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

摘要

The nitrogen (δ15N) and triple oxygen (δ17O andδ18O) isotopic composition of nitrate (NO₃−) was measuredyear-round in the atmosphere and snow pits at Dome C, Antarctica(DC, 75.1° S, 123.3° E), and in surface snow on a transectbetween DC and the coast. Comparison to the isotopic signal in atmosphericNO₃− shows that snow NO₃− is significantly enriched inδ15N by >200‰ and depleted in δ18O by<40‰. Post-depositional fractionation in Δ17O(NO₃−)is small, potentially allowing reconstruction of past shifts in tropospheric oxidationpathways from ice cores. Assuming a Rayleigh-type process we find fractionationconstants ε of −60±15‰, 8±2‰ and1±1‰, for δ15N, δ18O and Δ17O,respectively. A photolysis model yields an upper limit for the photolyticfractionation constant 15ε of δ15N, consistentwith lab and field measurements, and demonstrates a high sensitivity of15ε to the incident actinic flux spectrum. The photolytic15ε is process-specific and therefore applies to any snowcovered location. Previously published 15ε values arenot representative for conditions at the Earth surface, but apply only to theUV lamp used in the reported experiment (Blunier et al., 2005; Jacobi et al., 2006).Depletion of oxygen stable isotopes is attributed to photolysis followed byisotopic exchange with water and hydroxyl radicals. Conversely, 15Nenrichment of the NO₃− fraction in the snow implies 15N depletionof emissions. Indeed, δ15N in atmospheric NO₃− shows a strongdecrease from background levels (4±7‰) to −35‰in spring followed by recovery during summer, consistent withsignificant snowpack emissions of reactive nitrogen. Field andlab evidence therefore suggest that photolysis is an important processdriving fractionation and associated NO₃− loss from snow.The Δ17O signature confirms previous coastal measurements thatthe peak of atmospheric NO₃− in spring is of stratospheric origin.After sunrise photolysis drives then redistribution of NO₃− from thesnowpack photic zone to the atmosphere and a snow surface skin layer,thereby concentrating NO₃− at the surface. Little NO₃− appears tobe exported off the EAIS plateau, still snow emissions from as far as 600 kminland can contribute to the coastal NO₃− budget.