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Caractérisations isotopiques des voies de formation du nitrate atmosphérique et de la photochimie du nitrate dans la neige

Abstract : 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 study, we derived an apparent ZPE-shift value, which better constrains the absorption cross-section of 15NO3-. This apparent shift is derived from the best fit between the experimental observations and calculated fractionations based on existing ZPE-shift model and it includes actual ZPE-shift and changes in width, asymmetry and amplitude in absorption cross-section during isotopic substitution. We have validated the newly derived apparent ZPE-shift by conducting a field study at Dome C, Antarctica. In this study, an experimental setup was built on-site and the effect of solar UV photolysis on snow nitrate was investigated. This study was based on a comparison of two snow pits filled with locally drifted snow and by allowing/blocking the solar UV. The 15N fractionation for the UV exposed samples (-67.9 ± 12 ‰) was in fairly good agreement with the ZPE-shift model estimate from this study (-55.4 ‰). These values are also within the range of apparent isotopic fractionation observed at Dome C in previous studies. Further calculations to better constrain the absorption cross-section of 15NO3- with the ZPE-shift are underway, and we propose that the newly derived apparent ZPE-shift value should be used in future studies. We believe that incorporating these new findings in models predicting the enrichments of 15N nitrate in ice cores will allow a quantitative interpretation of the information preserved in ice.
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Tesfaye Berhanu. Caractérisations isotopiques des voies de formation du nitrate atmosphérique et de la photochimie du nitrate dans la neige. Sciences de la Terre. Université de Grenoble, 2013. Français. ⟨NNT : 2013GRENU018⟩. ⟨tel-00934489⟩



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