The interaction of ozone with alumina has been examined at ambient temperature as a function of ozone concentration and relative humidity. The experiments used diffuse reflectance FTIR spectroscopy in a small flow reactor, which provided control of the temperature, pressure, and composition of the gas mixture to which the sample was continuously exposed. Treatment of alumina with ozone produced a new spectroscopic feature at 1380 cm(-1), which we attribute to an aluminum oxide species formed by interaction of O3 with Lewis acid sites on the alumina surface. After exposure of the alumina sample to O3 was stopped, subsequent exposure of the sample to humidified nitrogen resulted in the slow removal of the peak at 1380 cm(-1). Simultaneously, the uptake of water by the alumina increased as indicated by the growth of the adsorbed water features which extend from approximately 3700 to 2500 cm(-1). Treatment of dry alumina with humidified ozone strongly inhibited both the rate of formation of the spectral feature at 1380 cm(-1) and its limiting extent of formation. These observations are analyzed in terms of the adsorption and surface reaction properties of ozone on alumina. The observation that the new oxide feature on alumina, produced by reaction with ozone, can be removed by water is important for assessing the ability of mineral dust aerosols to process atmospheric trace gases over a significant time scale. We believe the work reported here to be the first direct and quantitative kinetic study of the competition between O3 and water for adsorption sites on alumina.

The rate coefficients for the reactions of atomic bromine with toluene, tetrahydrofuran, and tetrahydropyran were measured from approximately 295 to 362 K using the relative rate method. Iso-octane was used as the reference compound for the reaction with toluene, and iso-octane and toluene were used as the reference compounds for the reaction with tetrahydrofuran; tetrahydrofuran was used as the reference compound for the reaction with tetrahydropyran. The rate coefficients were found to be unaffected by changes in pressure and oxygen concentration. The rate coefficient ratios were converted to absolute values using the absolute rate coefficient for the reaction of Br with the reference compound. The absolute rate coefficients, in the units cm(3) molecule(-1) s(-1), for the reaction of Br with toluene are given by k(T) = (3.7 +/- 1.7) x 10(-12) exp(-(1.63 +/- 0.15) x 10(3)/T), for the reaction of Br with tetrahydrofuran by k(T) = (3.7 +/- 2.7) x 10(-10) exp(-(2.20 +/- 0.22) x 10(3)/T), and for the reaction of Br with tetrahydropyran by k(T) = (3.6 +/- 1.8) x 10(-10) exp(-(2.35 +/- 0.16) x 10(3)/T). The uncertainties represent one standard deviation. The Arrhenius parameters for these reactions are compared with results in the literature for dimethyl ether, diethyl ether, and a series of saturated hydrocarbons, and the effects of structural variation on these parameters are identified.

The rate coefficient for the reaction of atomic bromine with 1,4-dioxane was measured from approximately 300 to 340 K using the relative rate method. Iso-octane and iso-butane were used as reference compounds, and the experiments were made in a bath of argon containing up to 210 Torr of O(2) at total pressures between 200 and 820 Torr. The rate coefficients were not affected by changes in pressure or O(2) concentration over our range of experimental conditions. The ratios of rate coefficients for the reaction of dioxane relative to the reference compound were put on an absolute basis by using the published absolute rate coefficients for the reference reactions. The variation of the experimentally determined rate coefficients with temperature for the reaction of Br with 1,4-dioxane can be given by k(1)(exp)(T) = (1.4 +/- 1.0) x 10(-11)exp[-23.0 +/- 1.8) kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1). We rationalized our experimental results in terms of transition state theory with molecular data from quantum chemical calculations. Molecular geometries and frequencies were obtained from MP2/aug-cc-pVDZ calculations, and single-point energies of the stationary points were obtained at CCSD(T)/CBS level of theory. The calculations indicate that the 1,4-dioxane + Br reaction proceeds in an overall endothermic addition-elimination mechanism via a number of intermediates. The rate-determining step is a chair-to-boat conformational change of the Br-dioxane adduct. The calculated rate coefficients, given by k(1)(calc)(T) = 5.6 x 10(-11)exp[-26.6 kJ mol(-1)/(RT)] cm(3) molecule(-1) s(-1), are in very good agreement with the experimental values. Comparison with results reported for the reactions of Br with other ethers suggests that this multistep mechanism differs significantly from that for abstraction of hydrogen from other ethers by atomic bromine.

The rate constants for the reactions of atomic bromine with dimethyl ether and diethyl ether were measured from approximately 300 to 350 K using the relative rate method. Both isooctane and isobutane were used as the reference reactants, and the rate constants for the reactions of these hydrocarbons were measured relative to each other over the same temperature range. The kinetic measurements were made by photolysis of dilute mixtures of bromine, the reference reactant, and the test reactant in mixtures of argon and oxygen at a total pressure of 1 atm. The resulting ratios of rate constants were combined with the absolute rate constant as a function of temperature for the reference reaction of Br with isobutane to calculate absolute rate constants for the reactions of Br with isooctane, dimethyl ether, and diethyl ether. The absolute rate constant, in the units cm3 molecule(-1) s(-1), for the reaction of Br with dimethyl ether was given by k = (3.8 +/- 2.4) x 10(-10) exp(-(3.54 +/- 0.21) x 10(3)/T) while for the reaction of Br with diethyl ether the rate constant is given by k = (2.8 +/- 2.7) x 10(-10) exp(-(2.44 +/- 0.32) x 10(3)/T). On the same basis, the rate constant for the reaction of Br with isooctane is given by k = (3.34 +/- 0.59) x 10(-12) exp(-(1.80 +/- 0.11) x 10(3)/T). In each case, the activation energy of the reaction is significantly smaller than the endothermicity of the reaction. This is discussed in terms of a complex mechanism for these reactions.

The reactions of Cl with tetrahydrofuran, tetrahydropyran, and dimethyl ether have been studied as a function of temperature, pressure, and O(2) concentration. The temperature was varied from approximately 280 to 360 K, the mole fraction of O(2) ranged from zero to approximately 0.6, and the experiments were made in a bath of argon at total pressures ranging from approximately 300 to 760 Torr. The rate coefficients were measured using the relative rate method with gas chromatographic analysis. The reaction of Cl with isobutane was the reference reaction, the rate coefficients for which were calibrated against the reaction of propane with chlorine atoms as a function of temperature. The rate coefficients were unaffected by the concentration of O(2) or by variation in pressure. The rate coefficient for the reaction of Cl with isobutane increased slightly with decreasing temperature. This weak temperature dependence of the rate coefficient was in satisfactory agreement with information in the literature and is represented in Arrhenius form by k(T) = (1.02(-0.25)(+0.32)) x 10(-10) exp(99 +/- 88/T) cm(3) molecule(-1) s(-1), where the uncertainties represent two standard deviations. The rate coefficients for the reactions of Cl with the ethers did not show a statistically significant dependence on temperature. Their average values over our range of temperature are: for Cl + tetrahydrofuran, k = (2.71 +/- 0.34) x 10(-10) cm(3) molecule(-1) s(-1); for Cl + tetrahydropyran, k = (2.03 +/- 0.82) x 10(-10) cm(3) molecule(-1) s(-1); and for Cl + dimethyl ether, k = (1.73 +/- 0.22) x 10(-10) cm(3) molecule(-1) s(-1), in which the uncertainties are again two standard deviations.