OH Profiles Research Articles

ChemInform Abstract: Rate Coefficients for the Reaction of OH with HONO Between 298 and 373 K.

Rate coefficients, k1, for the reaction OH + HONO H2O + NO2, have been measured over the temperature range 298 to 373 K. The OH radicals were produced by 266 nm laser photolysis of O3 in the presence of a large excess of H2O vapor. The temporal profiles of OH were measured under pseudo-first-order conditions, in an excess of HONO, using time resolved laser induced fluorescence. The measured rate coefficient exhibits a slight negative temperature dependence, with k1 = (2.8 ± 1.3) × 10−12 exp((260 ± 140)/T) cm3 molecule−1 s−1. The measured values of k1 are compared with previous determinations and the atmospheric implications of our findings are discussed.

Measurements of the reaction of OH with n-butanol at high-temperatures

We present the first high-temperature measurements of the reaction rate of OH radical with n-butanol using a shock tube and OH laser absorption technique. Measurements were conducted in the temperature range 1017–1269K, for pressures near 2.25atm, and with an initial n-butanol concentration of 147ppm in argon. OH radicals were produced by shock-heating tert-butyl-hydroperoxide, and monitored by laser absorption near 306.7nm. A rate constant for the overall reaction of OH with n-butanol was extracted by matching modeled and measured OH profiles in the reflected-shock region. Current data are in excellent agreement with a recent theoretical calculation.

Turbulent Spray Flames of Acetone and Ethanol Approaching Extinction

This paper presents measurements of mean temperature, axial velocity, turbulence, and droplets fields in pilot-stabilized jet flames of dilute sprays where acetone or ethanol is used as liquid fuel. Laser-induced fluorescence (LIF) is employed to image hydroxyl radical OH which marks the existence of hot regions or reaction zones. Depending on the fuel used, droplets within the flow are delineated by imaging LIF from acetone or Mie scattering. As the jet velocity is increased, the flames gradually approach blow-off in a region further downstream from the stabilizing pilot. It is found that the mean velocity, turbulence, and droplet fields are somewhat similar for both ethanol and acetone flames, and these do not change much with increasing jet velocity, particularly when normalized with the relevant parameters. However, the temperature and reactive fields are varying and undergo departure indicative of nonpremixed to premixed flame behavior depending on the vapor pressure of the fuel and proximity to blow-off. Broad regions of OH as well as breaks in the OH profile marking possible local extinction are observed in the ethanol flames.

Structure of hydrogen bonds in liquid water at subpicosecond time scales

A theory is presented showing how infrared pump–probe spectroscopy can be used to measure subpicosecond variations of the oxygen–oxygen distribution function in liquid water, Δ g OO( R, t). For times t > t c , where t c > 100 − 200 fs is the pump–probe time delay, the profiles of OH stretching bands of HDO/D 2O can be brought into a full coincidence with Δ g OO( R, t). Infrared pump–probe spectroscopy thus plays the role of time-resolved X-ray diffraction in this particular case. This method has many advantages. Not only it permits to obtain structural data concerning the OH⋯ O bonds at subpicosecond time scales, but also is non destructive in its essence; this is particularly important when studying biological water. Finally, pump–probe spectroscopy is very economical as compared with time-resolved X-ray techniques. Although theoretical, the present work may have an important impact in experimental biology.

Constraining the concentration of the hydroxyl radical in a stratocumulus-topped marine boundary layer from sea-to-air eddy covariance flux measurements of dimethylsulfide

Abstract. The hydroxyl radical (OH) is an important oxidant in the troposphere due to its high reactivity and relative abundance. Measuring the concentration of OH in situ, however, is technically challenging. Here we present a simple method of estimating an OH-equivalent oxidant concentration ("effective OH") in the marine boundary layer (MBL) from the mass balance of dimethylsulfide (DMS). We use shipboard eddy covariance measurements of the sea-to-air DMS flux from the Vamos Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) in October and November of 2008. The persistent stratocumulus cloud-cover off the west coast of South America and the associated strong inversion between MBL and the free troposphere (FT) greatly simplify the dynamics in this region and make our budget estimate possible. From the observed diurnal cycle in DMS concentration, the nighttime entrainment velocity at the inversion is estimated to be 4 mm s−1. We calculate 1.4(±0.2)×106 OH molecules cm−3 from the DMS budget, which represents a monthly effective concentration and is well within the range of previous estimates. Furthermore, when linearly proportioned according to the intensity of solar flux, the resultant diel OH profile, together with DMS surface and entrainment fluxes, enables us to accurately replicate the observed diurnal cycle in DMS (correlation coefficient over 0.9). The nitrate radical (NO3) is found to have little contribution to DMS oxidation during VOCALS-REx. An upper limit estimate of 1 pptv of bromine oxide radical (BrO) would account for 30% of DMS oxidation and lower the OH concentration to 1.0)×106 OH molecules cm−3. Our effective OH estimate includes the oxidation of DMS by such radicals.

Development of Laser Ionization Mass Spectrometer for detection of unstable species in flames

Results are presented here on the detection of unstable species formed in flames using a novel Laser Ionization Mass Spectrometry (LIMS) system. The chemical species generated in atmospheric pressure flames were directly introduced into a mass spectrometer operated at high vacuum conditions using a newly developed interface, in which the ionization was induced by laser irradiation. Optimum conditions for the experimental conditions were investigated in order to obtain mass spectra with adequate signal to noise (S/N) ratios. In addition, the distribution profiles of various species in the flame was measured and visualized. The distribution of OH profiles was also measured under the same conditions using Planer Laser Induced Fluorescence (PLIF) diagnostics. The results showed that the profiles of OH distribution by LIMS were in good agreement with those obtained using PLIF diagnostics.

South Pole Antarctica observations and modeling results: New insights on HO x radical and sulfur chemistry

Measurements of OH, H 2SO 4, and MSA at South Pole (SP) Antarctica were recorded as a part of the 2003 Antarctic Chemistry Investigation (ANTCI 2003). The time period 22 November, 2003–2 January, 2004 provided a unique opportunity to observe atmospheric chemistry at SP under both natural conditions as well as those uniquely defined by a solar eclipse event. Results under natural solar conditions generally confirmed those reported previously in the year 2000. In both years the major chemical driver leading to large scale fluctuations in OH was shifts in the concentration levels of NO. Like in 2000, however, the 2003 observational data were systematically lower than model predictions. This can be interpreted as indicating that the model mechanism is still missing a significant HO x sink reaction(s); or, alternatively, that the OH calibration source may have problems. Still a final possibility could involve the integrity of the OH sampling scheme which involved a fixed building site. As expected, during the peak in the solar eclipse both NO and OH showed large decreases in their respective concentrations. Interestingly, the observational OH profile could only be approximated by the model mechanism upon adding an additional HO x radical source in the form of snow emissions of CH 2O and/or H 2O 2. This would lead one to think that either CH 2O and/or H 2O 2 snow emissions represent a significant HO x radical source under summertime conditions at SP. Observations of H 2SO 4 and MSA revealed both species to be present at very low concentrations (e.g., 5 × 10 5 and 1 × 10 5 molec cm −3, respectively), but similar to those reported in 2000. The first measurements of SO 2 at SP demonstrated a close coupling with the oxidation product H 2SO 4. The observed low concentrations of MSA appear to be counter to the most recent thinking by glacio-chemists who have suggested that the plateau's lower atmosphere should have elevated levels of MSA. We speculate here that the absence of MSA may reflect efficient atmospheric removal mechanisms for this species involving either dynamical and/or chemical processes.

Numerical simulation of the effect of the addition of NO and NO2 on a rich hydrogen flame using the tracer method

The effect of the addition of nitric oxides (NO and NO2) on rich hydrogen-air flames was studied using the tracer method in numerical simulation. It is shown that the effects of these additives are not similar. Both oxides promote the formation of OH and H2O in the low-temperature zone of the front. The addition of NO reduces the first maximum of the OH profile and the burning velocity. The addition of NO2 increases the first maximum of the OH profile and does not change the burning velocity.

Relation between the OH stretching frequency and the OO distance in time-resolved infrared spectroscopy of hydrogen bonding

A non-empirical theory is presented to study the relation between the OH stretching frequency and the OO distance in ultrafast laser spectra of water. Diluted solutions HDO / D 2 O rather than pure H 2 O were considered to switch off resonant vibrational interactions between water molecules; the local structure of water as well as the OO distribution functions remain unchanged in this substitution. Only times superior to 100 – 200 fs are considered to avoid perturbations generated by collisions between water molecules. It is then shown that the Novak–Mikenda type relations between the OH stretching frequency and the OO distance largely survive when going from equilibrium to laser perturbed non-equilibrium systems. It is also shown that temporally varying infrared pump–probe profiles of OH stretching bands in HDO / D 2 O closely parallel the oxygen–oxygen distribution functions of these solutions. Infrared pump–probe spectroscopy can thus replace time-resolved X-ray diffraction in this particular case.

Formally direct pathways and low-temperature chain branching in hydrocarbon autoignition: the cyclohexyl + O2 reaction at high pressure

The OH concentration in the Cl-initiated oxidation of cyclohexane has been measured between 6.5-20.3 bar and in the 586-828 K temperature range by a pulsed-laser photolytic initiation-laser-induced fluorescence method. The experimental OH profiles are modeled by using a master-equation-based kinetic model as well as a comprehensive literature mechanism. Below approximately 700 K OH formation takes place on two distinct time-scales, one on the order of microseconds and the other over milliseconds. Detailed modeling demonstrates that "formally direct" chemical activation pathways are responsible for the OH formation on short timescales. These results establish that formally direct pathways are surprisingly important even for relatively large molecules at the pressures of practical combustors. It is also shown that remaining discrepancies between model and experiment are attributable to low-temperature chain branching from the addition of the "second oxygen" to hydroperoxycyclohexyl radicals.

Validation of Aura Microwave Limb Sounder OH measurements with Fourier Transform Ultra‐Violet Spectrometer total OH column measurements at Table Mountain, California

The first seasonal and interannual validation of OH measurements from the Aura Microwave Limb Sounder (MLS) has been conducted using ground‐based OH column measurements from the Fourier Transform Ultra‐Violet Spectrometer (FTUVS) over the Jet Propulsion Laboratory's Table Mountain Facility (TMF) during 2004–2007. To compare with FTUVS total column measurements, MLS OH vertical profiles over TMF are integrated to obtain partial OH columns above 21.5 hPa, which covers nearly 90% of the total column. The tropospheric OH and the lower stratopheric OH not measured by MLS are estimated using GEOS (Goddard Earth Observing System)‐Chem and a Harvard 2‐D model implemented within GEOS‐Chem, respectively. A number of field observations and calculations from a photochemical box model are compared to OH profiles from these models to estimate the variability in the lower atmospheric OH and thus the uncertainty in the combined total OH columns from MLS and models. In general, the combined total OH columns agree extremely well with TMF total OH columns, especially during seasons with high OH. In winter with low OH, the combined columns are often higher than TMF measurements. A slightly weaker seasonal variation is observed by MLS relative to TMF. OH columns from TMF and the combined total columns from MLS and models are highly correlated, resulting in a mean slope of 0.969 with a statistically insignificant intercept. This study therefore suggests that column abundances derived from MLS vertical profiles have been validated to within the mutual systematic uncertainties of the MLS and FTUVS measurements.

One-step reduced kinetics for lean hydrogen–air deflagration

A short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown to provide good predictions of hydrogen–air lean-flame burning velocities. This mechanism is further simplified by noting that over a range of conditions of practical interest, near the lean flammability limit all reaction intermediaries have small concentrations in the important thin reaction zone that controls the hydrogen–air laminar burning velocity and therefore follow a steady state approximation, while the main species react according to the global irreversible reaction 2H 2 + O 2 → 2H 2O. An explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation is derived from the seven-step detailed mechanism, for application near the flammability limit. The one-step results are used to calculate flammability limits and burning velocities of planar deflagrations. Furthermore, implications concerning radical profiles in the deflagration and reasons for the success of the approximations are clarified. It is also demonstrated that adding only two irreversible direct recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible shuffle reaction, improves predictions of O and OH profiles. The new reduced-chemistry descriptions can be useful for both analytical and computational studies of lean hydrogen–air flames, decreasing required computation times.

Validation of Aura Microwave Limb Sounder OH and HO2 measurements

The Microwave Limb Sounder (MLS) instrument on the Aura satellite obtains global measurements of both OH and HO2 radicals. This paper describes the precision and systematic errors of the MLS version v2.2 of the retrieval software. Estimated systematic errors are less than 8% for OH over 32–0.003 hPa and HO2 over 6.8–0.21 hPa. Comparison of measurements from MLS OH and HO2 profiles and three balloon‐based instruments show good agreement among themselves and with a photochemical model using standard chemistry (i.e., recommended rate constants). Similarly, good agreement is seen between column OH found by integrating satellite profiles and ground‐based measurements of column OH. The agreement between measured and modeled OH and HO2 is improved following perturbations to the rate constants O + OH and OH + HO2 that are within the recommended uncertainties. Measurements of OH obtained a decade ago by the Middle Atmosphere High‐Resolution Spectrograph Investigation (MAHRSI) are smaller than MLS measurements by 20% at 70 km, are similar to MLS data near 50 km, and are 50% larger than MLS observations near 42 km. The MLS and MAHRSI measurements of OH overlap at the limit of their respective 2‐σ uncertainties. Most importantly, we find the shape of the OH profile measured by MLS is simulated well using standard chemistry.

6.7 GHz methanol absorption toward the Seyfert 2 galaxy NGC 3079

The detection of the 6.7 GHz line of methanol (CH3OH) is reported for the first time toward an object beyond the Magellanic Clouds. Using the Effelsberg 100 m telescope, two absorption features were identified toward the Seyfert 2 galaxy NGC 3079. Both components probably originated on lines-of-sight toward the central region, presumably absorbing the radio continuum of the nuclear sources A, B, and E of NGC 3079. One absorption feature, at the systemic velocity, is narrow and may arise from gas not related to the nuclear environment of the galaxy. The weaker blue-shifted component is wider and may trace outflowing gas. Total A-type CH3OH column densities are estimated to be between a few times 10 13 and a few times 10 15 cm −2 . Because of a highly frequencydependent continuum background, the overall similarity of Hi, OH, and CH3OH absorption profiles hints at molecular clouds that cover the entire area occupied by the nuclear radio continuum sources (∼ 4p c).

Detailed chemical kinetic oxidation mechanism for a biodiesel surrogate

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of methyl decanoate, a surrogate for biodiesel fuels. This model has been built by following the rules established by Curran and co-workers for the oxidation of n-heptane and it includes all the reactions known to be pertinent to both low and high temperatures. Computed results have been compared with methyl decanoate experiments in an engine and oxidation of rapeseed oil methyl esters in a jet-stirred reactor. An important feature of this mechanism is its ability to reproduce the early formation of carbon dioxide that is unique to biofuels and due to the presence of the ester group in the reactant. The model also predicts ignition delay times and OH profiles very close to observed values in shock tube experiments fueled by n-decane. These model capabilities indicate that large n-alkanes can be good surrogates for large methyl esters and biodiesel fuels to predict overall reactivity, but some kinetic details, including early CO 2 production from biodiesel fuels, can be predicted only by a detailed kinetic mechanism for a true methyl ester fuel. The present methyl decanoate mechanism provides a realistic kinetic tool for simulation of biodiesel fuels.

On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O<sub>3</sub>, HO<sub>x</sub>, NO<sub>x</sub> and the Hg lifetime

Abstract. A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer (BL). The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of ~1010 molecule cm−2 s−1 from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the Antarctic boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I2O2, I2O3, I2O4 and I2O5), and rapid recycling of HOI and INO3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I2O5 in the lowest 10 m of the boundary layer. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O3 from the free troposphere appears to be required to account for the observation that on occasion there is little O3 depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of OH and HO2 and the NO2/NO ratio. The average Hg0 lifetime against oxidation is also predicted to be about 10 h during springtime. An important result from the model is that very large fluxes of iodine precursors into the boundary layer are required to account for the observed levels of IO. The mechanisms which cause these emissions are unknown. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of approximately 4x10(16) molecule cm(-3) in N2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k2=(3.22+/-0.46)x10(-11) and (2.12+/-0.12)x10(-11) cm3 molecule(-1) s(-1) at T=69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633-5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k3<1x10(-12) cm3 molecule(-1) s(-1), consistent with the presence of a small activation barrier of approximately 5 kJ mol(-1) between the reagents and the OH-C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k4=(5.08+/-0.65), (5.02+/-1.11) and (3.11+/-0.09)x10(-12) cm3 molecule(-1) s(-1) were obtained at T=69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k1=(7.8+/-2.5)x10(-11) cm3 molecule(-1) s(-1) was obtained for the reaction of OH with t-butyl-hydroperoxide at T=86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors.

Pressure dependence of NO formation in laminar fuel-rich premixed CH 4/air flames

Effects of pressure on NO formation in CH 4/air flames at a fixed equivalence ratio of 1.3 are investigated. The axial profiles of temperature, OH, CH, and NO mole fractions are measured using laser-induced fluorescence and compared with one-dimensional flame calculations. The measured and calculated temperature, CH, and NO profiles in free flames are observed to vary upon increasing the pressure from 40 to 75 Torr, following a scaling law derived for a chemical mechanism containing only second-order reactions. At pressures 300–760 Torr, the measurements and calculations in burner-stabilized flames show increasing flame temperature and NO mole fractions when the mass flux is increased linearly with pressure, while the CH profiles remain unchanged. The observed deviation from the scaling law in the temperature profiles arises from the increasing contribution of three-body reactions to the flame front propagation velocity, leading to a decrease in the degree of burner stabilization. The deviation from the pressure scaling law for the NO mole fractions is due to the temperature dependence of the rate coefficient for the reaction between CH and N 2 and the fact that the temperature profiles themselves do not scale. In contrast, the surprisingly good scaling of the CH mole fractions with pressure indicates the dominant role of two-body reactions participating in the chain of chemical reactions leading to CH formation. The calculations using GRI-Mech 3.0 substantially overpredict (up to 50%) the measured nitric oxide concentrations for all pressures studied. The observed differences in the NO mole fraction may be addressed by improving the CH prediction.

Measurements and Automated Mechanism Generation Modeling of OH Production in Photolytically Initiated Oxidation of the Neopentyl Radical

Production of OH in the reaction of the neopentyl radical with O2 has been measured by a laser photolysis/cw absorption method for various pressures and oxygen concentrations at 673, 700, and 725 K. The MIT Reaction Mechanism Generator (RMG) was used to automatically generate a model for this system, and the predicted OH concentration profiles are compared to present and literature experimental results. Several reactions significantly affect the OH profile. The experimental data provide useful constraints on the rate coefficient for the formally direct chemical activation reaction of neopentyl radical with O2 to form OH (CH3)3CCH2 + O2 --> OH + 3,3-dimethyloxetane (Rxn 1) At 673 K and 60 Torr, log k(1) (cm(3) molecule(-1) s(-1)) = -13.7 +/- 0.5. Absolute absorbance measurements on OH and I indicate that the branching ratio for R + O2 to OH is about 0.03 under these conditions. The data suggest that the ab initio neopentyl + O2 potential energy surface of Sun and Bozzelli is accurate to within 2 kcal mol(-1).

Extended OH(1720 MHz) maser emission from supernova remnants

AbstractCompact OH(1720 MHz) masers have proven to be excellent signposts for the interaction of supernova remnants with adjacent molecular clouds. Less appreciated has been the weak, extended OH(1720 MHz) emission which accompanies strong compact maser sources. Recent single-dish and interferometric observations reveal the majority of maser-emitting supernova remnants(SNRs) have accompanying regions of extended maser emission. Enhanced OH abundance created by the passing shock is observed both as maser emission and absorption against the strong background of the remnant. Modeling the observed OH profiles gives an estimate of the physical conditions in which weak, extended maser emission arises. I will discuss how we can realize the utility of this extended maser emission, particularly the potential to measure the strength of the post-shock magnetic field via Zeeman splitting over these large-scales.