An isotopic approach towards understanding nitrate formation pathways and revealing the photochemistry of nitrate in snow

• Main Author: Berhanu, Tesfaye
• Other Authors: Laboratoire de glaciologie et géophysique de l'environnement (LGGE), Observatoire des Sciences de l'Univers de Grenoble (OSUG), Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Université Joseph Fourier - Grenoble 1 (UJF)-Institut polytechnique de Grenoble - Grenoble Institute of Technology (Grenoble INP )-Institut national des sciences de l'Univers (INSU - CNRS)-Institut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA)-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de Grenoble, Joël Savarino
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2013
• Subjects: Stable isotopes; Nitrate in snow; Photolysis; Isotopic fractionation; Isotopes stables; Le nitrate dans la neige; La photolyse; Fractionnement isotopique; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; ice core
• Online Access: https://theses.hal.science/tel-00934489; https://theses.hal.science/tel-00934489/document; https://theses.hal.science/tel-00934489/file/pdf2star-1385455886-38784_BERHANU_2013_archivage.pdf

Nitrate, the end product of oxidation of atmospheric NOX (= NO + NO2), is one of the most abundant anions present in polar snow and ice. Its stable isotope ratios (δ18O, δ15N and Δ17O) have been widely used to constrain its sources and oxidation pathways. In addition, the nitrate archived in deep ice cores may be an important metric to constrain past climatic conditions. However, deposition of nitrate in polar regions with low snow accumulation is reversible due to post-depositional processes, and interpretation of this record is complicated. Currently, there exist deep ice core records of nitrate encompassing climatic information of millennial time scales, and their interpretation relies on careful quantification of post-depositional effects. We have experimentally studied the 17O-excess transfer from ozone during the gas phase NO2 + O3 → NO3 + O2 reaction, which is an important nighttime nitrate formation pathway. From this study, we have determined the ∆17O transfer function given by: ∆17O(O3*) = (1.23 ± 0.19) × ∆17O(O3)bulk + (9.02 ± 0.99). We have also evaluated the intramolecular oxygen isotope distribution of ozone and have observed the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The findings from this study will have an important implication for understanding nitrate formation pathways via different NOX oxidation mechanisms. The impact of photolysis on the amount and stable isotope enrichments of nitrate is investigated in this PhD study based on laboratory and field experiments. A laboratory study was conducted by irradiating a natural snow from Dome C with a Xe UV lamp and a selection of UV-filters (280 nm, 305 nm and 320 nm). Based on the oxygen and nitrogen isotope ratio measurements, wavelength dependent isotopic fractionations were determined. Accordingly, in the presence of high-energy UV light, isotopic fractionation is shifted towards less negative values and the reverse for lower energy UV photons. Based the isotopic fractionations obtained in the laboratory ...