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

Abstract : The nitrogen ( 15N) and triple oxygen ( 17/18O) isotopic composition of nitrate (NO−3 ) was measured year-round in the atmosphere and snow pits at Dome C (DC, 75.1 S, 123.3 E), and in surface snow on a transect between DC and the coast. Snow pit profiles of 15N ( 18O) in NO−3 5 show significant enrichment (depletion) of >200 (<40) ‰ compared to the isotopic signal in atmospheric NO−3 , whereas post-depositional fractionation in É17O(NO−3 ) is small, allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find in the DC04 (DC07) pit fractionation factors " of −50±10 (−71±12) ‰, 6±3 (9±2) ‰ and 10 1±0.2 (2±0.6) ‰, for 15N, 18O and É17O, respectively. A photolysis model reproduces " for 15N within the range of uncertainty at DC and for lab experiments reported by Blunier et al. (2005), suggesting that the current literature value for photolytic isotopic fractionation in snow is significantly underestimated. Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15N enrichment of the NO−3 15 fraction in the snow implies 15N depletion of emissions. Indeed, 15N in atmospheric NO−3 shows a strong decrease from background levels (4.4±6.8‰) to −35.1‰ in spring followed by recovery during summer, consistent with significant snow pack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis dominates fractionation and associated NO−3 20 loss from snow in the low-accumulation regions of the East Antarctic Ice Sheet (EAIS). The É17O signature confirms previous coastal measurements that the peak of atmospheric NO−3 in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO−3 from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO−3 at the surface. Little NO−3 is 25 exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO−3 budget.
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M.M. Frey, J. Savarino, S. Morin, J. Erbland, J.M. F. Martins. Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling. Atmospheric Chemistry and Physics, European Geosciences Union, 2009, 9 (22), pp.8681-8696. ⟨10.5194/acp-9-8681-2009⟩. ⟨insu-00412552⟩

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