Thesis of Maxime Infuso

Defense of thesis of Maxime Infuso - laboratory PhLAM

Unravelling the atmospheric iodine chemistry using molecular simulations

Abstract :

Oceans, through algae and phytoplankton activities, are the main source of iodine, including organic compounds as CH3I. In the recent past, iodine has played a critical role in health issues through historical events such as the Fukushima nuclear crisis or open-air bomb testing. Iodine, among other halogens, also participates in the catalytic destruction of ozone. The details of the interactions between iodinated compounds and aerosols in the troposphere remain largely unknown. In particular, the modification of the chemical speciation or effects of the environing molecules/aerosols on the iodine chemistry have consequences on its reactivity. Therefore, the knowledge of iodine atmospheric chemistry is essential to better understand general atmospheric phenomena. In this context, this thesis aims to improve the iodine atmospheric chemistry state of knowledge using theoretical simulations, focusing on interactions/chemical reactions between methyl iodide (CH3I) and its surrounding.

In a first part, the adsorption of gaseous iodomethane (CH3I) on model sea-salt aerosols (NaCl) at various humidities is investigated. We performed periodic density functional theory (DFT) as well as classical molecular dynamics (MD) calculations to investigate the influence of water coverage. To this aim, we parametrized a flexible non-polarizable force field for iodomethane. This force field shows good performances in describing the interactions with water and sea-salt surfaces. Simulations show that the presence of water tends to stabilize CH3I at the salt surface.

The lifetime of CH3I in the atmosphere (in gas phase or adsorbed on aerosols) may also be altered by its reaction with gas phase radicals. The reaction of CH3I with OH is thus investigated both in gas phase and in presence of water by means of quantum mechanical calculations. The presence of an additional water molecule favors the hydrogen abstraction by OH radical under atmospheric conditions. In other words, H2O plays the role of a catalyst in this atmospheric chemical reaction.

Finally, through cooperation with the laboratory ‘Physics of the Interactions of Ions and Molecules (PIIM)‘, we investigated the adsorption of methyl iodide on amorphous solid water (ASW) surfaces. In agreement with previous studies, we highlighted the importance of configuration sampling when dealing with amorphous interfaces. Using classical MD and quantum mechanical calculations, we have computed theoretical spectra for ASW surfaces with and without adsorbed CH3I, which could be directly compared with experimental ones. Adsorption of CH3I induces a redshift of about 20 cm-1 of the dangling OH stretching mode.

This thesis combines several theoretical methods to study the reactivity and capture of molecules by surfaces of atmospheric interest. The approaches applied in this work can be extended to other systems providing valuable interpretation of the spectra and experimental data.

Keywords : Atmosphere,iodine,Aerosols,Moecular simulations