No abstract

Rate constants of Br atom reactions have been determined using a relative kinetic method in a 20 1 reaction chamber at total pressures between 25 and 760 torr in Nz + Oz diluent over the temperature range 293-355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O2 present in the reaction system.

The kinetics of the reactions of 10, BrO, and C10 with DMS have been investigated at 298 K and total pressures of 0.5-6.8 mbar He using the discharge flow-mass spectrometry technique to monitor the halogen oxide radicals. Rate coefficients of (8.8 (2.7 f 0.5) x cm3 molecule-' 8-l have been obtained for the reactions of 10, BrO, and C10 with DMS, respectively. The result for I 0 with DMS is in good agreement with two other recent studies on this reaction but is nearly three orders of magnitude slower than two earlier studies, one of which was from this laboratory. The earlier studies are now thought to be in error because of heterogeneous and secondary chemistry occurring in the systems investigated. The rate coefficient for BrO with DMS is in excellent agreement with a previously reported preliminary value from this laboratory. However, the rate coefficient for C10 with DMS is a factor of 4 lower than our previously determined value. The new data are considered more reliable in view of the much improved experimental technique in the present study. DMSO was detected as a product in all of these reactions. A semi-quantitative determination of the DMSO yield was only possible in the case of the reaction of I0 with DMS where a yield of 84 f 40% was found.2.1) x and (9.5 2 2.0) x

The rate constant of the reaction of OH with DMS has been measured relative to OH + ethene in a 420 1 reaction chamber a t 760 torr total pressure and 298 t 3 K in N L + 0, buffer gas using the 254 nm photolysis of H202 as the OH source. In agreement with a recent absolute rate determination of the reaction the measured effective rate constant was found to increase with increasing partial pressure of 0, in the system, for 760 torr air a rate constant of (8.0 ? 0.5) x lo-'' cm' s Product studies have been performed on the reaction in air using FTIR absorption spectrometry for detection of reactants and products. On a molar basis, SO, was formed with a yield of 70% and dimethyl sulfone (CH,S02CH,) with a yield of approximately 2O'Z. These results are considerably different to those obtained in other product studies which were carried out in the presence of NO,. These differences are compared and their relevance for the atmospheric oxidation mechanisms of DMS is discussed. was obtained.

The reactions of OH radicals with SO2, H 2 S, thiophenol, and a series of aliphatic thiols (1-5 C-atoms) have been investigated in 201 and 381 reaction chambers at 1 atm total pressure and 300 K using a competitive kinetic technique. Initially, OH radicals were produced by photolysis of CH3ONO/NO mixtures in air. Applying this OH source rate constants for OH with SO2, H2S , and thiophenol in synthetic air were determined to be (1.1-+0.2) x l0 -12, (5.5-+0.8) ×10 -12 and (1.1 +-0.2) × 10 -it cm 3 s -1 , respectively. However, when this method was applied to the aliphatic thiols the rate constants obtained were found to be dependent on the partial pressures of Oz and NO. These effects have been attributed to the built-up of a radical species, not yet identified, which leads to uncontrolled chain reactions in the system. Using the photolysis of H202 at wavelengths greater than 260 nm as the OH source in 1 atm N:, rate constants for the 1-5 aliphatic thiols in the range 2.9 to 5.6 × 10 -it cm 3 s -~ were obtained. The rate constants obtained in the present study are compared with recent literature values.