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

Abstract

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.

Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

“…Model calculations are constrained and validated with existing observations of atmospheric NO (Erbland et al, 2013). The diurnal temperature variation is ∼ 10 K in summer, spring (midSeptember until mid-November) and autumn (February to March).…”

“…The relative contribution of photochemical and physical processes has been a matter of debate (Röthlisberger et al, 2000). Isotopic studies have shown that photolysis of NO − 3 is the dominating loss process of NO − 3 in snow (Frey et al, 2009;Erbland et al, 2013). Based on a typical photolysis rate coefficient of nitrate, J NO − 3 ≈ 1 × 10 −7 s −1 (at the surface in Dome C at a solar zenith angle of 52 • , France et al, 2011), the characteristic time for nitrate photolysis is ∼ 10 7 s. Thus, the characteristic time of nitrate photolysis is much larger compared to other physical processes near the snowpack surface, such as grain surface adsorption and solid-state diffusion (Table 1).…”

Surface Snow, Firn and Ice Core Composition in Polar Areas in Relation to Atmospheric Aerosol and Gas Concentrations: Critical Aspects

“…The relative importance of the photochemical and physical loss processes at a given site is a key issue to improve the interpretation of the nitrate ice core and determine the nitrate transfer function at the air-snow interface. This requires a knowledge of the nitrogen isotopic fractionation constant associated with each loss process [111].…”

“…At Dome C, photolysis should dominate nitrate loss [111] [121]. Field studies in the Antarctic as well as laboratory experiments have shown that the photochemical reduction of nitrates takes place in the superficial liquid-like layer of snow [126] [127].…”

“…During a campaign carried out at Dome C (January 2009-January 2010), Erbland et al [111] compared the particulate nitrate and gaseous HNO 3 in the atmosphere with the nitrate concentration of surface snow samples (referred to as the "skin layer") collected every three days. 3 …”