Yuri Bedjanian

Heterogeneous reaction of ozone with hydrocarbon flame soot

The reaction of ozone with toluene and kerosene flame soot was studied over the temperature range 240 to 350 K using a low pressure (a few Torr) flow reactor coupled to a modulated molecular beam mass spectrometer. A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well controlled and adjustable combustion conditions. Soot was found to be deactivated in reaction with ozone, the uptake coefficient (γ) being dependent on the time of exposure. The values of (1.8u2006±u20060.7)u2006×u200610−4 and (3.8u2006±u20061.5)u2006×u200610−4 independent of temperature in the range 240–350 K were determined for the initial uptake coefficient of ozone on toluene and kerosene soot, respectively. The process of soot ageing (deactivation) was parameterized, the uptake coefficient being expressed as a function of time and gas phase ozone concentration: γu2006=u2006γ0/(1u2006+u2006γ0k[O3]t), with temperature independent values of ku2006=u2006(1.1u2006±u20060.4)u2006×u200610−10 and (6.2u2006±u20062.5)u2006×u200610−11 cm3molecule−1s−1, for toluene and kerosene soot, respectively. From the soot surface saturation experiments the following maximum number of ozone molecules taken up were determined: ≃7u2006×u20061014 for toluene and ≃9u2006×u20061014 molecule cm−2 for kerosene soot. Experiments on soot ageing confirmed that soot deactivation occurs under real ambient conditions. The present results support current considerations that heterogeneous loss of ozone on soot has negligible impact on ozone concentration throughout the atmosphere.

Experimental study of the interaction of HO2 radicals with soot surface

The reaction of HO2 with toluene and kerosene flame soot was studied over the temperature range 240-350 K and at P = 0.5-5 Torr of helium using a discharge flow reactor coupled to a modulated molecular beam mass spectrometer. A flat-flame burner was used for the preparation and deposition of soot samples from premixed flames of liquid fuels under well controlled and adjustable combustion conditions. The independent of temperature in the range 240-350 K value of gamma = (7.5 +/- 1.5) x 10(-2) (calculated with geometric surface area) was found for the uptake coefficient of HO2 on kerosene and toluene soot. No significant deactivation of soot surface during its reaction with HO2 was observed. Experiments on soot ageing under ambient conditions showed that the reactivity of aged soot is similar to that of freshly prepared soot samples. The results show that the HO2 + soot reaction could be a significant loss process for HOx in the urban atmosphere with a potential impact on photochemical ozone formation. In contrast this process will be negligible in the upper troposphere even in flight corridors.

KINETIC STUDY OF THE BR + IO, I + BRO AND BR + I2 REACTIONS. HEAT OF FORMATION OF THE BRO RADICAL

Abstract Using the discharge-flow mass spectrometric method, the rate coefficients for the reactions Br + IO → I + BrO (2), I + BrO → Br + IO (3) and Br + I 2 → I + IBr (4) have been measured at 298 K and 1 Torr: k 2 = (2.3 ± 0.3) × 10 −11 , k 3 = (1.45 ± 0.20) × 10 −11 and k 4 = (1.20 ± 0.15) × 10 −10 cm 3 molecule −1 s −1 . The value of the enthalpy of BrO formation has been derived from the kinetic data obtained for reactions (2) and (3): ΔH f,298 (BrO) = 28.6 ± 1.4 kcal mol −1 .

Kinetic study of the reactions of OH and OD with HBr and DBr

Abstract The kinetics of the reactions of OH and OD radicals with HBr and DBr, OHxa0+xa0HBr (1), ODxa0+xa0HBr (3), OHxa0+xa0DBr (4) and ODxa0+xa0DBr (5), have been studied by the mass spectrometric discharge-flow method at temperatures between 230 and 360xa0K and at total pressure of 1xa0Torr of helium. The following Arrhenius expressions were obtained: k1xa0≈xa0k3xa0=xa0(5.3xa0±xa01.2)xa0×xa010−12xa0exp{(225xa0±xa060)/T} and k4xa0≈xa0k5xa0=xa0(4.3xa0±xa01.2)xa0×xa010−12xa0exp{(125xa0±xa080)/T}xa0cm3xa0s−1 per molecule. The isotope exchange reactions ODxa0+xa0HBrxa0→xa0OHxa0+xa0DBr (3′) and OHxa0+xa0DBrxa0→xa0ODxa0+xa0HBr (4′) were found to be slow and the upper limits for the rate constants of these channels were measured at Txa0=xa0298xa0K: k3′

Kinetic and mechanistic study of the X and XO (X=Cl, Br) reactions with dimethyl sulfoxide

The kinetics and mechanism of the reactions of dimethyl sulfoxide (DMSO) with Cl, Br, ClO and BrO have been studied by the mass spectrometric discharge-flow method at 298 K and at a total pressure of 1 Torr of helium. The rate coefficient of the reaction Clu2006+u2006DMSOu2006→u2006products (1) was measured under pseudo-first-order conditions either in excess of DMSO or in excess of Cl atoms: k1u2006=u2006(2.05u2006±u20060.35)u2006×u200610−11 cm3 molecule−1 s−1 n(quoted uncertainty includes estimated systematic errors). Both HCl and CH3 were detected as products of reaction (1) and the branching ratios 0.91u2006±u20060.15 and 0.10u2006±u20060.02, respectively, were found for the channels forming these species. For the reaction Bru2006+u2006DMSOu2006→u2006products (2) the rate constants for the HBr and CH3 forming channels (2a and 2b, respectively) were determined from the kinetics of formation of these products: k2au2006=u2006(1.1u2006±u20060.3)u2006×u200610−14 and k2bu2006=u2006(1.2u2006±u20060.3)u2006×u200610−15 cm3 molecule−1 s−1. For the reactions ClOu2006+u2006DMSOu2006→u2006products (3) and BrOu2006+u2006DMSOu2006→u2006products (4), only upper limits, k3u2006<u20061.6u2006×u200610−14 and k4u2006<u20064u2006×u200610−14 cm3 molecule−1 s−1, could be determined.

KINETIC STUDY OF THE REACTIONS BR + IBR I + BR2 AND I + BR2 BR + IBR

The kinetics of the title reactions have been studied using the discharge-flow mass spectrometic method at 296 K and 1 torr of helium. The rate constant obtained for the forward reaction Br+IBrI+Br2 (1), using three different experimental approaches (kinetics of Br consumption in excess of IBr, IBr consumption in excess of Br, and I formation), is: k1=(2.7±0.4)×10−11 cm3 molecule−1s−1. The rate constant of the reverse reaction: I+Br2Br+IBr (−1) has been obtained from the Br2 consumption rate (with an excess of I atoms) and the IBr formation rate: k−1=(1.65±0.2)×10−13 cm3molecule−1s−1. The equilibrium constant for the reactions (1,−1), resulting from these direct determinations of k1 and k−1 and, also, from the measurements of the equilibrium concentrations of Br, IBr, I, and Br2, is: K1=k1/k−1=161.2±19.7. These data have been used to determine the enthalpy of reaction (1), ΔH298°=−(3.6±0.1) kcal mol−1 and the heat of formation of the IBr molecule, ΔHf,298°(IBr)=(9.8±0.1) kcal mol−1.