Sophie Sobanska

Heteroaggregates of bromomethane and water were studied experimentally via matrix-isolation IR spectroscopy and theoretically using the ωB97X-D functional with the aug-cc-pVTZ basis set for all atoms except for bromine, for which the aug-cc-pVTZ-PP basis set was employed. Bromomethane and water mixtures at various ratios were diluted in argon and deposited onto a window cooled to 4-10 K. The resulting matrices were studied by FTIR spectroscopy both before and after annealing to 30-35 K. The 1:1 CH₃Br:H₂O complex was clearly identified in the FTIR spectra, even in matrices containing only traces of water. The lowest-energy 1:2 CH₃Br:H₂O heteroaggregate was also observed in the FTIR spectra of the mixtures. Despite the high water concentration, no clear evidence has been found for the presence of heteroaggregates containing larger numbers of water molecules. However, our results demonstrate that homoaggregates of CH₃Br are no longer formed once water molecules are present. To aid in the interpretation of the results in terms of heteroaggregates, Ar-matrices containing only CH₃Br were also studied to investigate homoaggregate formation. The experimental IR findings were compared with simulations of theoretical IR spectra for dimers and trimers. In addition to identifying the previously reported low-energy head-to-tail isomer of (CH₃Br)₂, further spectral features were attributed to a bromomethane trimer. IR bands corresponding to this species were detected in the more concentrated matrix and grew upon annealing. The homoaggregates of chloromethane and the CH₃Cl:H₂O heteroaggregates were also studied for comparison. This work highlights the affinity of halomethanes for water whatever the halogen (i.e., Cl, Br, and I), implying a probable modification of halo methane reactivity in the atmosphere that should be considered in the atmospheric studies.

The hydration mechanism of 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a relevant marker of secondary organic aerosol formation from the atmospheric oxidation of α-pinene, has been investigated using the matrix-isolation infrared spectroscopy technique. The experimental results were supported by theoretical calculations. Monomers of MBTCA and heterocomplexes MBTCA-n(H₂O) were identified. The presence of intramolecularly H-bonded monomers was experimentally identified, in accordance with the calculations. Heterocomplexes, i.e., aggregates with water, were found much more stable than homocomplexes. The results clearly demonstrate the affinity of MBTCA carboxylic functions for water molecules, and thus its ability to form an H-bond network, leading to the solvation of the molecule. This is the first experimental observation, evidencing the primary hydration stages of one relevant marker of secondary organic aerosol formation and aging. Although fundamental, such a molecular study helps in a better understanding of critical steps in cloud condensation nuclei formation.

The broad band UV photochemistry kinetics of acetylacetaldehyde, the hybrid form between malonaldehyde and acetylacetone (the two other most simple molecules exhibiting an intramolecular proton transfer), trapped in four cryogenic matrices: neon, nitrogen, argon and xenon, has been followed by IRTF and UV spectroscopies. After deposition, only the two chelated forms are observed while they isomerize upon UV irradiation toward nonchelated species. From previous UV irradiation effects, we have already identified several nonchelated isomers, capable, in turn, of isomerizing and fragmenting, even fragmentation seems to be most unlikely due to cryogenic cages confinement. Based on these findings, we have attempted an approach to the reaction path of electronic relaxation. Indeed, we have demonstrated, in previous works, that in the case of malonaldehyde, this electronic relaxation pathway proceeds through singlet states while it proceeds through triplet ones in the case of acetylacetone. We observed CO and CO₂ formations when photochemistry is almost observed among nonchelated forms, i.e. when parent molecule is almost totally consumed. In order to identify a triplet state transition, we have tried to observe a "heavy atom effect" by increasing the weight of the matrix gas, from Ne to Xe, and to quench T₁ state by doping the matrices with O₂. It appears that as in the case of acetylacetone, it is the nonchelated forms which fragment. It also appears that these fragmentations certainly take place in the T₁ triplet state and originate in an ∏* ← n transition.

Gaseous iodomethane (CH ₃ I) once emitted in the atmosphere will interact very little with sea salt aerosol particles. However, a low amount of CH ₃ I is irreversibly adsorbed at the particle surface.

The rapid industrialization and urbanization of intra- and peri-urban areas at the world scale are responsible for the degradation of the quality of edible crops, because of their contamination with airborne pollutants. Their consumption could lead to serious health risks. In this work, we aim to investigate the phytotoxicity induced by foliar transfer of atmospheric particles of industrial/urban origin. Leaves of cabbage plants (Brassica oleracea var. Prover) were contaminated with metal-rich particles (PbSO₄ CuO and CdO) of micrometer size. A trichloroacetic acid (TCA) treatment was used to inhibit the synthesis of the epicuticular waxes in order to investigate their protective role against metallic particles toxicity. Besides the location of the particles on/in the leaves by microscopic techniques, photosynthetic activity measurements, genotoxicity assessment, and quantification of the gene expression have been studied for several durations of exposure (5, 10, and 15 days). The results show that the depletion of epicuticular waxes has a limited effect on the particle penetration in the leaf tissues. The stomatal openings appear to be the main pathway of particles entry inside the leaf tissues, as demonstrated by the overexpression of the BolC.CHLI1 gene. The effects of particles on the photosynthetic activity are limited, considering only the photosynthetic F_v/F_m parameter. The genotoxic effects were significant for the contaminated TCA-treated plants, especially after 10 days of exposure. Still, the cabbage plants are able to implement repair mechanisms quickly, and to thwart the physiological effects induced by the particles. Finally, the foliar contamination by metallic particles induces no serious damage to DNA, as observed by monitoring the BolC.OGG1 gene.