Reported in this study are the results from a gas kinetics — aerosol laboratory study involving the OH induced oxidation of SO2. At tropospheric and lower stratospheric pressures, reaction (1) is neither a third order process nor is it a simple bimolecular reaction. The effective bimolecular k value at one atmosphere (N2) pressure has been estimated by these authors to be 9 × 10−13cm³/molec/s. Based on estimated k values and concentrations for several trace gases, it is suggested here that the resulting HSO3 radical from reaction (1) under atmospheric conditions would react predominantly with O2 in both the lower stratosphere and the troposphere. It is further suggested that the product HSO5 radical would undergo hydration in both atmospheric regions.
The reactions of hydroxyl radicals with eight substituted aromatic hydrocarbons and four olefins were studied utilizing the flash photolysis-resonance fluorescence technique. The rate constants were measured at 298°K using either Ar or He as the diluent gas. The values of the rate constants (k X 10l2) in the units of cm3/molec. sec are
The excited singlet state of monochlorobenzene couples with the decomposing levels to give phenyl radicals and chlorine atoms and with the triplet manifold.11 The probability of the transition from the excited singlet state to the decomposing levels should be dependent on the vibrational energy since the photodecomposition quantum yield increases with increasing the excitation energy. The vibrational relaxation in the solid matrix, being faster than radiationless decay processes from the excited singlet state, can transfer the molecule to the lowest excited singlet state which results in the increased fluorescence quantum yield in the solid matrix. Experiments in the vapor phase, such as wavelength and pressure effects on , must clarify complicated mechanisms mentioned above and are now undertaken.9
The flash photolysis-resonance fluorescence technique has been utilized to study the kinetics of hydroxyl radical reactions with ethylene and acetylene at 300 K over a wide range of experimental conditions. (1) OH+C2H4 →k1 Products (e.g., C2H5O), (2) OH+C2H2 →k1 Products (e.g.,C2H2O+H). The bimolecular rate constant for Reaction (1) was observed to increase from (2.24–5.33) × 10−12 cm3 molecule−1⋅ sec−1 as the total pressure varied from (3–300) torr of helium. The rate of Reaction (2) was invarient with total pressure, and the value obtained for k2 was (1.65±0.15) × 10−13 cm2 molecule⋅sec−1. The observed pressure dependency of Reaction (1) brings the existing literature values for k1 into close agreement. Reaction (2) has been shown to be particularly sensitive to secondary processes and this is discussed in some detail.
The technique of flash photolysis–resonance fluorescence has been utilized to study the temperature dependences of two chlorine atom reactions of considerable fundamental importance to stratospheric chemistry. These reactions have been studied using a wide range of experimental conditions to ensure the absence of complicating secondary processes. The reactions of interest with their corresponding rate constants are expressed in units of cm3 molecule−1 s−1: Cl+O3→ClO O2 (k1), ΔU°298=−164 kJ mol−1, k1 = (3.08±0.30) ×10−11 exp[−(576±60/RT)], (220–350) K; Cl+CH4→CH3+HCl (k2), ΔU°298=−164 kJ mol−1, k2 = (7.44±0.75) ×10−12 exp[−(2437±110/RT)], (218–401) K. In addition, the followong reaction was studied at 300 K: Cl+H2O2→HCl+HO2 (K3), ΔU°298=−56.8 kJ mol−1, k3?5.8×10−13 (±factor 2.0), 300 K. A direct implication of the new rate data is the need to revise downward by a factor of 2.4 to 3 the magnitude of the ozone perturbation predicted by earlier model calculations due to the presence of ClOx species in the stratosphere.
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