Study of OH Reactions at High Pressures by Excimer Laser Photolysis — Dye Laser Fluorescence

Abstract

Reactions of hydroxyl radicals have been studied up to total pressures of 1 atm and in the presence of strongly quenching gases by using excimer laser photolysis of H 2 0 2 or H N 0 3 for the generation and pulsed dye laser fluorescence for monitoring the concentration of the radicals as a function of reaction time. Results are reported of a study of the effects of molecular oxygen on the reactions of OH with acetylene and with some sulfur containing compounds (e.g. CS2).The high sensitivity of the pulsed dye laser induced fluorescence technique (LIF) allows absolute rate constant measurements of OH reactions under tropospheric conditions, i.e. up to total pressures of 1 atm and in the presence of strongly quenching gases like O2 and H 2 0 . We report on measurements in which excimer laser photolysis is used for the pulsed generation of OH radicals from H 2 0 2 or HN'03 parent gases and where a pulsed, frequency doubled, high energy dye laser serves for monitoring the time decay of the OH concentration in an excess of reactant gas by LIF of the OH(A2Ct t , X2H) transition near 300 nm [l].In order to check the method and the apparatus, test measurements were performed in synthetic air at a total pressure of 1 atm with H2 and some simple alkanes and alkenes. The results (Table 1) are in reasonable agreement with literature data. cm6 s-', Cm3 S -' , cm3 s -' which are in good agreement with recent results of Wahner ut al. [S] obtained with a laser photolysis/laser absorption technique. In the presence of oxygen, the OH decay curves were found to be bi-exponential (Fig. 2). Such behaviour is characteristic for systems in which O H radicals are regencrated by fast secondary reactions. This was verified by n-C4Hlo -The reaction of OH with CzH2 was studied in detail to clarify some discrepancies in the literature data on the rate constant of this reaction [2 -63 in particular concerning the different results for its pressure dependence for He and Ar buffer gases and the contribution of a bimolecular process to the reaction. Fig. 1 shows the results for He, Ar, and N2.Similar pressure dependences are observed for these inert who measured pressure independent rate constants in He buffer gas. Also, the zero pressure intercept, i.e. the pressure independent bimolecular contribution to the rate constant, is found to be about a factor of 10 lower than given in the

Kinetics of the gas-phase reactions of OH radicals with alkanes and cycloalkanes

“…The absolute rate constant measurements carried out over the temperature range 231-509 K by Schmidt et al (1985), Droege and Tully (1986b), Abbatt et al (1990), Schiffman et al (1991, Talukdar et al (1994), Donahue et al (1998) and Chuong and Stevens (2002) are in good agreement, with earlier absolute rate measurements of Greiner (1970), Perry et al (1976) and Paraskevopoulos and Nip (1980) at room temperature being ∼10-15% higher than these more recent studies. Figure 7 shows an Arrhenius plot of the absolute rate constants of Droege and Tully (1986b), Abbatt et al (1990), Talukdar et al (1994) and Donahue et al (1998) together with the relative rate data of Baker et al (1970) (as re-evaluated by Baldwin and Walker, 1979), Hucknall et al (1975) and DeMore and Bayes (1999).…”

“…Over the temperature range ∼200-800 K the absolute rate constants of Overend et al (1975), Howard and Evenson (1976b), Leu (1979), Lee and Tang (1982), Margitan and Watson (1982), Tully et al (1983Tully et al ( , 1986a, Smith et al (1984), Devolder et al (1984), Schmidt et al (1985), Baulch et al (1985), Stachnik et al (1986), Bourmada et al (1987), Wallington et al (1987), Lafage et al (1987), Zabarnick et al (1988), Abbatt et al (1990), Schiffman et al (1991, Dóbé et al (1991Dóbé et al ( , 1992, Sharkey and Smith (1993), Talukdar et al (1994), Crowley et al (1996), Donahue et al (1996Donahue et al ( , 1998 and Clarke et al (1998) are in good agreement. Because several of these studies involved measurement of the rate constant for the reaction of OH radicals with ethane at one temperature (generally room temperature) as a check on the experimental technique used (Leu, 1979;Lee and Tang, 1982;Margitan and Watson, 1982;Devolder et al, 1984;Bourmada et al, 1987;Lafage et al, 1987;Zabarnick et al, 1988;Dóbé et al, 1991Dóbé et al, , 1992, the rate constants from the more extensive absolute studies of Tully et al (1983Tully et al ( , 1986a, ...…”

Reactions of OH radicals with inorganic compounds in the gas phase

“…The values of 4.8 x 1011, calculated from [215], lie only slightly outside the recommended region. The proposed mechanism [212,[216][217][218] for the subsequent atmospheric oxidation of SO2 involves the following steps, and is based on monitoring OH decays in the presence of 02 and small amounts of NO, and computer simulations of the system. In this mechanism OH is regenerated and oxidizes SO2 catalytically.…”

Mechanism of the Reaction of OH with Alkynes in the Presence of Oxygen

“…[5][6][7] (HCO) 2 is the simplest -dicarbonyl and its atmospheric significance stems from its role in aerosol formation, [8][9][10] and use as a marker of biogenic emission. 11,12 In addition, (HCO) 2 photochemistry is a recognised source of HOx (OH and HO 2 ) radicals in the troposphere.…”