Rate coefficients for the reactions of OH with butanols from 298 K to temperatures relevant for low‐temperature combustion

Rate coefficients for the reactions of OH with n, s, and iso‐butanol have been measured over the temperature range 298 to ∼650 K. The rate coefficients display significant curvature over this temperature range and bridge the gap between previous low‐temperature measurements with a negative temperature dependence and higher temperature shock tube measurements that have a positive temperature dependence. In combination with literature data, the following parameterizations are recommended: k1,OH + n‐butanol(T) = (3.8 ± 10.4) × 10−19T2.48 ± 0.37exp ((840 ± 161)/T) cm3 molecule−1 s−1 k2,OH + s‐butanol(T) = (3.5 ± 3.0) × 10−20T2.76 ± 0.12exp ((1085 ± 55)/T) cm3 molecule−1 s−1 k3,OH + i‐butanol(T) = (5.1 ± 5.3) × 10−20T2.72 ± 0.14exp ((1059 ± 66)/T) cm3 molecule−1 s−1 k4,OH + t‐butanol(T) = (8.8 ± 10.4) × 10−22T3.24 ± 0.15exp ((711 ± 83)/T) cm3 molecule−1 s−1 Comparison of the current data with the higher shock tube measurements suggests that at temperatures of ∼1000 K, the OH yields, primarily from decomposition of β‐hydroxyperoxy radicals, are ∼0.3 (n‐butanol), ∼0.3 (s‐butanol) and ∼0.2 (iso‐butanol) with β‐hydroxyperoxy decompositions generating OH, and a butene as the main products. The data suggest that decomposition of β‐hydroxyperoxy radicals predominantly occurs via OH elimination

A new instrument for time-resolved measurement of HO2 radicals

OH and HO2 radicals are closely coupled in the atmospheric oxidation and combustion of volatile organic compounds (VOCs). Simultaneous measurement of HO2 yields and OH kinetics can provide the ability to assign site-specific rate coefficients that are important for understanding the oxidation mechanisms of VOCs. By coupling a fluorescence assay by gaseous expansion (FAGE) laser-induced fluorescence (LIF) detection system for OH and HO2 with a high-pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-1st-order loss processes up to ∼100 000 s−1 and to determine HO2 yields via time-resolved measurements. This time resolution allows discrimination between primary HO2 from the target reaction and secondary production from side reactions. The apparatus was characterized by measuring yields from the reactions of OH with H2O2 (1:1 link between OH and HO2), with C2H4∕O2 (where secondary chemistry can generate HO2), with C2H6∕O2 (where there should be zero HO2 yield), and with CH3OH∕O2 (where there is a well-defined HO2 yield). As an application of the new instrument, the reaction of OH with n-butanol has been studied at 293 and 616 K. The bimolecular rate coefficient at 293 K, (9.24±0.21)×10−12 cm3 molec.−1 s−1, is in good agreement with recent literature, verifying that this instrument can measure accurate OH kinetics. At 616 K the regeneration of OH in the absence of O2, from the decomposition of the β-hydroxy radical, was observed, which allowed the determination of the fraction of OH reacting at the β site (0.23±0.04). Direct observation of the HO2 product in the presence of oxygen has allowed the assignment of the α-branching fractions (0.57±0.06) at 293 K and (0.54±0.04) at 616 K, again in good agreement with recent literature; branching ratios are key to modelling the ignition delay times of this potential “drop-in” biofuel

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures.

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by approximately a factor of 18 between 293 and 54 K for ethanol, and by ∼10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provides evidence for two reaction channels: a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH-alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol

The Gas Consumption History to z ~ 4

Using the observations of the star formation rate and HI densities to z ~ 4, with measurements of the Molecular Gas Depletion Rate (MGDR) and local density of H_2 at z = 0, we derive the history of the gas consumption by star formation to z ~ 4. We find that closed-box models in which H_2 is not replenished by HI require improbably large increases in rho(H_2) and a decrease in the MGDR with lookback time that is inconsistent with observations. Allowing the H_2 used in star formation to be replenished by HI does not alleviate the problem because observations show that there is very little evolution of rho(HI) from z = 0 to z = 4. We show that to be consistent with observational constraints, star formation on cosmic timescales must be fueled by intergalactic ionized gas, which may come from either accretion of gas through cold (but ionized) flows or from ionized gas associated with accretion of dark matter halos. We constrain the rate at which the extraglactic ionized gas must be converted into HI and ultimately into H_2. The ionized gas inflow rate roughly traces the SFRD: about 1 - 2 x 10^8 M_sun Gyr^-1 Mpc^-3 from z ~ 1 - 4, decreasing by about an order of magnitude from z=1 to z=0 with details depending largely on MGDR(t). All models considered require the volume averaged density of rho(H_2) to increase by a factor of 1.5 - 10 to z ~ 1.5 over the currently measured value. Because the molecular gas must reside in galaxies, it implies that galaxies at high z must, on average, be more molecule rich than they are at the present epoch, which is consistent with observations. These quantitative results, derived solely from observations, agree well with cosmological simulations.Comment: 11 pages, 6 figures. Accepted for publication in the Astrophysical Journal

OH Production from the Photolysis of Isoprene-derived Peroxy Radicals: Cross-sections, quantum yields and atmospheric implications

In environments with high concentrations of biogenic volatile organic compounds and low concentrations of nitrogen oxides (NOx = NO + NO2), significant discrepancies have been found between measured and modeled concentrations of hydroxyl radical (OH). The photolysis of peroxy radicals from isoprene (HO-Iso-O2) in the near ultraviolet represents a potential source of OH in these environments, yet has not been considered in atmospheric models. This paper presents measurements of the absorption cross-sections for OH formation (σRO2,OH) from the photolysis of HO-Iso-O2 at wavelengths from 310–362.5 nm via direct observation by laser-induced fluorescence of the additional OH produced following laser photolysis of HO-Iso-O2. Values of σRO2,OH for HO-Iso-O2 ranged from (6.0 ± 1.6) × 10-20 cm2 molecule-1 at 310 nm to (0.5 ± 0.15) × 10-20 cm2 molecule-1 at 362.5 nm. OH photodissociation yields from HO-Iso-O2 photolysis, ϕOH,RO2, were determined via comparison of the measured values of σRO2,OH to the total absorption cross-sections for HO-Iso-O2 (σRO2), which were obtained using a newly-constructed spectrometer. ϕOH,RO2 was determined to be 0.13 ± 0.037 at wavelengths from 310–362.5 nm. To determine the impact of HO-Iso-O2 photolysis on atmospheric OH concentrations, a modeling case-study for a high-isoprene, low-NOx environment (namely, the 2008 Oxidant and Particle Photochemical Processes above a South-East Asian Tropical Rainforest (OP-3) field campaign, conducted in Borneo) was undertaken using the detailed Master Chemical Mechanism. The model calculated that the inclusion of HO-Iso-O2 photolysis in the model had increased the OH concentration by only 1% on average from 10:00–16:00 local time. Thus, HO-Iso-O2 photolysis alone is insufficient to resolve the discrepancy seen between measured OH concentrations and those predicted by atmospheric chemistry models in such environments

On the mechanism of iodine oxide particle formation

The formation of atmospherically relevant iodine oxides IxO y (x = 1,...,3, y = 1,...,7) has been studied experimentally using time-of-flight mass spectrometry combined with a soft ionisation source, complemented with ab initio electronic structure calculations of ionisation potentials and bond energies at a high level of theory presented in detail in the accompanying paper (Galvez et al., 2013). For the first time, direct experimental evidence of the I2Oy (y = 1,...,5) molecules in the gas phase has been obtained. These chemical species are observed alongside their precursors (IO and OIO) in experiments where large amounts of aerosol are also generated. The measured relative concentrations of the I xOy molecules and their dependence on ozone concentration have been investigated by using chemical modelling and rate theory calculations. It is concluded that I2O4 is the most plausible candidate to initiate nucleation, while the contribution of I2O5 in the initial steps is likely to be marginal. The absence of large I 3Oy (y = 3,...,6) peaks in the mass spectra and the high stability of the I2O4-I2O4 dimer indicate that dimerisation of I2O4 is the key step in iodine oxide particle nucleation

Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm

Iodine oxides (IxOy) play an important role in the atmospheric chemistry of iodine. They are initiators of new particle formation events in the coastal and polar boundary layers and act as iodine reservoirs in tropospheric ozone-depleting chemical cycles. Despite the importance of the aforementioned processes, the photochemistry of these molecules has not been studied in detail previously. Here, we report the first determination of the absorption cross sections of IxOy, x=2, 3, 5, y=1–12 at λ=355 nm by combining pulsed laser photolysis of I2∕O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration. The oxides selected for absorption cross-section determinations are those presenting the strongest signals in the mass spectra, where signals containing four iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, σ355nm,IO=(1.2±0.1) ×10−18 cm2, which is found to be in good agreement with the most recent literature. The results obtained are σ355nm,I2O3<5×10−19 cm2 molec.−1, σ355nm,I2O4= (3.9±1.2)×10−18 cm2 molec.−1, σ355nm,I3O6= (6.1±1.6)×10−18 cm2 molec.−1, σ355nm,I3O7= (5.3±1.4)×10−18 cm2 molec.−1, and σ355nm,I5O12= (9.8±1.0)×10−18 cm2 molec.−1. Photodepletion at λ=532 nm was only observed for OIO, which enabled determination of upper limits for the absorption cross sections of IxOy at 532 nm using OIO as an actinometer. These measurements are supplemented with ab initio calculations of electronic spectra in order to estimate atmospheric photolysis rates J(IxOy). Our results confirm a high J(IxOy) scenario where IxOy is efficiently removed during daytime, implying enhanced iodine-driven ozone depletion and hindering iodine particle formation. Possible I2O3 and I2O4 photolysis products are discussed, including IO3, which may be a precursor to iodic acid (HIO3) in the presence of HO2

Temperature and Pressure Dependent Kinetics of QOOH Decomposition and Reaction with O2: Experimental and Theoretical Investigations of QOOH Radicals Derived from Cl + (CH3)3COOH

QOOH radicals are key species in autoignition, produced by internal isomerisations of RO2 radicals, and are central to chain branching reactions in low temperature combustion. The kinetics of QOOH radical decomposition and reaction with O2 have been determined as a function of temperature and pressure, using observations of OH radical production and decay following H-atom abstraction from tertiary-butyl hydroperoxide ((CH3)3COOH) by Cl atoms to produce QOOH (.CH2(CH3)2COOH) radicals. The kinetics of QOOH decomposition have been investigated as a function of temperature (251 to 298 K), and pressure (10 to 350 Torr), in helium and nitrogen bath gases, and those of the reaction between QOOH and O2 have been investigated as a function of temperature (251 to 304 K), and pressure (10 to 100 Torr) in He and N2. Decomposition of the QOOH radicals was observed to display temperature and pressure dependence, with a barrier height for decomposition of (44.7 ± 4.0) kJ mol-1 determined by master equation fitting to the experimental data. The rate coefficient for the reaction between QOOH and O2 was determined to be (5.6 ± 1.7) × 10-13 cm3 s-1 at 298 K, with no significant dependence on pressure, and can be described by the Arrhenius parameters A = (7.3 ± 6.8) × 10-14 cm3 s-1 and Ea = -(5.4 ± 2.1) kJ mol-1 in the temperature range 251 to 304 K. This work represents the first measurements of any QOOH radical kinetics as a function of temperature and pressure

Measurements of Low Temperature Rate Coefficients for the Reaction of CH with CH₂O and Application to Dark Cloud and AGB Stellar Wind Models

Rate coefficients have been measured for the reaction of CH radicals with formaldehyde, CH₂O, over the temperature range of 31–133 K using a pulsed Laval nozzle apparatus combined with pulsed laser photolysis and laser-induced fluorescence spectroscopy. The rate coefficients are very large and display a distinct decrease with decreasing temperature below 70 K, although classical collision rate theory fails to reproduce this temperature dependence. The measured rate coefficients have been parameterized and used as input for astrochemical models for both dark cloud and Asymptotic Giant Branch stellar outflow scenarios. The models predict a distinct change (up to a factor of two) in the abundance of ketene, H₂CCO, which is the major expected molecular product of the CH + CH₂O reaction

A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides

Emitted from the oceans, iodine-bearing molecules are ubiquitous in the atmosphere and a source of new atmospheric aerosol particles of potentially global significance. However, its inclusion in atmospheric models is hindered by a lack of understanding of the first steps of the photochemical gas-to-particle conversion mechanism. Our laboratory results show that under a high humidity and low HOx regime, the recently proposed nucleating molecule (iodic acid, HOIO2) does not form rapidly enough, and gas-to-particle conversion proceeds by clustering of iodine oxides (IxOy), albeit at slower rates than under dryer conditions. Moreover, we show experimentally that gas-phase HOIO2 is not necessary for the formation of HOIO2-containing particles. These insights help to explain new particle formation in the relatively dry polar regions and, more generally, provide for the first time a thermochemically feasible molecular mechanism from ocean iodine emissions to atmospheric particles that is currently missing in model calculations of aerosol radiative forcing