Emmanuel Assaf

The Reaction between CH3O2 and OH Radicals: Product Yields and Atmospheric Implications

The reaction between CH3O2 and OH radicals has been shown to be fast and to play an appreciable role for the removal of CH3O2 radials in remote environments such as the marine boundary layer. Two different experimental techniques have been used here to determine the products of this reaction. The HO2 yield has been obtained from simultaneous time-resolved measurements of the absolute concentration of CH3O2, OH, and HO2 radicals by cw-CRDS. The possible formation of a Criegee intermediate has been measured by broadband cavity enhanced UV absorption. A yield of ϕHO2 = (0.8 ± 0.2) and an upper limit for ϕCriegee = 0.05 has been determined for this reaction, suggesting a minor yield of methanol or stabilized trioxide as a product. The impact of this reaction on the composition of the remote marine boundary layer has been determined by implementing these findings into a box model utilizing the Master Chemical Mechanism v3.2, and constraining the model for conditions found at the Cape Verde Atmospheric Observatory in the remote tropical Atlantic Ocean. Inclusion of the CH3O2+OH reaction into the model results in up to 30% decrease in the CH3O2 radical concentration while the HO2 concentration increased by up to 20%. Production and destruction of O3 are also influenced by these changes, and the model indicates that taking into account the reaction between CH3O2 and OH leads to a 6% decrease of O3.

Cross Section of OH Radical Overtone Transition near 7028 cm–1 and Measurement of the Rate Constant of the Reaction of OH with HO2 Radicals

The absorption cross section of an overtone transition of OH radicals at 7028.831 cm(-1) has been measured using an improved experimental setup coupling laser photolysis to three individual time-resolved detection techniques. Time-resolved relative OH radical profiles were measured by laser-induced fluorescence (LIF), and their absolute profiles have been obtained by cw-cavity ring-down spectroscopy (cw-CRDS). HO2 radicals were quantified simultaneously at the well-characterized absorption line at 6638.21 cm(-1) by a second cw-CRDS absorption path. Initial OH concentrations and thus their absorption cross sections have been deduced from experiments of 248 nm photolysis of H2O2: OH and HO2 profiles have been fitted to a simple kinetic model using well-known rate constants. The rate constant of the reaction between OH and HO2 radicals turned out to be sensitive to the deduction of the initial OH concentration and has been revisited in this work: OH decays have been observed in the presence of varying excess HO2 concentrations. A rate constant of (1.02 ± 0.06) × 10(-10) cm(3) s(-1) has been obtained, in good agreement with previous measurements and recent recommendations. An absorption cross section of σOH = (1.54 ± 0.1) × 10(-19) cm(2) at a total pressure of 50 Torr helium has been obtained from consistent fitting of OH and HO2 profiles in a large range of concentrations.

H-Abstraction by OH from Large Branched Alkanes: Overall Rate Measurements and Site-Specific Tertiary Rate Calculations

Reaction rate coefficients for the reaction of hydroxyl (OH) radicals with nine large branched alkanes (i.e., 2-methyl-3-ethyl-pentane, 2,3-dimethyl-pentane, 2,2,3-trimethylbutane, 2,2,3-trimethyl-pentane, 2,3,4-trimethyl-pentane, 3-ethyl-pentane, 2,2,3,4-tetramethyl-pentane, 2,2-dimethyl-3-ethyl-pentane, and 2,4-dimethyl-3-ethyl-pentane) are measured at high temperatures (900-1300 K) using a shock tube and narrow-line-width OH absorption diagnostic in the UV region. In addition, room-temperature measurements of six out of these nine rate coefficients are performed in a photolysis cell using high repetition laser-induced fluorescence of OH radicals. Our experimental results are combined with previous literature measurements to obtain three-parameter Arrhenius expressions valid over a wide temperature range (300-1300 K). The rate coefficients are analyzed using the next-nearest-neighbor (N-N-N) methodology to derive nine tertiary (T003, T012, T013, T022, T023, T111, T112, T113, and T122) site-specific rate coefficients for the abstraction of H atoms by OH radicals from branched alkanes. Derived Arrhenius expressions, valid over 950-1300 K, are given as (the subscripts denote the number of carbon atoms connected to the next-nearest-neighbor carbon): T003 = 1.80 × 10-10 exp(-2971 K/T) cm3 molecule-1 s-1; T012 = 9.36 × 10-11 exp(-3024 K/T) cm3 molecule-1 s-1; T013 = 4.40 × 10-10 exp(-4162 K/T) cm3 molecule-1 s-1; T022 = 1.47 × 10-10 exp(-3587 K/T) cm3 molecule-1 s-1; T023 = 6.06 × 10-11 exp(-3010 K/T) cm3 molecule-1 s-1; T111 = 3.98 × 10-11 exp(-1617 K/T) cm3 molecule-1 s-1; T112 = 9.08 × 10-12 exp(-3661 K/T) cm3 molecule-1 s-1; T113 = 6.74 × 10-9 exp(-7547 K/T) cm3 molecule-1 s-1; T122 = 3.47 × 10-11 exp(-1802 K/T) cm3 molecule-1 s-1.

Experimental and Theoretical Investigation of the Reaction NO + OH + O2 → HO2 + NO2

Photolysis of NO2 is the only major pathway for O3 formation as products from the reaction of OH and NO under atmospheric conditions in competition to the formation of HONO has been investigated experimentally and theoretically. Experiments have been carried out by directly measuring the formation of HO2 radicals using laser photolysis coupled to cw-CRDS. OH radicals have been generated from the reaction of F atoms with H2O, and absolute HO2 and OH profiles have been recorded at different NO concentrations. The potential energy surface has been calculated and the rate constant has been obtained from RRKM master equation modeling. Both experiment and theory show that the OH + NO reaction in the presence of O2 bath gas is not a competitive source of HO2 + NO2.