The heterogeneous reaction of O3 with lauric acid/oleic acid (LA/OA) mixtures and myristic acid/oleic acid (MA/OA) mixtures were studied as a function of composition, physical state, and microstructure at 298 K. Lauric acid and myristic acid are both alkanoic acids, whereas oleic acid is an alkenoic acid. Additionally, we investigated the uptake of O3 by multicomponent mixtures that closely represent the composition of meat-cooking aerosols. These measurements were performed with a rotating-wall flow-tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficients (gamma) of O3 on liquid LA/OA and MA/OA solutions range from 4 x 10(-4) to 7.2 x 10(-4). The gamma values measured for solid-liquid LA/OA and MA/OA mixtures (which consist of solid LA or solid MA in equilibrium with a liquid) range from 2 x 10(-5) to 1.7 x 10(-4). These experiments show that only 7% solid by mass in the solid-liquid mixture can decrease gamma by an order of magnitude compared to the liquid mixtures. The gamma values for solid-liquid mixtures that closely represent the composition of meat-cooking aerosols range from 1.6 x 10(-5) to 6.9 x 10(-5). We found that gamma of solid-liquid mixtures depends on the microstructure of the mixtures, which in turn depends on the method of preparing the films. Furthermore, experiments employing solid-liquid mixtures show an increase in gamma with increasing film age. This can be explained either by the formation of a nonequilibrium phase followed by its relaxation to the stable phase or by Ostwald's ripening, which refers to a change in the solid microstructure due to a tendency to minimize the total surface free energy of the solid. We used the obtained gamma values to estimate OA lifetimes for polluted atmospheric conditions. For liquid solutions, the lifetimes were on the order of a few minutes. The lifetimes derived for solid-liquid mixtures are up to 75 min, significantly longer than for liquid solutions. Our study emphasizes the effect of the physical state and microstructure of multicomponent mixtures on the heterogeneous chemistry.

The kinetics of the reactions of atomic bromine with ethene, 2-methylpropane, dimethyl ether, and diethyl ether have been studied at 298 K in a 70-L photochemical reaction chamber with the relative rate method. Chemical analysis was by gas chromatography with a flame ionization detector. The effects of bromine and oxygen partial pressure and of the choice of reference reactant on the numerical values of the rate constants obtained have been evaluated. The results are analyzed to obtain limiting values of the rate constant ratio that are independent of the concentration of O 2 and do not depend on the mechanism used to represent the overall reaction. The following rate constants were obtained at 298 K by using the well-established rate constant for the reaction of Br with 2-methylpropane as the primary reference: Br + (CH 3 ) 2 O, k 298 ) 9.43 × 10 5 L mol -1 s -1 ( 2%; Br + (C 2 H 5 ) 2 O, k 298 ) 1.41 × 10 7 L mol -1 s -1 ( 6%. With the rate constants for Br + (CH 3 ) 2 O as a secondary reference value, the rate constant at 298 K for Br + (C 2 H 5 ) 2 O was 1.27 × 10 7 L mol -1 s -1 ( 8% calculated at a limiting high concentration of O 2 . When Br + (C 2 H 5 ) 2 O was measured relative to Br + C 2 H 4 , the rate constant at 298 K, calculated for limiting high concentrations of O 2 , ranged from 4.7 × 10 7 to 1.2 × 10 7 L mol -1 s -1 depending on the rate constant chosen for the reference reaction. The sensitivity to oxygen concentration of the rate constant for the reaction of Br with ethene depends on the concentration of Br 2 used as the photolytic source of atomic bromine. When the reaction of Br with an organic reactant is sensitive to the presence of oxygen, measurements at O 2 concentrations much larger than those in synthetic air may be required to obtain reliable rate constants for the initial reaction of Br with the organic reactant.

The pyrolysis of vinyl bromide has been examined in the temperature range 637−733 K and at pressures from 6 to 86 kPa. The yields of the major hydrocarbon products, C2H2, C2H4, and 1,3-C4H6, are second order in vinyl bromide over the entire range of temperatures investigated. At the higher temperatures, initiation by molecular elimination of HBr dominates, while at lower temperatures a free radical initiation channel becomes increasingly important. Our data for the overall process leading to HBr fit the relation ln(k) = (30.7 ± 4.8) − ((26.6 ± 3.3) × 103)/T, with the rate constant in the units L mol-1 s-1, indicating an activation energy of 220 kJ mol-1 ± 12% for the HBr elimination. A simple Arrhenius extrapolation is close to previous results at temperatures from 800 to over 2000 K. The combination of our data and the earlier measurements of the HBr elimination is reasonably represented by ln(k) = 37 − (3 × 104)/T. Our data suggest that the free radical pathway is disproportionation rather than unimolecular cleavage of the C−Br bond, a situation analogous to that in the low-temperature thermal decomposition of ethylene. Kinetic analysis indicates that the activation energy of this new free radical initiation channel is approximately 150 kJ mol-1, much less than the C−Br bond energy.