Georges Le Bras

Role of the BRO + HO2 reaction in the stratospheric chemistry of bromine

The impact of new laboratory data for the reaction BrO + HO2 → HOBr + O2 has been estimated in a one-dimensional photochemical modelling of bromine/ozone stratospheric chemistry. The reported 6 fold increase in the measured value for the rate constant of this reaction significantly increases both the HOBr mixing ratio and the global ozone depletion due to bromine compounds (the calculated ozone loss increases from 1.14% to 1.45% for a 20 ppt total bromine content). The higher rate constant makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction. The influence of the existing uncertainty in the photolysis rate of HOBr is also analysed.

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 (1.8 ± 0.7) × 10−4 and (3.8 ± 1.5) × 10−4 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: γ = γ0/(1 + γ0k[O3]t), with temperature independent values of k = (1.1 ± 0.4) × 10−10 and (6.2 ± 2.5) × 10−11 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: ≃7 × 1014 for toluene and ≃9 × 1014 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.

The BrO + Ch3O2 reaction: Kinetics and role in the atmospheric ozone budget

Using the discharge flow method with detection of species by mass spectrometry and laser-induced fluorescence, the kinetics of the reaction BrO + CH 3 O 2 → products has been studied at 298 K. The rate constant obtained is (5.7 ± 0.6)x10 -2 cm3 molecules -1 s -1 . The mechanism of the reaction has been also investigated. These laboratory data are used to discuss the possible impact of this reaction on the ozone budget in different parts of the atmosphere. The reaction is likely to be negligible in the stratosphere, but is potentially significant in the remote marine boundary layer. The BrO + CH 3 O 2 reaction, together with other BrO + RO 2 reactions, may also significantly contribute to the ozone loss events observed in the Arctic troposphere in spring.

Water Vapor Effect on the HNO3 Yield in the HO2 + NO Reaction: Experimental and Theoretical Evidence

The influence of water vapor on the production of nitric acid in the gas-phase HO(2) + NO reaction was determined at 298 K and 200 Torr using a high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer. The yield of HNO(3) was found to increase linearly with the increase of water concentration reaching an enhancement factor of about 8 at [H(2)O] = 4 x 10(17) molecules cm(-3) ( approximately 50% relative humidity). A rate constant value k(1bw) = 6 x 10(-13) cm(3) molecule(-1) s(-1) was derived for the reaction involving the HO(2)xH(2)O complex: HO(2)xH(2)O + NO --> HNO(3) (1bw), assuming that the water enhancement is due to this reaction. k(1bw) is approximately 40 times higher than the rate constant of the reaction HO(2) + NO --> HNO(3) (1b), at the same temperature and pressure. The experimental findings are corroborated by density functional theory (DFT) calculations performed on the H(2)O/HO(2)/NO system. The significance of this result for atmospheric chemistry and chemical amplifier instruments is briefly discussed. An appendix containing a detailed consideration of the possible contribution from the surface reactions in our previous studies of the title reaction and in the present one is included.

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 .

Kinetics of Cl atom reactions with butadienes including isoprene

Abstract The laser photolysis-resonance fluorescence technique was used to determine the rate constants for the reactions of Cl atoms with 1,3-butadiene, 2-methyl-1,3 butadiene (isoprene) and 2,3-dimethyl-1,3-butadiene in the pressure range 15–60 Torr and T=(298±2) K. The obtained rate constants (in units of 10−10 cm3 molecule−1 s−1) were as follows: 1,3-butadiene (3.48±0.10), isoprene (3.61±0.10) and 2,3-dimethyl-1,3-butadiene (3.63±0.14). These kinetic data are discussed.

Desorption of polycyclic aromatic hydrocarbons from soot surface: pyrene and fluoranthene.

The kinetics of thermal desorption of two four-ring polycyclic aromatic hydrocarbons, fluoranthene, and pyrene from well-characterized laboratory-generated kerosene soot surface was studied over the temperature range 260-320 K in a low-pressure flow reactor combined with an electron-impact mass spectrometer. Two methods were used to measure the desorption rate constants: monitoring of the surface-bound fluoranthene and pyrene decays due to desorption using off-line HPLC measurements of their concentrations in soot samples, and monitoring of the desorbed molecules in the gas phase using in situ mass spectrometric detection. Results obtained with the two methods were in good agreement and yielded the following Arrhenius expressions for the desorption rate constants: k(des) (fluoranthene) = 4 x 10(14) exp[-(93900 +/- 1700)/RT] and k(des) (pyrene) = 6 x 10(14) exp[-(95200 +/- 1800)/RT] (k(des) are in units of s(-1), and activation energies are in J mol(-1)). In addition, the combined uptake coefficient of fluoranthene and pyrene on soot (calculated using specific surface area) was estimated to be near 5 x 10(-3) at T = 310 K.

Laboratory investigation of the photooxidation of formaldehyde combining FTIR analysis of stable species and HO2 detection by the chemical amplifier technique

Abstract The photodissociation of formaldehyde in air at one atmosphere pressure and room temperature has been investigated in a 977 L photoreactor by in situ analysis of stable species by FTIR absorption spectroscopy and of peroxy radicals, HO 2 , by the chemical amplifier technique, after sampling. The photodissociation coefficient of the channel H 2 CO+ hν (+O 2 ) → 2 HO 2 +CO (1) determined experimentally was found consistent with that calculated from the absorption spectrum of H 2 CO and the spectral distribution of the photolysis light. The rate constant k −3 of the thermal decomposition of the adduct formed in the reaction HO 2 +H 2 CO↔HOCH 2 O 2 (3, −3) was determined and found at the lower limit of the range of the literature values. This study confirms the potentiality of the chemical amplifier technique in combination with other analytical techniques to investigate chemical mechanisms of atmospheric interest in photoreactors.

Rate constant for the reaction of OH radical with HFC‐365mfc (CF3CH2CF2CH3)

Rate constant for the reaction of OH with HFC-365mfc (CF3CH2CF2CH3) has been measured at a total pressure of 100 Torr, using the laser photolysis - laser induced fluorescence technique. The obtained kinetic data were used to derive the following Arrhenius expression over the temperature range 269–370 K : k = (1.68±0.21) × 10−12 exp[-(1585±80)/T] cm³ molecule−1 s−1. At 298 K, the measured rate coefficient was : k = (8.69±0.74) × 10−15 cm³ molecule−1 s−1. This study is the first investigation to be reported on this reaction. Using our kinetic data, the tropospheric lifetime of CF3CH2CF2CH3 is estimated to be 7.8 years.