OH Radical Concentration Research Articles

Rate constants for the gas‐phase reaction of CF3CF2CF2CF2CF2CHF2 with OH radicals at 250–430 K

AbstractThe rate constants k1 for the reaction of CF3CF2CF2CF2CF2CHF2 with OH radicals were determined by using both absolute and relative rate methods. The absolute rate constants were measured at 250–430 K using the flash photolysis–laser‐induced fluorescence (FP‐LIF) technique and the laser photolysis–laser‐induced fluorescence (LP‐LIF) technique to monitor the OH radical concentration. The relative rate constants were measured at 253–328 K in an 11.5‐dm3 reaction chamber with either CHF2Cl or CH2FCF3 as a reference compound. OH radicals were produced by UV photolysis of an O3–H2O–He mixture at an initial pressure of 200 Torr. Ozone was continuously introduced into the reaction chamber during the UV irradiation. The k1 (298 K) values determined by the absolute method were (1.69 ± 0.07) × 10−15 cm3 molecule−1 s−1 (FP‐LIF method) and (1.72 ± 0.07) × 10−15 cm3 molecule−1 s−1 (LP‐LIF method), whereas the K1 (298 K) values determined by the relative method were (1.87 ± 0.11) × 10−15 cm3 molecule−1 s−1 (CHF2Cl reference) and (2.12 ± 0.11) × 10−15 cm3 molecule−1 s−1 (CH2FCF3 reference). These data are in agreement with each other within the estimated experimental uncertainties. The Arrhenius rate constant determined from the kinetic data was K1 = (4.71 ± 0.94) × 10−13 exp[−(1630 ± 80)/T] cm3 molecule−1 s−1. Using kinetic data for the reaction of tropospheric CH3CCl3 with OH radicals [k1 (272 K) = 6.0 × 10−15 cm3 molecule−1 s−1, tropospheric lifetime of CH3CCl3 = 6.0 years], we estimated the tropospheric lifetime of CF3CF2CF2CF2CF2CHF2 through reaction with OH radicals to be 31 years. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 26–33, 2004

Determination of the Rate Constant for the Radical−Radical Reaction CN(X2Σ+) + OH(X2Π) at 292 K

The rate constant for the reaction of the cyano radical, CN(X{sup 2}{Sigma}), with the hydroxyl radical, OH(X{sup 2}{Pi}), has been measured to be (1.4 {+-} 0.48) x 10{sup -10} cm{sup 3} molecule{sup -1} s{sup -1} at a temperature of 292 {+-} 2 K. The error estimate includes both systematic and random error at the level of one standard deviation. The measurements were carried out by simultaneously monitoring the temporal dependencies of the CN and OH radical concentrations on the same photolytic laser pulse that created the transient species. The rate constant was determined by two independent methods of data analysis: one based on the simulation of CN and OH concentration profiles using a detailed kinetic mechanism and the other based on a new integrated-profiles analysis (Int. J. Chem. Kinet. 1998, 30, 47).

Hohenpeissenberg Photochemical Experiment (HOPE 2000): Measurements and photostationary state calculations of OH and peroxy radicals

Abstract. Measurements of OH, total peroxy radicals, non-methane hydrocarbons (NMHCs) and various other trace gases were made at the Meteorological Observatory Hohenpeissenberg in June 2000. The data from an intensive measurement period characterised by high solar insolation (18-21 June) are analysed. The maximum midday OH concentration ranged between 4.5x106 molecules cm-3 and 7.4x106 molecules cm-3. The maximum total ROx (ROx =OH+RO+HO2+RO2) mixing ratio increased from about 55 pptv on 18 June to nearly 70 pptv on 20 and 21 June. A total of 64 NMHCs, including isoprene and monoterpenes, were measured every 1 to 6 hours. The oxidation rate of the NMHCs by OH was calculated and reached a total of over 14x106 molecules cm-3 s-1 on two days. A simple photostationary state balance model was used to simulate the ambient OH and peroxy radical concentrations with the measured data as input. This approach was able to reproduce the main features of the diurnal profiles of both OH and peroxy radicals. The balance equations were used to test the effect of the assumptions made in this model. The results proved to be most sensitive to assumptions about the impact of unmeasured volatile organic compounds (VOC), e.g. formaldehyde (HCHO), and about the partitioning between HO2 and RO2. The measured OH concentration and peroxy radical mixing ratios were reproduced well by assuming the presence of 3 ppbv HCHO as a proxy for oxygenated hydrocarbons, and a HO2/ RO2 ratio between 1:1 and 1:2. The most important source of OH, and conversely the greatest sink for peroxy radicals, was the recycling of HO2 radicals to OH. This reaction was responsible for the recycling of more than 45x106 molecules cm-3 s-1 on two days. The most important sink for OH, and the largest source of peroxy radicals, was the oxidation of NMHCs, in particular, of isoprene and the monoterpenes.

Multiwavelength ultraviolet absorption spectroscopy of NO and OH radical concentration applied to a high-swirl diesel-like system

Ultraviolet absorption measurements were carried out in an optical research engine by exploiting the emitted plasma kernel of laser-induced breakdown. Temporal evolution and spatial distribution of OH radical and NO absolute concentrations were evaluated by a numerical procedure for retrieving optical data. Spectral measurements were made in an optically accessible external combustion chamber obtained by modifying a single cylinder, four-stroke diesel engine. The experiments were performed using a commercial diesel fuel at two engine speeds varying the ignition delay and fixing the injected fuel amount. In-chamber NO concentrations for each engine operating condition were correlated with those measured at the exhaust by a conventional method.

Hydroxyl radicals in microwave photocatalysis. Enhanced formation of OH radicals probed by ESR techniques in microwave-assisted photocatalysis in aqueous TiO 2 dispersions

Formation of OH radicals during TiO 2 photocatalyzed oxidations driven by simultaneous UV and microwave radiation was probed by the electron spin resonance technique employing a novel setup in which the ESR sample (contained the DMPO spin-trap agent and P-25 TiO 2 or pure anatase or pure rutile in aqueous media) could be irradiated by both UV light and microwave radiation; the microwave generator was coupled through an N connector to the ESR sample cavity and to a photoreactor in which phenol was photooxidized by simultaneous irradiation with UV light and microwaves to assess whether the photooxidative process correlated with the concentration of OH radicals formed.

Oxidation of Tetrachloroethylene in a Bubble Column Photochemical Reactor Applying the UV/H2O2 Technique

AbstractMass transfer of tetrachloroethylene (PCE) gas, followed by a free‐OH radical reaction in the liquid phase, was studied in a bubble column reactor equipped with a UV light source and containing aqueous H2O2 as the reacting medium. Degradation of PCE in the liquid phase was found to follow pseudo‐first‐order kinetics, and the optimal ratio of H2O2/PCE leading to the highest oxidation rate could be successfully obtained from the analysis of the steady‐state OH radical concentration. Absorption of PCE gas was in the kinetic slow regime and could be predicted based on the ratio of the apparent rate constant of PCE liquid degradation to the volumetric mass transfer coefficient (kapp/KLa). The presence of small amounts of PCE in the liquid bulk had positive effect on the mass transfer over the range studied and need to be taken into account.

Prediction of the disinfection and oxidation efficiency of full-scale ozone reactors

The efficiency of a two-step ozonation with regard to disinfection and oxidation of micropollutants; was assessed for a river-water treatment plant (Limmat River, Zurich, Switzerland). The assessment was based on laboratory experiments to estimate transient ozone and OH radical concentrations coupled with hydraulic modelling of the ozone reactors. The laboratory experiments were performed for various temperatures, ozone dosages and pretreatments to mimic the full-scale treatment. The hydraulics were determined by a conservative tracer test. The kinetic data for disinfection and oxidation were taken from the literature. The inactivation of microorganisms (Cryptosporidium parvum oocysts, Bacillus subtilis spores, Giardia lamblia cysts and polio virus) was more critical at low temperature (5degreesC) than at high temperature (20degreesC). For Cryptosporidium parvum oocysts it was calculated to be less than 1 log for the two ozonation steps, and an overall ozone dose of 1.5 mg/l (5degreesC). For the same microorganism it was >3.5-log inactivation at 20degreesC. For other microorganisms, the calculated inactivation was greater than or equal to2 log (5degreesC) with B. subtilis spores being the most resistant. The oxidation of micropollutants was much less affected by variations in the temperature. The fraction of oxidation increased in the following order: between 15 and 25% of tetrachloroethene, 15 and 25% of methyl tertiary butyl ether, 30 and 40% of atrazine, 80 and 90% of p-xylene and between 97 and 100% of trim ethyl benzene were oxidized. For the oxidation of methyl tertiary butyl ether, the formation of degradation products was modelled in the pre-ozonation step. For all investigated treatment conditions the overall bromate formation was less than or equal to2.5 mug/l.

NO x emissions in n-heptane/air partially premixed flames

NO x emissions in n-heptane/air partially premixed flames (PPFs) in a counter-flow configuration have been investigated. The flame is computed using a detailed mechanism that combines the Held’s mechanism for n-heptane and the Li and Williams’ mechanism for NO x. The combined mechanism contains 54 species and 327 reactions. Based on a detailed analysis, dominant mechanisms responsible for NOx formation and destruction in PPFs are found to be thermal, prompt, and reburn mechanisms. The dominant reactions associated with these mechanisms are also identified. The effects of strain rate (a s) and equivalence ratio (φ) on NO x emissions are characterized for conditions in which the flame contains two spatially separated reaction zones; a rich premixed zone on the fuel side and a non-premixed zone on the air side. For most conditions, except for relatively high level of partial premixing, the NO formation rate in the non-premixed zone is significantly higher than that in the rich premixed zone. Within the rich premixed zone, the contribution of thermal NO to total NO x is higher than that of prompt NO, while in the non-premixed zone, the prompt NO is the major contributor. The behavior is related to the transport of acetylene from the rich premixed to the non-premixed zone, and higher concentrations of CH, O, and OH radicals in the latter zone. A notable result in this context is that the existence of CH does not automatically imply that prompt NO will form. The existence of O and OH is also necessary, in addition to CH, to form prompt NO. The relative contributions of thermal and prompt mechanisms to total NO x are generally insensitive to variations in a s, but show strong sensitivity to variations in φ. There is a NO x destruction region sandwiched between the rich premixed and the non-premixed reaction zones. The NO x destruction occurs mainly through the reburn mechanism. The NO x emission index (EINOx) is computed as a function of φ and a s. These results are qualitatively in accord with previous numerical and experimental results for methane-air PPFs.

Measurements of OH and HO2 radical concentrations and photolysis frequencies during BERLIOZ

This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser‐induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope [OH]/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.

New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

AbstractWe have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O3 in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O3/O2 (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53–1.2) × 1011 radicals cm−3] can be produced during the irradiation in continuous, steady‐state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF3 (HFC‐23) with OH radicals, k1. We obtained k1(298 K) = (3.32 ± 0.20) × 10−16 and determined the temperature dependence of k1 to be (0.48 ± 0.13) × 10−12 exp[−(2180 ± 100)/T] cm3 molecule−1 s−1 at 253–328 K using CHF2CF3 (HFC‐125) and CHF2Cl (HCFC‐22) as reference compounds in CHF3–reference–H2O gas mixtures. The value of k1 obtained in this study is in agreement with previous measurements of k1. This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ∼10−16 cm3 molecule−1 s−1, provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O3 interfere the measurement when selecting this technique. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 317–325, 2003

312 微速空気流中に形成される液滴火炎内の化学種計測

An experimental study was performed to clarify the flame structure of burning fuel droplet. Fuel employed was mixture of n-heptane and benzen. Both OH radical and soot concentration distributions were measured along the radial direction passing the front stagnation point of a fuel droplet flame in steady air flow. The laser induced fluorescence technique was applied for measurement of OH radical concentration distributions. The laser light scattering technique was applied for measurement of soot concentration distributions. An Nd : YAG laser-pumped pulsed dye laser with frequency doubling was used for these measurements as the light source. The wavelength of the UV light was adjusted to 283.65nm. The results showed that the soot concentration increased with increasing benzen volume concentration, while the OH radical concentration kept almost constant.

膜リアクタ・デバイス ナノ多孔性酸化チタン膜を用いた光触媒膜型反応

A TiO2 photocatalytic membrane reactor, in which permeation of organic molecules through a membrane and a photocatalytic reaction occur simultaneously, is reviewed for liquid phase as well as gas-phase reactions. The advantages of the system are (1) the forced transport of reactants by convection to the TiO2 membranes, (2) the oxidation reaction on the outer and inner surface of the porous TiO2 membranes where high concentrations of OH radicals would be expected, (3) the potential for obtaining a permeate stream oxidized with OH radicals after a one-pass permeation through the TiO2 membranes, and (4) improved selectivity combined with the molecular sieving mechanism. For the liquid phase reactions of trichloroethylene (TCE), a decomposition ratio in excess of 95% in the permeate stream to that in feed stream was achieved by a TiO2 photocatalytic membrane reactor, when approximately 1 ppm TCE solutions was fed under blacklight irradiation. For the case of gas-phase reactions, photocatalysis with membrane permeation resulted in a large decomposition rate and the formation of a large amount of intermediate products, compared to photocatalysis without membrane permeation.

Combustion Control and Mechanism with Direct Injection of Reaction Suppressors in a Premixed Charge Compression Ignition Engine.

Direct in-cylinder injection of reaction suppressors was effective to successfully control igition timing and suppress rapid combustion in a premixed charge compression ignition (PCCI) engine. The experimental results showed that the reaction suppressor injection significantly recuced the heat release at low-temperature oxidation reactions which suppressed an increase in the charge temperature after the onset of low-temperature oxidation reactions and in the rapid combustion. Chemical kinetic modeling showed a reduction of OH radical concentration before the onset of low temperature oxidation with injection of suppressors. Among the various ignition suppressors, alcohols had a greater impact on OH radical reduction resulting in stronger ignition suppression than other oxygenated and unoxygenated hydrocarbons, water, and hydrogen. The mechanism of the low temperature oxidation suppression with alcohols was considered to be caused by the consumption of OH radical reacting with hydrogen atom in alcohol molecules.

Direct evidence for destruction of polychlorobiphenyls by OH radicals in the subtropical troposphere.

Although laboratory studies have indicated that OH radicals readily oxidize polychorobiphenyls (PCBs) in the gas-phase, this mechanism has never been positively demonstrated under real atmospheric conditions. By applying elaborated sampling and analytical techniques we achieved for the first time simultaneously a field study of the diurnal atmospheric cycle of OH radical and PCBs in a remote site of eastern Mediterranean. In all cases, the concentration of sigmaPCB (sum of 27 congeners) showed a characteristic depletion during daytime, while the concentration of OH radicals was at the maximum levels. By assuming that the depletion of PCBs was caused solely by the destruction from OH radicals, PCB-OH rate constants (KOH) of different PCB congeners were determined from field data by applying the relative rate method. Our field-determined KOH values were notably coherent with those previously measured in laboratory experiments. In all measurement periods, the KOH values consistently decreased in sequence for those compounds showing an increasing degree of chlorination on the biphenyl group. By taking into account KOH values and latitude-dependent concentration of OH radicals, it was estimated that, near to tropical and subtropical regions, the atmospheric lifetimes of PCBs 8 and 110 should be substantially low (10 and 20 days, respectively). A significant fraction of PCBs should be destroyed during their residence over tropical/subtropical regions, due to the intensive destruction by OH.

OH radical generation by atmospheric pressure pulsed discharge plasma and its quantitative analysis by monitoring CO oxidation

OH radicals play a very important role in non-thermal plasma chemical reactions for decomposition of gaseous pollutants or synthesis of methanol from methane, etc. In this paper, the CO oxidation monitoring method, which has been used in atmospheric chemistry, was examined to measure the concentration of OH radicals produced by a pulsed discharge plasma. The concentration of OH radicals in the discharge plasma of H2O/Ar mixture gas was estimated by measuring the amount of CO2 produced through oxidation of CO by OH radicals. The experimental results and the calculated results showed that it is possible to measure OH radicals using this simple method. In this experimental work, the maximum concentration of OH radicals produced by the pulsed discharge plasma of H2O/Ar was measured to be 9.4×1014 molecules cm−3 pulse−1. Within the temperature range investigated in this study (50–150°C), the formation of OH radicals increased with increase in the specific input energy (discharge energy dissipated in unit volume of the gas; SIE) value and the content of H2O; in contrast, it decreased with increase in the gas temperature.

Estimating the contribution of degradation in air and deposition to the deep sea to the global loss of PCBs

A series of theoretical and semi-empirical approaches, including the zonally averaged global fate and transport model Globo-POP, were used to estimate the rate of loss of selected polychlorinated biphenyl (PCB) congeners from the global environment, and the relative contribution of atmospheric degradation and transfer to the deep sea to this loss. The atmospheric fate of PCBs is highly dependent on the OH radical concentration, temperature and the latter's influence on the gas/particle partitioning equilibrium. The PCB loss was found to be very congener specific. Atmospheric reaction with the OH radical is the major loss process for the lighter and intermediate PCB congeners, whereas particle-bound transfer to the deep sea dominates the loss of the congeners with a large number of chlorine substituents. As concentrations in air and seawater decline more rapidly than in the terrestrial environment, other loss processes such as degradation in soils are gaining in relative importance with time. Estimated lifetimes of the PCBs in the global environment are on the order of decades, with significant differences between congeners. Research needs are identified that would allow improvements of these estimates.

Behavior and effect on NOx formation of OH radical in methane-air diffusion flame with steam addition

This paper presents the results of fundamental research on gas turbine cogeneration systems with steam injection that attains high thermal efficiency and low pollutant emissions. The concentration of OH radical may be affected significantly in these systems. To prove the behavior of OH radical in the combustion zone and to clarify the relationship between OH radical production and NOx emission, the characteristics of methane-air counterflow flames with steam addition have been simulated numerically by applying GRI-Mech detailed chemical kinetics. To investigate the chemical reaction effects of steam addition on the OH radical and NO formation with the effect of the temperature decrease because of steam addition excluded, the maximum flame temperature was adjusted to the same value by varying the initial temperature. The calculated results clearly show that with steam addition, the decomposition reaction of steam is suppressed and the production rate of the OH radical is decreased regardless of the increase in steam concentration. These results can be explained by the significant decrease in flame temperature for the same initial temperatures of fuel and oxidizer. For the same flame temperature, the OH concentration becomes higher. The increase in the OH concentration because of steam addition in itself results in the increase in NOx emissions. However, because of the decrease in CH concentration with steam addition, the production rates of HCN and N radicals decrease dramatically, and NO formation is suppressed as a whole. These results may be useful to predict NOx emissions in a high-temperature gas turbine system with steam or water addition.

Laser-Induced Fluorescence Instrument for the Detection of Tropospheric OH Radicals

Abstract An instrument for the measurement of troposphric OH radical concentrations using a laser-induced fluorescence technique has been developed. Ambient air is expanded through a pinhole into a low-pressure fluorescence cell and irradiated by the second harmonic of a dye laser at a high repetition rate of 1 kHz. The OH radicals are electronically excited using A2Σ+(ν′ = 0) ← X2Π (ν′′ = 0) transitions around 308 nm. The fluorescence from OH radicals is collected by a lens system and detected by a photomultiplier. The photoelectron pulses from the photomultiplier are processed by a photon-counting system. The dynode gate of the photomultiplier and counting gate systems are made to minimize the detection of the chamber and Rayleigh scattering signal. The sensitivity of the developed instrument is calibrated with two methods: one is a long-path absorption technique and the other is a titration technique with simultaneous photolysis of water vapor and oxygen. It is found that the background signal is mainly produced by after-pulse effects in the photomultiplier for the fluorescence detection with a dynode gate system. The minimum detection limit is determined to be 7.0×105 molecules cm-3 for OH radicals with a signal-to-noise ratio of 2 and a signal integration time of 60 s.

Evidence for destruction of PCBs by the OH radical in urban atmospheres

Evidence for reaction of polychlorinated biphenyl (PCB) congeners with the hydroxyl (OH) radical in the troposphere was observed in diurnal variations in ambient gas-phase PCB concentrations at three urban sampling sites located in the Chicago, IL; Baltimore, MD; and Jersey City, NJ urban/industrial areas. The magnitude of the depletion of individual PCB congeners decreased by about 10–20% for each additional chlorine substituent, reflecting slower reaction rates for higher MW congeners with the OH radical. Octa- and nonachlorobiphenyls, which are largely unreactive with the OH radical, were used as tracers to investigate the effects of dilution on diurnal variation. The environmental rate constants for disappearance of the PCBs range from about 1.0 day −1 for trichlorobiphenyls to about 0.3 day −1 for hexachlorobiphenyls. Assuming a OH radical concentration of 3×10 6 molecules cm −3, the second-order rate constants for reaction of specific congeners with the OH radical are consistent with laboratory measurements. More importantly, the relative reactivity of PCB homologues agrees well with the relationship predicted by other researchers from laboratory measurements, suggesting that losses of PCBs during daytime tropospheric transport are due at least in part to reactions with the OH radical.

The Water Degradation Yield and Spatial Distribution of Primary Radicals in the Near-Discharge Volume of an Electrolytic Cathode

The features of combination and scavenging reactions of OH radicals were experimentally investigated in comparison with the decomposition of water by the action of anodic microdischarges on an electrolytic cathode. The assumption was proved that the formation of primary water degradation products occurs mainly in the liquid phase in tracks of fast particles entering the solution from the discharge plasma. The local concentration of OH radicals in tracks was estimated at the level of 10–2 mol/l; although the peripheral regions of tracks can overlap because of the close arrangement of tracks in the near-discharge layer of the solution, the central regions of increased OH concentration may be considered as isolated.