Air-snow transfer of nitrate on the East Antarctic Plateau - Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer

作者：Erbland J^*; Vicars W C; Savarino J; Morin S; Frey M M; Frosini D; Vince E; Martins J M F

来源：Atmospheric Chemistry and Physics, 2013, 13(13): 6403-6419.

DOI：10.5194/acp-13-6403-2013

Here we report the measurement of the comprehensive isotopic composition (delta N-15, Delta O-17 and delta O-18) of nitrate at the air-snow interface at Dome C, Antarctica (DC, 75 degrees 06%26apos;S, 123 degrees 19%26apos;E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66 degrees S and 78 degrees S. In most of the snow pits, nitrate loss (either by physical release or UV photolysis of nitrate) is observed and fractionation constants associated are calculated. Nitrate collected from snow pits on the plateau (snow accumulation rate below 50 kg m(-2)a(-1)) displays average fractionation constants of (-59 +/- 10) parts per thousand, (+2.0 +/- 1.0)parts per thousand and (+8.7 +/- 2.4)parts per thousand for delta N-15, Delta O-17 and delta O-18, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (-16 +/- 14) parts per thousand, (-0.2 +/- 1.5)parts per thousand and (+3.1 +/- 5.8) parts per thousand, for delta N-15, Delta O-17 and delta O-18, respectively. Our observations corroborate that photolysis (associated with a N-15/N-14 fractionation constant of the order of -48 parts per thousand according to Frey et al. (2009)) is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a close relationship between the Delta O-17 of atmospheric nitrate and Delta O-17 of nitrate in skin layer snow, suggesting a photolytically driven isotopic equilibrium imposed by nitrate recycling at this interface. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift of the order of 25 parts per thousand between the delta N-15 values in the atmospheric and skin layer nitrate at DC. Asymptotic delta N-15(NO3-) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(delta N-15(as.) + 1) = (5.76 +/- 0.47) . (kg m(-2) a(-1)/A)+(0.01 +/- 0.02)), confirming the strong relationship between the snow accumulation rate and the degree of isotopic fractionation, consistent with previous observations by Freyer et al. (1996). Asymptotic Delta O-17(NO3-) values on the plateau are smaller than the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photoproducts and water molecules from the surrounding ice. However, the apparent fractionation in Delta O-17 is small, thus allowing the preservation of a portion of the atmospheric signal.

• 出版日期2013
• 单位British Antarctic Survey