Measurement and modelling of UV radiation penetration and photolysis rates of nitrate and hydrogen peroxide in Antarctic sea ice: An estimate of the production rate of hydroxyl radicals in first-year sea ice

Abstract

Sea ice may be an oxidising medium owing to sunlight-driven reactions occurring within the ice. UV light transmission and albedo (320–450 nm) are reported for first-year sea ice in Terra Nova Bay, Antarctica, in conjunction with depth integrated photolysis rates for OH radical production from photolysis of hydrogen peroxide (H 2O 2) and nitrate anion (NO 3 −). The albedo is 0.70–0.75 and the transmission is characterised by an e-folding depth of ∼50 cm or an extinction coefficient of ∼2 m −1. A coupled atmosphere-snowpack radiation-transfer model (TUV-snow) was applied to the experimental measurements so that scattering and absorption cross-sections of the ice could be deduced. These cross-sections were used to model actinic flux (spherical irradiance) profiles in the ice, and thus illustrate the enhancement of actinic flux around a depth of 10 cm in the sea ice for zenith angles smaller than 50°. The actinic flux-depth profiles demonstrate how extinction coefficients (measured at solar zenith angles greater than 50°) for the top 10–20 cm of sea ice are much larger than extinction coefficients measured deeper in the ice. The TUV model was also used to calculate photolysis frequencies for nitrate anions and hydrogen peroxide, which produce hydroxyl radicals within the sea ice. The depth integrated photolysis rate of hydrogen peroxide is an order of magnitude larger than the depth integrated photolysis rate of nitrate. However, the low concentrations of hydrogen peroxide in sea water and ice relative to nitrate result in a higher rate of production of OH radicals for nitrate than hydrogen peroxide. Approximate upper limits for depth integrated rate of production OH from nitrate photolysis in a 1 m deep sea ice block were found to be 0.06–2 μmol m −2 h −1 for solar zenith angles of 85–45°, respectively. The depth integrated production rate of OH radical from hydrogen peroxide photolysis is 0.01–0.3 μmol m −2 h −1 for solar zenith angles of 85–45°, respectively. Photolysis of nitrate in ice is an efficient route to the production of hydroxyl radicals and an oxidising ice. It must be stressed that these depth integrated rates of production are, however, approximate upper limits because the low porosity and the microphysical, chemical and photochemical process occurring in sea ice may reduce the depth integrated rate of production. In conclusion we suggest that first-year sea ice may be an efficient medium for photochemistry and that 85% of ice photochemistry may occur in the top 1 m of sea ice, assuming that the concentration of chromophore (nitrate or hydrogen peroxide) and photochemical efficiency are independent of depth in the sea ice.