OH Radical Concentration Research Articles

Rates and regimes of photochemical ozone production over Central East China in June 2006: a box model analysis using comprehensive measurements of ozone precursors

Abstract. An observation-based box model approach was undertaken to estimate concentrations of OH, HO2, and RO2 radicals and the net photochemical production rate of ozone at the top of Mount Tai, located in the middle of Central East China, in June 2006. The model calculation was constrained by the measurements of O3, H2O, CO, NO, NO2, hydrocarbon, HCHO, and CH3CHO concentrations, and temperature and J values. The net production rate of ozone was estimated to be 6.4 ppb h−1 as a 6-h average (09:00–15:00 CST), suggesting 58±37 ppb of ozone is produced in one day. Thus the daytime buildup of ozone recorded at the mountain top as ~23 ppb on average is likely affected by in situ photochemistry as well as by the upward transport of polluted air mass in the daytime. On days with high ozone concentrations (hourly values exceeding 100 ppb at least once), in situ photochemistry was more active than it was on low ozone days, suggesting that in situ photochemistry is an important factor controlling ozone concentrations. Sensitivity model runs for which different NOx and hydrocarbon concentrations were assumed suggested that the ozone production occurred normally under NOx-limited conditions, with some exceptional periods (under volatile-organic-compound-limited conditions) in which there was fresh pollution. We also examined the possible influence of the heterogeneous loss of gaseous HO2 radicals in contact with aerosol particle surfaces on the rate and regimes of ozone production.

A DEMS Study of Methanol and Formic Acid Oxidation on Boron-Doped Diamond Electrode

The electrochemical oxidation of methanol and formic acid in 1 M is studied on boron-doped diamond electrode using differential electrochemical mass spectrometry (DEMS). DEMS is used to identify products and intermediates of the reaction monitored online during the potential sweep. The measurements show that the oxidation of both methanol and formic acid proceeds close to the potential of the oxygen evolution reaction, resulting in the appearance of an oxidation wave. During the oxidation, the oxygen evolution reaction shifts toward a higher potential, i.e., becomes a secondary reaction. Depending on the potential, either the formation of intermediates (partial oxidation) or the complete mineralization of organics, simultaneous with oxygen evolution, is observed. Diffusion limitation of the oxidation is clearly observed in parallel to the increase in formation. Methylformate is formed from methanol at potentials at which the local concentration of OH radicals is not sufficient for a complete mineralization. Formic acid oxidation proceeds in two waves; before the onset of oxygen evolution, an intermediate (probably oxalic acid) is produced in parallel to formation. Only in parallel to oxygen evolution is mineralization to is complete.

Characterization of Methylene Blue Decomposition on Fe-ACF/TiO2 Photocatalysts Under UV Irradiation with or Without H2O2

The photocatalysts of Fe-ACF/TiO compositeswere prepared by the sol-gel method and characterized by BET, XRD, SEM, and EDX. It showed that the BET surface area was related to adsorption capacity for each composite. The SEM results showed that ferric compound and titanium dioxide were distributed on the surfaces of ACF. The XRD results showed that Fe-ACF/TiO composite only contained an anatase structure with a Fe mediated compound. EDX results showed the presence of C, O, and Ti with Fe peaks in Fe-ACF/ TiO composites. From the photocataytic degradation effect, TiO on activated carbon fiber surface modified with Fe (Fe-ACF/TiO ) could work in the photo-Fenton process. It was revealed that the photo-Fenton reaction gives considerable photocatalytic ability for the decomposition of methylene blue (MB) compared to non-treated ACF/TiO , and the photo-Fenton reaction was improved by the addition of H O. It was proved that the decomposition of MB under UV (365 nm) irradiation in the presence of H O predominantly accelerated the oxidation of Fe to Fe and produced a high concentration of OH radicals.

Atmospheric trace gas measurements during SEEDS-II over the northwestern pacific

Abstract Atmospheric trace gas measurements were conducted during SEEDS-II. Atmospheric dimethylsulfide (DMS) was continuously measured by GC-MS during the R/V Hakuho cruise. Further, ambient air was sampled into canisters (42 samples) and analyzed by GC-MS and GC-FID for various biogenic and anthropogenic volatile organic compounds (VOCs) after the cruise. CO, O3, SO2, and NOx were monitored continuously aboard the ship. A fertilization experiment was conducted in a high-nitrate, low-chlorophyll (HNLC) region (48°N, 165°E). The atmospheric concentrations inside a patch (fertilized area) were compared with those outside it (natural area); however, clear differences were not observed for biogenic trace gasses (DMS, CH3Cl, CH3I, isoprene, and alkenes) in the atmosphere. However, a high DMS concentration was observed over the northwestern Pacific Ocean. The fertilized area was also observed by R/V Kilo Moana, and DMS was measured by GC-FPD. A good agreement was observed between the results of the measurements made aboard the two independent ships by different measurement methods. The atmospheric SO2 concentration was compared with the atmospheric DMS concentration. The SO2 concentration was found to vary with the atmospheric DMS concentration. A diurnal variation of the atmospheric DMS concentration was observed around the fertilized region. The DMS content tends to increase during the night and decrease during the day. A box model calculation was conducted to explain the diurnal variation of the atmospheric DMS. Since there was no diurnal variation of the wind speed, a constant DMS flux from the ocean surface was assumed. Further, the atmospheric OH radical concentration was assumed to be dependent on sunlight. The box model can roughly reproduce the atmospheric DMS diurnal variation mainly caused by its removal reaction with OH radicals.

A study of OH radicals in an atmospheric AC discharge plasma using near infrared diode laser cavity ringdown spectroscopy combined with optical emission spectroscopy

Simultaneous measurements of absolute concentrations of H2O and OH radicals in an atmospheric AC discharge using continuous wave cavity ringdown spectroscopy (cw-CRDS) are reported. Formation of OH radicals and plasma temperatures are characterized by optical emission spectroscopy. The concentration of OH radical at the edge of the discharge plume at 380 K is measured by the cw-CRDS technique to be 1.1 ×1015 molecule cm-3. Ringdown measurements of the H2O (120-000) band and the OH first overtone around 1515 nm enable us to determine an OH generation yield, \(\Phi =\frac{N_{\rm OH} }{N_{\rm H_2 O}}\), to be 4.8 ×10-3, where NOH and \( N_{H_2 O} \) are the number densities of OH and H2O, respectively. The minimum detectable absorption coefficient of the cw-CRDS system is 8.9 ×10-9 cm-1, which corresponds to a 1σ detection limit of OH number density of 1.2 ×1013 molecule cm-3 in the discharge. This experimental approach is demonstrated for the first time ever in an AC discharge, and can be applied in general to a variety of atmospheric plasmas to help study OH formation mechanisms and OH-related plasma applications.

Measurements of combustion properties in a microwave enhanced flame

Microwave induced flame speed enhancement is quantified in a laminar, premixed CH 4/air wall stagnation flat flame. Experiments were performed in a high Q microwave cavity with the cavity tuned so that the maximum microwave field is located in the vicinity of a flat flame front. Equivalence ratios were varied between 0.6 and 0.8. When the flame is radiated by a continuous wave microwave field of approximately 5 kV/cm, the flame front is observed to move towards the burner exit and stabilize at a standoff distance corresponding to a flame speed increase of up to 20%. No microwave discharge is observed, indicating that the enhanced flame speed arises from microwave energy deposited directly into the reaction zone through coupling to the weakly ionized gas in that region. Laser diagnostics were performed to quantify temperature increase, the laminar flame speed enhancement, and changes in the OH radical concentration through filtered Rayleigh scattering, particle image velocimetry, and planar laser induced fluorescence, respectively. These measurements indicate that microwave radiation may prove to be an effective means to non-invasively control and enhance flame stability in combustors.

Characteristics of ambient C 2–C 11 non-methane hydrocarbons in metropolitan Nagoya, Japan

To understand the characteristics of non-methane hydrocarbon (NMHC) abundance in an urban air of Nagoya, one of the metropolitan areas of Japan, 48 species of C 2–C 11 NMHCs were measured with a measurement system, developed in this study, by using gas chromatography with flame ionization detection (GC/FID) continuously for one year from December 2003 to November 2004. Annual mean concentration of NMHCs in normal and propylene equivalent (PE) in Nagoya was compared with those in four urban areas of Seoul, London, Lille, and Dallas to extract characteristics of urban air. While the absolute values of the normal and PE concentrations of alkanes, alkenes, alkyne, and aromatics were significantly different among these urban areas, the proportions of each chemical group to the total NMHC were not so different. In Nagoya, the total normal concentration was high from November to February and low from June to August. The pattern of the seasonal variation was influenced mainly by that of alkanes. On the other hand, the total PE concentration was high from July to December and low from January to June. The pattern of the seasonal variation was influenced mainly by those of alkenes and aromatics. Particularly the normal concentration of isoprene was high from May to September because of large emission associated with activity of plants. As the results, in summer, the PE concentration of isoprene was especially high, and its contribution to the total NMHCs measured in this study was approximately 40%. The total PE concentrations were high in summer when the concentration of OH radicals is also high, suggesting that the productions of ozone and secondary organic aerosol (SOA) are likely to be promoted in summer of Nagoya.

Optical diagnostics of a low power—low gas flow rates atmospheric-pressure argon plasma created by a microwave plasma torch

We employ a suite of optical techniques, namely, visual imaging, optical emission spectroscopy and cavity ringdown spectroscopy (CRDS), to characterize a low power, low gas flow rates, atmospheric-pressure argon microwave induced plasma. The plasma is created by a microwave plasma torch, which is excited by a 2.45 GHz microwave with powers ranging from 60 to 120 W. A series of plasma images captured in a time-resolution range of as fine as 10 µs shows that the converging point is actually a time-averaged visual effect and the converging point does not exist when the plasma is visualized under high time resolution, e.g. <2 ms. Simulations of the emission spectra of OH, N2 and in the range 200–450 nm enable the plasma electronic excitation temperature (Texc) to be determined at 8000–9000 K, while the vibrational temperature (Tv), the rotational temperature (Tr) and the gas temperature (Tg) at different locations along the axis of the plasma column are all determined to be in the range 1800–2200 K. Thermal equilibrium properties of the plasma are discussed. OH radical concentrations along the plasma column axis are measured by CRDS and the concentrations are in the range 1.6 × 1013–3.0 × 1014 cm−3 with the highest density at the tail of the plasma column. The upper limit of electron density ne is estimated to be 5.0 × 1014 cm−3 from the Lorentzian component of the broadened lineshape obtained by ringdown spectral scans of the rovibrational line S21 of the OH A–X (0–0) band.

Modelling trends in OH radical concentrations using generalized additive models

Abstract. During the TORCH campaign a zero dimensional box model based on the Master Chemical Mechanism was used to model concentrations of OH radicals. The model provided a close overall fit to measured concentrations but with some significant deviations. In this research, an approach was established for applying Generalized Additive Models (GAM) to atmospheric concentration data. Two GAM models were fitted to OH radical concentrations using TORCH data, the first using measured OH data and the second using MCM model results. GAM models with five smooth functions provided a close fit to the data with 78% of the deviance explained for measured OH and 83% for modelled OH. The GAM model for measured OH produced substantially better predictions of OH concentrations than the original MCM model results. The diurnal profile of OH concentration was reproduced and the predicted mean diurnal OH concentration was only 0.2% less than the measured concentration compared to 16.3% over-estimation by the MCM model. Photolysis reactions were identified as most important in explaining concentrations of OH. The GAM models combined both primary and secondary pollutants and also anthropogenic and biogenic species to explain changes in OH concentrations. Differences identified in the dependencies of modelled and measured OH concentrations, particularly for aromatic and biogenic species, may help to understand why the MCM model predictions sometimes disagree with measurements of atmospheric species.

Global sensitivity analysis for model-based prediction of oxidative micropollutant transformation during drinking water treatment

This study quantifies the uncertainty involved in predicting micropollutant oxidation during drinking water ozonation in a pilot plant reactor. The analysis is conducted for geosmin, methyl tert-butyl ether (MTBE), isopropylmethoxypyrazine (IPMP), bezafibrate, β-cyclocitral and ciprofloxazin. These compounds are representative for a wide range of substances with second order rate constants between 0.1 and 1.9 × 10 4 M −1 s −1 for the reaction with ozone and between 2 × 10 9 and 8 × 10 9 M −1 s −1 for the reaction with OH-radicals. Uncertainty ranges are derived for second order rate constants, hydraulic parameters, flow- and ozone concentration data, and water characteristic parameters. The uncertain model factors are propagated via Monte Carlo simulation and the resulting probability distributions of the relative residual micropollutant concentrations are assessed. The importance of factors in determining model output variance is quantified using Extended Fourier Amplitude Sensitivity Testing (Extended-FAST). For substances that react slowly with ozone (MTBE, IPMP, geosmin) the water characteristic R ct-value (ratio of ozone- to OH-radical concentration) is the most influential factor explaining 80% of the output variance. In the case of bezafibrate the R ct-value and the second order rate constant for the reaction with ozone each contribute about 30% to the output variance. For β-cyclocitral and ciprofloxazin (fast reacting with ozone) the second order rate constant for the reaction with ozone and the hydraulic model structure become the dominating sources of uncertainty.

Flame characteristics of hydrogen-enriched methane–air premixed swirling flames

The effect of hydrogen addition in methane–air premixed flames has been examined from a swirl-stabilized combustor under unconfined flame conditions. Different swirlers have been examined to investigate the effect of swirl intensity on enriching methane–air flame with hydrogen in a laboratory-scale premixed combustor operated at 5.81 kW. The hydrogen-enriched methane fuel and air were mixed in a pre-mixer and introduced into the burner having swirlers of different swirl vane angles that provided different swirl strengths. The combustion characteristics of hydrogen-enriched methane–air flames at fixed thermal load but different swirl strengths were examined using particle image velocimetry (PIV), OH chemiluminescence, gas analyzers, and micro-thermocouple diagnostics to provide information on flow field, combustion generated OH radical and gas species concentration, and temperature distribution, respectively. The results show that higher combustibility of hydrogen assists to promote faster chemical reaction, raises temperature in the reaction zone and reduces the recirculation flow in the reaction zone. The upstream of flame region is more dependent on the swirl strength than the effect of hydrogen addition to methane fuel. At lower swirl strength condition the NO concentration in the reaction zone reduces with increase in hydrogen content in the fuel mixture. Higher combustibility of hydrogen accelerates the flow to reduce the residence time of hot product gases in the high temperature reaction zone. At higher swirl strength the NO concentration increases with increase in hydrogen content in the fuel mixture. The effect of dynamic expansion of the gases with hydrogen addition appears to be more dominant to reduce the recirculation of relatively cooler gases into the reaction zone. NO concentration also increases with decrease in the swirl strength.

Chemical ionisation mass spectrometer for measurements of OH and Peroxy radical concentrations in moderately polluted atmospheres

A new version of an atmospheric pressure chemical ionisation mass spectrometer has been developed for ground based in situ atmospheric measurements of OH and total peroxy (HO2 + organic peroxy) radicals. Based on the previously developed principle of chemical conversion of OH radicals to H2SO4 in reaction with SO2 and detection of H2SO4 using an ion molecule reaction with NO 3 ─ , the new instrument is equipped with a turbulent chemical conversion reactor allowing for measurements in moderately polluted atmosphere at NO concentrations up to several ppb. Unlike other similar devices, where the primary NO 3 ─ ions are produced using radioactive ion sources, the new instrument is equipped with a specially developed corona discharge ion source. According to laboratory measurements, the overall accuracy and detection limits are estimated to be, respectively, 25% and 2 × 105 molecule cm-3 for OH and 30% and 1 × 105 molecule cm-3 for HO2 at 10 min integration times. The detection limit for measurements of OH radicals under polluted conditions is 5 × 105 molecules cm-3 at 10 min integration times. Examples of ambient air measurements during a field campaign near Paris in July 2007 are presented demonstrating the capability of the new instrument, although with reduced performance due to the employment of non isotopic SO2.

The Potential of Pulsed Underwater Streamer Discharges as a Disinfection Technique

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we investigate the effectiveness of pulsed underwater streamer discharges for water disinfection and its scalability to large throughputs. For this paper, we have built a coaxial streamer reactor containing a central cylindrical anode covered by a thin porous layer of almandine. The reactor was supplied by electric pulses with amplitudes of up to 100 kV and pulse durations between 200 and 400 ns at a repetition rate of up to 20 Hz. Different chemical probes have been used as the main diagnostic to determine the production rates and the accumulation of oxidants in the bulk water. It has been found that the number and the length of streamers scaled linearly with the applied voltage amplitude, whereas the pulse duration only affected the streamer length. All oxidant production rates scaled linearly with the number-length product of streamers. OH-radical concentrations of up to 100 mM appear at the streamer-water-boundary. However, because of the limited volume of streamers and the limited range of OH radicals, the most important oxidant with respect to decontamination is <formula formulatype="inline"><tex>$\hbox{H}_{2}\hbox{O}_{2}$</tex></formula> which accumulates in the water and, by interacting with the intense UV radiation and the shock waves from the streamers, produces OH radicals in the bulk water. Placing water-filled cuvettes from different materials in the reactor, we found that the contribution of shock waves to the production of oxidants in the bulk water seems more important than the contribution of UV radiation. The production of oxidants can be enhanced by percolating suitable gases through the active zone of the reactor. Most effective is the use of oxygen. About 20 <formula formulatype="inline"><tex>$\hbox{J/cm}^{3}$</tex> </formula> of electrical energy was required to reduce the concentration of <emphasis emphasistype="italic"> Pseudomonas putida</emphasis> bacteria by six orders of magnitude. However, up to 180 <formula formulatype="inline"> <tex>$\hbox{J/cm}^{3}$</tex></formula> of specific energy input was necessary to reduce the bacterial freight in waste water from a municipal sewage plant by just two orders of magnitude. This has been explained by the high conductivity of waste water which drastically reduces the length of streamers for the same electrical parameters. Therefore, it seems unlikely that underwater streamer discharges alone can become competitive with other advanced oxidation processes like ozonation, UV irradiation, or irradiation with megaelectronvolt electron beams. However, it seems conceivable that a combination of underwater streamer discharges with ozonation or other processes can lead to a more effective and economic decontamination technique. </para>

Method of reconstruction of the chemical composition and combustion efficiency of hydrogen-air mixtures from incomplete experimental data

A principal possibility of approximate reconstruction of the chemical composition in combustion of a hydrogen-air mixture at the end of the scramjet duct from incomplete experimental data (measured concentration of OH radicals and temperature) under the assumption of detailed chemical equilibrium of exchange reactions is analyzed. A closed algebraic system of equations including the concentration of OH radicals and the temperature as parameters is derived in this approximation. A code for solving this system numerically is developed for approximate determination of reaction completeness from the measured temperature and concentration of OH radicals. The model was tested by results of exact thermodynamic calculations of the Jouguet state and overdriven waves in a stoichiometric hydrogen-air mixture and various hydrogen-oxygen mixtures. In the range of pressures of 0.2 to 500 atm and temperatures of 2500 to 3500 K, this method allows the molecular weight and heat release to be reconstructed with accuracy sufficient for gas-dynamic calculations.

Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X2Πi (ν' = 2 ←ν′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10-8 cm-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H2O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 1013 molecules cm-3 to (3.7 ± 0.1) × 1013 molecules cm-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O2 and N2 addition on the production of OH radicals have been also investigated.

Effects of hydrogen addition to methane on the flame structure and CO emissions in atmospheric burners

Hydrogen addition to methane will have an important role to reach a fully developed hydrogen economy. The effects of this addition on the flame structure and CO emissions were evaluated in two different atmospheric burners. Four fuels with the following composition were used: 100%CH 4, 2%H 2+98%CH 4, 6%H 2+94%CH 4 and 15%H 2+85%CH 4. In a single-port atmospheric burner, a decreasing trend in the height of the blue cone with hydrogen addition was determined. The increase in the laminar burning velocity was identified as the main effect on the behavior of this parameter. In a drilled-port atmospheric burner, a significant reduction in CO emissions with hydrogen addition was achieved under two operating conditions: (1) keeping the primary air ratio constant and (2) keeping the primary air ratio and the thermal input constant. The results obtained were consistent with previous experimental studies. This reduction is attributed to a higher concentration of OH radicals as a result of hydrogen addition.

Direct Detection of OH Radicals and Indirect Detection of H2O2 Molecules in the Gas Phase near a TiO2 Photocatalyst Using LIF

The formation of OH radicals and its diffusion into the gas phase during the excitation of TiO2 in the presence of H2O has been studied using the very sensitive and selective detection method of laser-induced fluorescence (LIF). The time-resolved evolution of the OH radical concentration has been observed at pressures between 4 and 600 Torr and at varying distances between the photocatalytic surface and the detection volume. OH radicals have been detected even at the highest pressures, opening the assumption that gas-phase reactions with OH radicals may very well be involved in the photocatalytic degradation of VOCs, even at atmospheric pressure. Interestingly, a second fluorescence signal peak has been observed at longer delays with respect to the excitation pulse. The use of different fluorescence laser energies leads us to interpret this second peak as H2O2, released from the surface into the gas phase and detected by a two-photon process; in fact, we use fluorescence laser fluencies high enough to photolyze H2O2 molecules at the fluorescence−excitation wavelength (282 nm) and excite the generated OH radicals within the same laser pulse.

Night‐time NO3 and OH radical concentrations in the United Kingdom inferred from hydrocarbon measurements

AbstractThe hydrocarbon decay method (Rivett et al., 2003) has been used to analyse hydrocarbon data from four contrasting sites in the United Kingdom to estimate night‐time levels of NO3 and OH. Remarkably consistent results emerge using alkenes, revealing NO3 and OH levels in the range of 0.01–10 ppt and 1 × 104 − 1 × 106 molecule cm−3, respectively. Weak seasonal cycles are observed where NO3 levels peak in spring and OH in summer. Analysis using alkanes suggests that Cl atom levels of around 2 × 104 molecule cm−3 may be present. How Cl may be formed at night in such high quantities is unknown and may not be the answer to the disparity between the two methods. Copyright © 2008 Royal Meteorological Society

Flame Extinction Limits of H2‐CO Fuel Blends

The flame extinction limits of syngas (H2‐CO) flames were measured using a twin-flame counterflow burner. Plots of extinction limits (%f: volumetric percent of fuel in air) versus global stretch rates were generated at different fuel blend compositions and were extrapolated to determine the flame extinction limit corresponding to an experimentally unattainable zero-stretch condition. The zero-stretch extinction limit of H2‐CO mixtures decreases with the increase in H2 concentration in the mixture. The average difference between the measured flame extinction limit and the Le Chatelier’s calculation is around 7% of the mean value. The measured OH chemiluminescence data indicates that regardless of blend composition the OH radical concentration reduces to a critical value prior to the flame extinction. The measured laminar flame velocity close to the extinction indicates that regardless of fuel composition, the premixed flame of hydrogen fuel blends extinguishes when the mixture laminar flame velocity falls below a critical value.

Antioxidant properties of gold nanoparticles studied by ESR spectroscopy

The Au-containing nanocomposites were synthesized by UV irradiation followed by the thermal treatment of chitosan oligomer solutions doped by HAuCl4. The size of the formed gold nanoparticles depends on the concentration of the dopant, which is proved by UV—Vis absorption spectroscopy and small-angle X-ray scattering (SAXS). The antioxidant activity of the gold nanoparticles with respect to hydroxy radicals significantly depends on the specific surface of the particles, which was found using the secondary radical spin-trapping technique. The change in the ·OH radical concentration was monitored by the intensity of the ESR signal of the adduct of the spin trap (α-phenyl-N-tert-butylnitrone) with the Me·radicals formed in the reaction of ·OH with DMSO.