Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

Abstract

The nitrogen (δ¹⁵N) and triple oxygen (δ¹⁷O and δ¹⁸O) isotopic composition of nitrate (NO₃^−) was measured year-round in the atmosphere and snow pits at Dome C, Antarctica (DC, 75.1° S, 123.3° E), and in surface snow on a transect between DC and the coast. Comparison to the isotopic signal in atmospheric NO₃^− shows that snow NO₃^− is significantly enriched in δ¹⁵N by >200‰ and depleted in δ¹⁸O by <40‰. Post-depositional fractionation in Δ¹⁷O(NO₃^−) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants ε of −60±15‰, 8±2‰ and 1±1‰, for δ¹⁵N, δ¹⁸O and Δ¹⁷O, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant ¹⁵ε of δ¹⁵N, consistent with lab and field measurements, and demonstrates a high sensitivity of ¹⁵ε to the incident actinic flux spectrum. The photolytic ¹⁵ε is process-specific and therefore applies to any snow covered location. Previously published ¹⁵ε values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005; Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, ¹⁵N enrichment of the NO₃^− fraction in the snow implies ¹⁵N depletion of emissions. Indeed, δ¹⁵N in atmospheric NO₃^− shows a strong decrease from background levels (4±7‰) to −35‰ in spring followed by recovery during summer, consistent with significant snowpack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis is an important process driving fractionation and associated NO₃^− loss from snow. The Δ¹⁷O signature confirms previous coastal measurements that the peak of atmospheric NO₃^− in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO₃^− from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO₃^− at the surface. Little NO₃^− appears to be exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO₃^− budget.