No abstract

A gas-phase kinetics study of the atmospherically important reaction between Cl 2 and dimethyl sulfide (DMS) Cl 2 + CH 3 SCH 3 → products ----- (1) has been made using a flow-tube interfaced to a photoelectron spectrometer. The rate constant for this reaction has been measured at 1.6 and 3.0 torr at T= (294±2) K as (3.4 ± 0.7) x 10 -14 cm 3 molecule -1 s -1 . Reaction (1) has been found to proceed via an intermediate, (CH 3 ) 2 SCl 2 , to give CH 3 SCH 2 Cl and HCl as the products. The mechanism of this reaction and the structure of the intermediate were investigated using electronic structure calculations. A comparison of the mechanisms of the reactions between Cl atoms and DMS, and Cl 2 and DMS has been made and the relevance of the results to atmospheric chemistry is discussed.

The ozone-ethene reaction has been investigated at low pressure in a flow-tube interfaced to a u.v. photoelectron spectrometer. Photoelectron spectra recorded as a function of reaction time have been used to estimate partial pressures of the reagents and products, using photoionization cross-sections for selected photoelectron bands of the reagents and products, which have been measured separately. Product yields compare favourably with results of other studies, and the production of oxygen and acetaldehyde have been measured as a function of time for the first time. A reaction scheme developed for the ozone-ethene reaction has been used to simulate the reagents and products as a function of time. The results obtained are in good agreement with the experimental measurements. For each of the observed products, the simulations allow the main reaction (or reactions) for production of that product to be established. The product yields have been used in a global model to estimate their global annual emissions in the atmosphere. Of particular interest are the calculated global annual emissions of formaldehyde (0.96 ± 0.10 Tg) and formic acid, (0.05 ± 0.01 Tg) which are estimated as 0.04% and 0.7% of the total annual emission respectively.

The burning of tobacco creates various types of free radicals that have been reported to be biologically active. Some radicals are transient but can initiate catalytic cycles that generate other free radicals. Other radicals are environmentally persistent and can exist in total particulate matter (TPM) for extended periods. In spite of their importance, little is known concerning the precursors of these radicals or under what pyrolysis/ combustion conditions they are formed. We performed studies of the formation of radicals from the gas-phase pyrolysis and oxidative pyrolysis of hydroquinone (HQ) and catechol (CT) between 750 and 1000 °C and phenol from 500 to 1000 °C. The initial electron paramagnetic resonance (EPR) spectra were complex, indicating the presence of multiple radicals. Using matrix annealing and microwave power saturation techniques, phenoxyl, cyclopentadienyl, and peroxyl radicals were identifiable, but only cyclopentadienyl radicals were stable above 750 °C.