Reactivity of the atmospheric radicals CH(3)S, CH(3)O and CH(3), with nitrogen dioxide and ozone: Experimental study by pulsed photolysis and laser-induced fluorescence (Methoxy radicals, Methylthio radicals, Methyl radicals)

In this work an experimental kinetic study on radicals of atmospheric interest such as, CH₃S, CH₃O and CH₃, with NO₂ and O₃ has been carried out as a function of temperature and total pressure. Kinetic parameters of these reactions are determined employing Laser Pulsed Photolysis to generate the transient species and Laser Induced Fluorescence (LIF) detection to monitor its concentration or that of a reaction product. The experimental system used to study these gas-phase atmospheric processes was designed and recently built in our laboratory. Concentrations of CH₃S(X2E) and CH₃O(X2E), generated in the photolysis of the corresponding photochemical precursor or produced in the reaction of CH₃ with NO₂ or O₃, were monitored by exciting the A2 A₁ ← X2E transition and by detection of the resulting LIF emission.; In the study of the kinetic behaviour of CH₃S, CH₃O and CH₃ radicals in the presence of NO₂ and CH ₃S radical in the presence of O₃ a slightly negative temperature dependence of their absolute rate constants has been observed. Kinetics on CH₃S with NO₂ and O₃ were independent on total pressure in the range studied (p_T = 30–300 Torr in He), indicating that the contribution of the possible recombination channel was negligible or the rate constants were in the high pressure limit. On the other hand, kinetics on CH₃ and CH₃O with NO₂ exhibits a strong dependence on buffer gas concentration, He, showing that the association reaction channel was important in these reactions. In both cases, measured rate coefficients are found in the fall-off region, between low- (k0) and high-pressure limits (k∞).; A treatment of the rate constants for these reactions, based on Troe's Formalism, yielded the low- and high-pressure limiting rate constants. In the case of CH₃O + NO₂ reaction, the temperature dependence in the 250–390 K range is also slightly negative and an expression of this behaviour is obtained. The rate constant for CH₃ + NO ₂ reaction has been determined at room temperature and as a function of total pressure (p_T = 20–100 Torr). Otherwise, limiting low-(k0) and high-pressure rate constant ( k∞) have been obtained using the conventional Troe's Formalism.; In this work, CH₃O radical has been detected by LIF in CH ₃ + _O3 reaction at room temperature and 100 Torr of total low CH₃O yield, estimated in less than 3% of total reaction routes.; In light of the rate constants obtained in normal conditions and taking into account the typical concentrations of NO₂ and O₃ in the troposphere, tropospheric lifetimes of CH₃, CH₃O and CH₃S radicals (τ) were estimated. We can conclude that in polluted areas the reaction with O₃ accounts for the main consumption of these radicals (τ < 3 s) instead of the reaction with NO₂ (τ < 5 min), whilst both removal processes are comparable in marine remote zones or non-polluted regions.