Isotope Fractionation of Nitrate During Volatilization in Snow: A Field Investigation in Antarctica

• Published in: Geophysical research letters 2019-03, Vol.46 (6), p.3287-3297
• Main Authors: Shi, Guitao, Chai, Jiajue, Zhu, Zhuoyi, Hu, Zhengyi, Chen, Zhenlou, Yu, Jinhai, Ma, Tianming, Ma, Hongmei, An, Chunlei, Jiang, Su, Tang, Xueyuan, Hastings, Meredith G.
• Format: Article
• Language: English
• Subjects: Acids; Antarctic snow; Atmospheric chemistry; Atmospheric composition; Constants; coupled cluster method; Depletion; Desorption; Exchanging; Field investigations; Field tests; Fractionation; Ice; Investigations; Isotope composition; Isotope fractionation; Isotopes; Low temperature; nitrate; Nitrates; Nitric acid; Nitric acids; Oxygen; Photolysis; Polar environments; Polar regions; Snow; Snowpack; stable isotope; Stable isotopes; Surface boundary layer; Surface layers; Volatilization
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL081968

Several postdepositional processes impact snow nitrate; however, only the isotopic effects of nitrate photolysis have been quantified. Here we discuss results from experiments in field Antarctic snow investigating isotopic fractionation of nitrate due to volatilization. At −35 °C, concentration and isotopic composition of nitrate remained constant during the 16‐day experiment. At −24 °C, 7.5% of nitrate was lost, synchronous with 1.5‰ decrease in δ18O and a constant δ15N. At −4 °C, 38% of nitrate was lost, and δ15N and δ18O decreased by 3.1 and 1.8‰, respectively. Results at −4 °C yield calculated fractionation constants close to theoretical estimates including equilibrium isotopic exchange between nitric acid and nitrate and the desorption of nitric acid from water in quasi‐liquid layers. This suggests that isotopic fractionation associated with nitrate volatilization across most of Antarctica, especially at sites with temperatures