INFUSO Maxime : Unravelling the atmospheric iodine chemistry using molecular simulations.
Résumé de thèse : The goal is to improve the understanding of the heterogeneous reactivity between gaseous iodinated species and aerosols present in the troposphere. To date, these heterogeneous interactions have not been considered in the atmospheric iodine dispersion models in case of a severe nuclear power plant accident. This is worrisome since such heterogeneous reactivity may play a major role in the iodine transport far from their emission sources. The importance of iodine in atmospheric chemistry has been highlighted by recent reviews. However, the atmospheric iodine heterogeneous reactivity studies have focused almost exclusively on determining the uptake coefficient of inorganic iodinated compounds (for example, I2, HI, and HOI) by water or ice. Those conditions are not fully relevant for our applications and have to be extended. Furthermore, photo-oxidation of gaseous CH3I and I2 in presence of O3 is known to produce IxOy aerosols, which are measured in the field campaigns. To the best of our knowledge, the influence of aerosols on the iodine photolysis processes in gas phase is not documented. Finally, the field measurements in Arctic and Antarctica pointed out the role of the low temperature in iodine chemistry in gas phase and in the formation of iodine-rich aerosols.As a result, this thesis will provide a set of reliable kinetic and mechanistic data on iodine reactivity with atmospheric aerosols in order to improve the relevance and accuracy of iodine chemistry in dispersion models. The work will be based on molecular simulations combining molecular dynamics simulations and electronic structure calculations. Systems associating both the main iodinated gaseous species and representative atmospheric aerosols will be carefully selected. Both molecular iodine (I2) and iodomethane (CH3I) are key iodine compounds of marine and biogenic origin that appears to be of central importance in understanding iodine chemistry in the troposphere. Further, in addition to their atmospheric interest, the reactivity of those compounds has gained much interest in the field of nuclear safety as they are the most probable gaseous iodine species to be released to the troposphere during a severe nuclear power plant accident of the type in Fukushima, Japan. As a result, this work will start on gaseous molecular iodine and iodomethane surface reactivity as a function of key inorganic and organic aerosols classes from the nanometric up to micrometric size. Model primary and secondary aerosols from marine origin will be considered, such as sodium chloride and low to high oxidized organic aerosols.
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