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

Abstract. Here we report the measurement of the comprehensive isotopic composition (δ¹⁵N, Δ¹⁷O and δ¹⁸O) of nitrate at the air–snow interface at Dome C, Antarctica (DC, 75°06' S, 123°19' E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66° S and 78° 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) ‰, (+2.0±1.0) ‰ and (+8.7±2.4)‰ for δ¹⁵N, Δ¹⁷O and δ¹⁸O, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (−16±14) ‰, (−0.2±1.5) ‰ and (+3.1±5.8) ‰, for δ¹⁵N, Δ¹⁷O and δ¹⁸O, respectively. Our observations corroborate that photolysis (associated with a ¹⁵N / ¹⁴N fractionation constant of the order of –48 ‰ 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 Δ¹⁷O of atmospheric nitrate and Δ¹⁷O 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 ‰ between the δ¹⁵N values in the atmospheric and skin layer nitrate at DC. Asymptotic δ¹⁵N(NO₃⁻) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(δ¹⁵N as. + 1) = (5.76±0.47) ċ (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 Δ¹⁷O(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 Δ¹⁷O is small, thus allowing the preservation of a portion of the atmospheric signal.