Photophysical oxidation of HCHO produces HO2 radicals.

Formaldehyde, HCHO, is the highest-volume carbonyl in the atmosphere. It absorbs sunlight at wavelengths shorter than 330 nm and photolyses to form H and HCO radicals, which then react with O2 to form HO2. Here we show HCHO has an additional HO2 formation pathway. At photolysis energies below the energetic threshold for radical formation we directly detect HO2 at low pressures by cavity ring-down spectroscopy and indirectly detect HO2 at 1 bar by Fourier-transform infrared spectroscopy end-product analysis. Supported by electronic structure theory and master equation simulations, we attribute this HO2 to photophysical oxidation (PPO): photoexcited HCHO relaxes non-radiatively to the ground electronic state where the far-from-equilibrium, vibrationally activated HCHO molecules react with thermal O2. PPO is likely to be a general mechanism in tropospheric chemistry and, unlike photolysis, PPO will increase with increasing O2 pressure.

Measurement of the Intramolecular Hydrogen-Shift Rate Coefficient for the CH3SCH2OO Radical between 314 and 433 K.

The intramolecular hydrogen-shift rate coefficient of the CH3SCH2O2 (methylthiomethylperoxy, MSP) radical, a product formed in the oxidation of dimethyl sulfide (DMS), was measured using a pulsed laser photolysis flow tube reactor coupled to a high-resolution time-of-flight chemical ionization mass spectrometer that measured the formation of the DMS degradation end product HOOCH2SCHO (hydroperoxymethyl thioformate). Measurements performed over the temperature range of 314-433 K yielded a hydrogen-shift rate coefficient of k1(T) = (2.39 ± 0.7) × 109 exp(-(7278 ± 99)/T) s-1 Arrhenius expression and a value extrapolated to 298 K of 0.06 s-1. The potential energy surface and the rate coefficient have also been theoretically investigated using density functional theory at the M06-2X/aug-cc-pVTZ level combined with approximate CCSD(T)/CBS energies yielding k1(273-433 K) = 2.4 × 1011 × exp(-8782/T) s-1 and k1(298 K) = 0.037 s-1 in fair agreement with the experimental results. Present results are compared with the previously reported values of k1(293-298 K).

Global airborne sampling reveals a previously unobserved dimethyl sulfide oxidation mechanism in the marine atmosphere.

Dimethyl sulfide (DMS), emitted from the oceans, is the most abundant biological source of sulfur to the marine atmosphere. Atmospheric DMS is oxidized to condensable products that form secondary aerosols that affect Earth's radiative balance by scattering solar radiation and serving as cloud condensation nuclei. We report the atmospheric discovery of a previously unquantified DMS oxidation product, hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), identified through global-scale airborne observations that demonstrate it to be a major reservoir of marine sulfur. Observationally constrained model results show that more than 30% of oceanic DMS emitted to the atmosphere forms HPMTF. Coincident particle measurements suggest a strong link between HPMTF concentration and new particle formation and growth. Analyses of these observations show that HPMTF chemistry must be included in atmospheric models to improve representation of key linkages between the biogeochemistry of the ocean, marine aerosol formation and growth, and their combined effects on climate.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments.

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.

The reaction of fluorine atoms with methanol: yield of CH3O/CH2OH and rate constant of the reactions CH3O + CH3O and CH3O + HO2.

Xenondifluoride, XeF2, has been photolysed in the presence of methanol, CH3OH. Two reaction pathways are possible: F + CH3OH → CH2OH + HF and F + CH3OH → CH3O + HF. Both products, CH2OH and CH3O, will be converted to HO2 in the presence of O2. The rate constants for the reaction of both radicals with O2 differ by more than 3 orders of magnitude, which allows an unequivocal distinction between the two reactions when measuring HO2 concentrations in the presence of different O2 concentrations. The following yields have then been determined from time-resolved HO2 profiles: φCH2OH = (0.497 ± 0.013) and φCH3O = (0.503 ± 0.013). Experiments under low O2 concentrations lead to reaction mixtures containing nearly equal amounts of HO2 (converted from the first reaction) and CH3O (from the second reaction). The subsequent HO2 decays are very sensitive to the rate constants of the reaction between these two radicals and the following rate constants have been obtained: k(CH3O + CH3O) = (7.0 ± 1.4) × 10-11 cm3 s-1 and k(CH3O + HO2) = (1.1 ± 0.2) × 10-10 cm3 s-1. The latter reaction has also been theoretically investigated on the CCSD(T)//M06-2X/aug-cc-pVTZ level of theory and CH3OH + O2 have been identified as the main products. Using µVTST, a virtually pressure independent rate constant of k(CH3O + HO2) = 4.7 × 10-11 cm3 s-1 has been obtained, in good agreement with the experiment.

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.

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.

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.

Rate Constant of the Reaction between CH3O2 Radicals and OH Radicals Revisited.

The reaction between CH3O2 and OH radicals has been studied in a laser photolysis cell using the reaction of F atoms with CH4 and H2O for the simultaneous generation of both radicals, with F atoms generated through 248 nm photolysis of XeF2. An experimental setup combining cw-Cavity Ring Down Spectroscopy (cw-CRDS) and high repetition rate laser-induced fluorescence (LIF) to a laser photolysis cell has been used. The absolute concentration of CH3O2 was measured by cw-CRDS, while the relative concentration of OH(v = 0) radicals was determined by LIF. To remove dubiety from the quantification of CH3O2 by cw-CRDS in the near-infrared, its absorption cross section has been determined at 7489.16 cm-1 using two different methods. A rate constant of k1 = (1.60 ± 0.4) × 10-10 cm3 s-1 has been determined at 295 K, nearly a factor of 2 lower than an earlier determination from our group ((2.8 ± 1.4) × 10-10 cm3 s-1) using CH3I photolysis as a precursor. Quenching of electronically excited I atoms (from CH3I photolysis) in collision with OH(v = 0) is suspected to be responsible for a bias in the earlier, fast rate constant.

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.