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

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JournalAtmospheric Environment
DatePublished - 1 Nov 2011
Issue number35
Volume45
Number of pages10
Pages (from-to)6393-6402
Original languageEnglish

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 CH I, C H I, 2-C H I and 1-C H I, 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 cm s at -3°C, the timescale of diffusion through 500mm 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.

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