Study of the atmospheric reactivity of halogenated compounds, mercury and pesticides
Halogenated compounds
The importance of gas-phase halogenated compounds (chlorine, bromine and iodine) in the atmosphere has been established since the 1970s with the discovery of the Antarctic ozone hole. These gases generate radicals with a wide range of applications for tropospheric and stratospheric chemistry: ozone budget, atmospheric concentrations (OH, NOx, volatile organic compounds), aerosol formation in the marine boundary layer, halogen interactions, climate change.
Many studies have already been carried out on halogen chemistry using global models. Most of them have focused on bromine and iodine, which are more active than chlorine due to the higher chemical stability of HCl compared to other HX acids (X = Br, I). In chemistry-transport models, there are a limited number of reactions, especially for organic halogen compounds. To date, data on atmospheric gas phase reactivity and gas-aerosol interactions remain incomplete and poorly understood. Quantum chemical tools are being used to better understand the observed reactivity trends and predict thermokinetic parameters of experimental data that are difficult or impossible to obtain. Recent work by our group has shown that the addition of the iodinated organic scheme to the atmospheric model strongly influences its chemical speciation (Fortin et al, Atm. Env., 2019, 214, 116838).
The objectives are to (i) update the reaction mechanism of halogenated compounds using a comprehensive literature review, (ii) incorporate the new reaction mechanism into atmospheric models, (iii) perform a kinetic analysis using a 0D model to establish key reaction pathways and identify data gaps, (iv) complete the state of the art with molecular modeling (v) evaluate with chemistry. . .the MOCAGE transport model the impact of the updated mechanism on global stratospheric and tropospheric air composition, particularly on the ozone layer. The new data collected will assist and guide the risk management community and government health and policy makers to better protect and serve the public interest.
Mercury
Oxidation of Hg(0) to Hg(II) in the atmospheric gas phase limits the rate of transfer in ecosystems. The overall oxidation of Hg(0) is primarily initiated by the bromine atom. The resulting BrHg radical reacts primarily with NO2 to form BrHgONO, which in turn is rapidly photolyzed to give BrHgO. The reaction of BrHgO with CO can be a reduction reaction of the oxidation number of mercury in the atmosphere. The potential energy surface is with potential energies calculated at the CCSD(T)/CBS level of theory including corrections due to relativistic effects from geometrical parameters optimized with the MP2 method. Kinetic calculations are performed by solving the master equation in order to determine the factors that can influence the global rate constant. This reaction is analogous to that of OH with CO. However, the reaction intermediate BrHgOCO is much more stable than HOCO with respect to CO2 loss. This leads to an immediate dissociation of BrHgOCO avoiding its stabilization in the atmosphere. Because of the value of the rate constant for the BrHgO + CO reaction and the abundance of CO in the atmosphere, this reaction can govern the atmospheric fate of BrHgO. The BrHg product can then dissociate to give Hg(0), which can then be transferred to ecosystems.
The study of the microhydration of oxygenated mercury compounds (BrHgO, HOHg, BrHgOH, BrHgOOH, Anti-BrHgONO, Syn-BrHgONO, BrHgNO2) is in progress.
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Pesticides
Pesticides are semi-volatile organic compounds emitted into the atmosphere through vapor movement during application, wind erosion from treated plants and soil, and post-application volatilization from the treated soil surface and plants. Environmental contamination from pesticides causes many health problems and will disrupt the balanced ecosystem. 80-90% of pesticides are volatilized within a few days of application and there is a maximum probability of air and environmental contamination.
The primary route of removal of pesticides from the atmosphere is by dry or wet deposition. The chemical reaction initiated by atmospheric oxidants (OH, O3, NO, NO2) is responsible for the transformation of pesticides. In the atmosphere, abiotic degradation of pesticides occurs mainly by photolysis and reactions with radicals. The products formed from these reactions can be dangerous and can cause several negative consequences. Recently, ANSES has identified 32 pesticides for which further investigations are needed to better understand their environmental properties.
Molecular simulations are being performed to determine the thermochemical properties and kinetic parameters in the gas and aqueous phases for the reactions of OH radicals with certain pesticides for which no data are available in the literature. The fate of the degradation products after the primary reaction is also studied.