusion through porous sea–ice. We conducted several experiments with artificial sea–ice in a cold chamber to assess the potential for dissolved gaseous halocarbons to percolate through brine channels within sea–ice to the overlying air. Physico-chemical properties of the hyper-saline brine, sea–ice and the under-lying seawater were measured to quantify the vertical transport of a comprehensive range of volatile organic iodinated compounds (VOICs), including CH3I, C2H5I, 2-C3H7I and 1-C3H7I, at air temperatures of −3 and −14 °C. We find that the vertical transport of VOICs through sea–ice provides a very small flux pathway for gas transport during periods of consolidated ice cover. The results suggest that VOIC gas transfer velocities from diffusion through the sea–ice alone are at least ∼60 times lower at −3 °C than gas exchange from leads and polynas during the winter (assuming a sea–ice fractional coverage of 0.1). Assuming 100% brine channel fractional connectivity and a diffusion coefficient (D) of 5 × 10−5 cm2 s−1 at −3 °C, the timescale of diffusion through 500 mm of first year sea–ice is ∼145 days. This has significant implications for in-situ VOIC losses within the brine from chlorination, hydrolysis and photolysis processes and it is unlikely that measurable concentrations of VOICs would survive vertical transport from the under-lying seawater to the surface sea–ice quasi-liquid layer.