Thesis of M.Assali

Thesis defense of M.Assali    - laboratory PC2A Abstract : Degradation of volatile organic pollutants, such as Volatils Organic Compounds (VOCs), under tropospheric conditions is usually initiated by the main oxidant which is the OH radical, followed by the formation of hydroproxy radicals HO2 and alkylperoxy radicals RO2 by reaction of products with oxygen. The fate of these radicals plays an important role in tropospheric chemistry. They are closely linked to the cycle that controls the oxidative capacity of the atmosphere and the formation of tropospheric ozone. In a polluted environment, the influence of peroxy radicals is well known and many experimental results are available in the literature. In a clean environment (with low nitrogen oxides NOx (x=1,2) concentration) the reactivity between HOx (x=1,2) and RO2 controls tropospheric chemistry. However, this chemistry is not yet well known. In the frame of this thesis, experimental kinetic studies have been carried out to better understand the oxidation mechanisms of these species. An experimental laser photolysis device coupled with time-resolved spectroscopic detection techniques: continuous wave Cavity Ring-Down Spectroscopy (cw-CRDS) allowing the detection of HO2 and RO2 radicals and Laser Induced Fluorescence (LIF) for the detection of OH radicals was used.
Different reaction systems were studied using the experimental technique mentioned above:1) the reaction of CH3C(O)O2 + CH3C(O)O2, and CH3C(O)O2 + CH3O2, 2) CH3C(O)CH2O2 + CH3C(O)CH2O2 and for the first time the reaction Cl + CH3C(O)CH2O2, 3) DO2 + DO2 and for the first time the reaction HO2 + DO2. The rate constants were determined for these six reactions at ambient temperature. For the first four different reaction pathways are possible, and we have also determined the branching ratio of the pathway leading to the formation of radicals for these reactions. Keywords : continuous–wave cavity ring-down spectroscopy,peroxy radicals,laser photolysis,absorrption cross section,rate constant