OH Radical Concentration Research Articles

Full-scale comparison of UV/H2O2 and UV/Cl2 advanced oxidation: The degradation of micropollutant surrogates and the formation of disinfection byproducts

The photolysis of chlorine by UV light leads to the formation of the hydroxyl radicals (OH) as well as reactive chlorine species (RCS) that can be effective as advanced oxidation processes (AOPs) for water treatment. Much of the research to date has been done at laboratory- or bench-scale. This study reports results from a model that demonstrates that the relative effectiveness of the UV/Cl2 AOP compared to the more traditional UV/H2O2 AOP is a function of optical path length. As such, the relative effectiveness of the two treatment options evaluated at small scale may not reflect the relative performance at full-scale, making results previously obtained at small-scale potentially less scalable. This study therefore compares the performance of UV/Cl2 to UV/H2O2 at a full-scale water treatment plant, using sucralose and caffeine as spiked surrogates for contaminants that are reactive solely to OH radicals, and to both OH and RCS, respectively. pH was varied between 6.5 and 8.0. The results demonstrated that when using a medium pressure UV lamp, UV/Cl2 might lead to approximately twice the production of OH radicals as UV/H2O2 at pH 6.5 when using the same molar oxidant concentration, but adding chlorine to the UV reactor at pH 8.0 had a negligible impact on OH radical concentration in comparison to UV alone. The study also confirmed previous small-scale results that RCS can be a major contributor to UV/Cl2 treatment for compounds such as caffeine that are susceptible to RCS, with UV/Cl2 effective at both pH 6.5 and 8.0 for such compounds. Disinfection byproducts were monitored, with adsorbable organohalide (AOX) formation increasing by approximately 10 μg-Cl/L due to chlorine photolysis, but only at pH 6.5 and not at pH 8.0. This implies that UV/Cl2 might increase AOX mostly due to reaction between OH and organic precursors to make them more reactive with chlorine, and not due to RCS. The formation of specific DBPs of current or emerging regulatory interest was minimal under all conditions, except for chlorate. Chlorate yields were in the order of 6–18% of the photolysed chlorine.

Dispersion of OH Radicals in Applications Related to Fear-Free Dentistry Using Cold Plasma

Cold atmospheric plasmas (CAPs) are being used in applications related to dentistry. Potential benefits include tooth whitening/bleaching, the sterilization of dental cavities, and root canal disinfection. Generated reactive species, such as hydroxyl (OH) radicals, play a critical role in the effectiveness of CAPs in dentistry. In the present work, the mandibular jaw and teeth were modeled. The propagation of CAP plume in ambient air was dynamically tracked using the level set method. The transport and dispersion OH radicals away from the nozzle and towards the teeth under treatment were also tracked. The distributions of concentration of OH radicals over the teeth were obtained for nozzle to tooth distances of 2 and 4 mm. The discharge of the OH radicals out of the nozzle was found to be asymmetrical. Interestingly, depending on the type of tooth treated, the dispersion of OH radicals out of the nozzle could be altered. The present model and obtained results could be useful for advancements towards a fear-free dentistry using CAPs.

Winter photochemistry in Beijing: Observation and model simulation of OH and HO2 radicals at an urban site

A field campaign was conducted from November to December 2017 at the campus of Peking University (PKU) to investigate the formation mechanism of the winter air pollution in Beijing with the measurement of hydroxyl and hydroperoxyl radical (OH and HO2) with the support from comprehensive observation of trace gases compounds. The extent of air pollution depends on meteorological conditions. The daily maximum OH radical concentrations are on average 2.0 × 106 cm−3 and 1.5 × 106 cm−3 during the clean and polluted episodes, respectively. The daily maximum HO2 radical concentrations are on average 0.4 × 108 cm−3 and 0.3 × 108 cm−3 during the clean and polluted episodes, respectively (diurnal averaged for one hour bin). A box model based on RACM2-LIM1 mechanism can reproduce the OH concentrations but underestimate the HO2 concentrations by 50% during the clean episode. The OH and HO2 concentrations are underestimated by 50% and 12 folds during the polluted episode, respectively. Strong dependence on nitric oxide (NO) concentration is found for both observed and modeled HO2 concentrations, with the modeled HO2 decreasing more rapidly than observed HO2, leading to severe HO2 underestimation at higher NO concentrations. The OH reactivity is calculated from measured and modeled species and inorganic compounds (carbon monoxide (CO), NO, and nitrogen dioxide (NO2)) make up 69%–76% of the calculated OH reactivity. The photochemical oxidation rate denoted by the OH loss rate increases by 3 times from the clean to polluted episodes, indicating the strong oxidation capacity in polluted conditions. The comparison between measurements at PKU site and a suburban site from one previous study shows that chemical conditions are similar in both urban and suburban areas. Hence, the strong oxidation capacity and its potential contribution to the pollution bursts are relatively homogeneous over the whole Beijing city and its surrounding areas.

Kinetic Modeling of NOx Formation and Consumption during Methanol and Ethanol Oxidation

ABSTRACTThis work presents a newly developed model for the oxidation of methanol and ethanol and their fuel interaction with NOx (NO and NO2) chemistry in jet-stirred and flow reactors, freely propagating and burner-stabilized premixed flames, as well as shock tubes. This work takes into account a very recent study on NO formation in pure methanol and ethanol burner-stabilized flames (Combust. Flame, 194, 363–375, 2018), augmented with so far unpublished experimental data. The paper mainly focuses on fuel interaction with nitrogen chemistry and NO formation in laminar premixed flames but also considers the formation and reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. In agreement with previous experimental work, we find that doping of fuel blends with NO shifts the onset of fuel oxidation to lower temperatures depending on the gas conditions. The model suggests that the reactivity promoting effect of NO is mainly due to the net increase of OH radical concentrations, which causes increased fuel oxidation via the NO/NO2 interconversion reaction channel, NO+HO2⇋NO2+OH, NO2+H⇋NO+OH, NO2+HO2⇋HONO+O2, followed by the thermal decomposition of HONO. In burner-stabilized premixed ethanol flames, NO is mainly formed via a NCN pathway for all equivalence ratios, while for methanol flames the NCN pathway is only favored at rich conditions and the N2O pathway is favored at lean conditions.

Tracing the sources and formation pathways of atmospheric particulate nitrate over the Pacific Ocean using stable isotopes

This paper presents isotopic compositions (δ15N and Δ17O values) of atmospheric particulate nitrate (p-NO3−) collected over the Pacific Ocean between 40°S and 68°N on two cruises using the Research Vessel Hakuho Maru. Six marine regions over the Pacific Ocean were categorized using backward trajectory analysis, and various ion concentrations were employed to assess the dominance of anthropogenic or non-anthropogenic sources. The δ15N(p-NO3−) values ranged from −15.4‰ to 2.5‰ over the Pacific Ocean between 40°S and 68°N. The range of the source δ15N(NOx) values were calculated for each sample based on isotopic exchange/fractionation during oxidation from NOx (NO and NO2) to HNO3, and a range of −14.1 to 6.9‰ was yielded over the Pacific Ocean. Compared to other regions, extremely low δ15N(NOx) values of −14.1‰ to −7.3‰ were found over the equatorial Pacific, and such values cannot be matched using conventional NOx apportionment. Emissions of specific oceanic nitrogen emissions of NH3 and alkyl nitrate are thus proposed for the equatorial Pacific. Results demonstrate the importance of the nitrogen recycling process between the ocean and lower atmosphere in the equatorial Pacific. Δ17O(p-NO3−) values and light flux were found to be generally well-correlated in most Pacific Ocean regions, and this result is consistent with the importance of photolysis for OH radical concentrations relative to the contribution of O3 in producing HNO3 from NOx. In contrast, Δ17O(p-NO3−) values for the eastern North Pacific region were higher than in other regions with similar light fluxes, which implies a contribution from halogens to p-NO3− formation.

Development of a field system for measurement of tropospheric OH radical using laser-induced fluorescence technique

A ground-based system for measuring tropospheric OH radical based on laser-induced fluorescence (AIOFM-LIF) was developed in this work. In this system, ambient air is expanded through a 0.4 mm nozzle to low pressure in a detection chamber, where OH radical is irradiated by the 308 nm laser pulse at a repetition rate of 8.5 kHz. Then, the resultant fluorescence corresponding to the A2Σ+(υ'=0)←X2Πi(ν''=0) transition at 308 nm is detected using gated photon counting. The AIOFM-LIF system was integrated into a mobile observing platform for the field observation following the series of laboratory characterization. A portable standard OH radical source by water photolysis-ozone actinometry was established and optimized for accurate system calibration. The factors affecting the system sensitivity were quantified. It was shown that the ultimate system sensitivity is 9.9 × 10-8 cps (molecules cm-3)-1 mw-1; the minimum detection limits are (1.84 ± 0.26) × 105 cm-3 and (3.69 ± 0.52) × 105 cm-3 at night and noon, respectively; and the whole error of AIOFM-LIF system is about 16%. Then, the system was deployed in Shenzhen, China, during the "A comprehensive STudy of the Ozone foRmation Mechanism in Shenzhen" (STORM) campaign. Valid OH radical concentrations for 31 days were obtained, and the peak of the daily average concentration was 6.6 × 106 cm-3 around 12:00. And a high correlation (R2 = 0.77) between OH and j(O1D) was also observed in this field campaign. The relationship between OH concentration and NOx was attentively discussed. The deployment of AIOFM-LIF system in STORM campaign has demonstrated its capability of measuring tropospheric OH radical with high sensitivity and accuracy in a polluted environment.

Heterogeneous OH oxidation of isoprene-epoxydiol-derived organosulfates: kinetics, chemistry and formation of inorganic sulfate

Abstract. Acid-catalyzed multiphase chemistry of epoxydiols formed from isoprene oxidation yields the most abundant organosulfates (i.e., methyltetrol sulfates) detected in atmospheric fine aerosols in the boundary layer. This potentially determines the physicochemical properties of fine aerosols in isoprene-rich regions. However, chemical stability of these organosulfates remains unclear. As a result, we investigate the heterogeneous oxidation of aerosols consisting of potassium 3-methyltetrol sulfate ester (C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relative humidity (RH) of 70.8 %. Real-time molecular composition of the aerosols is obtained by using a Direct Analysis in Real Time (DART) ionization source coupled to a high-resolution mass spectrometer. Aerosol mass spectra reveal that 3-methyltetrol sulfate ester can be detected as its anionic form (C5H11SO7-) via direct ionization in the negative ionization mode. Kinetic measurements reveal that the effective heterogeneous OH rate constant is measured to be 4.74±0.2×10-13 cm3 molecule−1 s−1 with a chemical lifetime against OH oxidation of 16.2±0.3 days, assuming an OH radical concentration of 1.5×106 molecules cm−3. Comparison of this lifetime with those against other aerosol removal processes, such as dry and wet deposition, suggests that 3-methyltetrol sulfate ester is likely to be chemically stable over atmospheric timescales. Aerosol mass spectra only show an increase in the intensity of bisulfate ion (HSO4-) after oxidation, suggesting the importance of fragmentation processes. Overall, potassium 3-methyltetrol sulfate ester likely decomposes to form volatile fragmentation products and aqueous-phase sulfate radial anion (SO4⚫-). SO4⚫- subsequently undergoes intermolecular hydrogen abstraction to form HSO4-. These processes appear to explain the compositional evolution of 3-methyltetrol sulfate ester during heterogeneous OH oxidation.

Planarization mechanism for 6H–SiC (0001) Si-faced surfaces using electrochemical reactions

We have developed a fast planarization method of 6H–SiC (0001) Si-faced surfaces using electrochemical reactions. In this method, a working electrode of a rotating Pt disc electrode is physically contacted with an SiC wafer immersed in an HF solution, and a potential of 2 or 3 V with respect to the Ag/AgCl reference electrode is applied to the Pt disk electrode. After the planarization process with the applied voltage of 3 V for 1 h, relatively flat surfaces with the 0.09 nm average roughness (Ra) value are obtained, but a 1 ∼ 2 nm carbon-rich layer is present on the surfaces. A subsequent planarization process with the applied voltage of 2 V for 1 h almost completely removes the carbon-rich layer, resulting in atomically flat surfaces with the 0.05 nm Ra value. Under the condition of the applied voltage of 2 V, the concentration of OH radicals generated during water electrolysis decreases to one-half of that generated under 3 V. The oxidation rate of C atoms is proportional to the concentration of OH radicals while that of Si atoms is proportional to the square of the concentration. Under the condition of the applied voltage of 2 V, therefore, oxidation of C atoms proceeds more dominantly than that of Si atoms, resulting in absence of the carbon-rich layer.

Development and Optimization of an Immobilized Photocatalytic System within a Stacked Frame Photoreactor (SFPR) Using Light Distribution and Fluid Mixing Simulation Coupled with Experimental Validation

Recently, photocatalytic reactors have been designed with a view toward overcoming mass transfer limitations especially in systems with immobilized catalysts. This paper reports the design of a titanium “bladed” propeller with TiO2 immobilized on the blades. To evaluate the propeller efficiency, modeling using COMSOL Multiphysics was validated experimentally using coumarin as a probe molecule allowing for OH radical quantification. Modeling of light distribution and catalyst irradiance at varying irradiation distance was performed using ray optics, which, alongside experimental work, showed that irradiation at 4 and 5 cm from the propeller yielded the highest irradiance (29.3 and 22.1 mW/cm2) and OH radical concentrations (5.38 and 5.56 μM, respectively). Propeller rotation was modeled and compared against experimental data to assess mass transfer limitations at varying rotation speeds. This showed that 300 rpm provided the highest rate of coumarin degradation (0.32 μM/min) despite the model showing higher fluid velocities at 400 rpm.

ROOOH: a missing piece of the puzzle for OH measurements in low-NO environments?

Abstract. Field campaigns have been carried out with the FAGE (fluorescence assay by gas expansion) technique in remote biogenic environments in the last decade to quantify the in situ concentrations of OH, the main oxidant in the atmosphere. These data have revealed concentrations of OH radicals up to a factor of 10 higher than predicted by models, whereby the disagreement increases with decreasing NO concentration. This was interpreted as a major lack in our understanding of the chemistry of biogenic VOCs (volatile organic compounds), particularly isoprene, which are dominant in remote pristine conditions. But interferences in these measurements of unknown origin have also been discovered for some FAGE instruments: using a pre-injector, all ambient OH is removed by fast reaction before entering the FAGE cell, and any remaining OH signal can be attributed to an interference. This technique is now systematically used for FAGE measurements, allowing the reliable quantification of ambient OH concentrations along with the signal due to interference OH. However, the disagreement between modelled and measured high OH concentrations of earlier field campaigns as well as the origin of the now-quantifiable background OH is still not understood. We present in this paper the compelling idea that this interference, and thus the disagreement between model and measurement in earlier field campaigns, might be at least partially due to the unexpected decomposition of a new class of molecule, ROOOH, within the FAGE instruments. This idea is based on experiments, obtained with the FAGE set-up of the University of Lille, and supported by a modelling study. Even though the occurrence of this interference will be highly dependent on the design and measurement conditions of different FAGE instruments, including ROOOH in atmospheric chemistry models might reflect a missing piece of the puzzle in our understanding of OH in clean atmospheres.

Preferential production of reactive species and bactericidal efficacy of gas-liquid plasma discharge

Disinfection of bacteria-contaminated water is crucial to public health. In this work, a gas-liquid phase air and oxygen plasma is designed to selectively and controllably generate reactive species and efficiently inactivate Staphylococcus aureus (S. aureus) in the liquid. Optical emission spectroscopy (OES) and fluorescent probes are used to analyze the formation of reactive species induced by the plasma in the gas-liquid and liquid phases. OH radicals are observed to be the predominant species in the liquid induced by the oxygen plasma. In the air plasma, owing to the presence of active nitrogen, nitrogen-containing reactive species including NO, HNO2, HNO3, and ONOOH are generated in the liquid. The bactericidal and biological effects associated with the formation of liquid products are investigated. The different bactericidal effects of the air and oxygen plasma (direct and indirect) treatment show that the inactivation efficacy is related to the species and content of the reactive oxygen and nitrogen species produced by the air and oxygen plasma in the liquid. The better inactivation effect of direct oxygen plasma treatment stems from the larger concentration of OH radicals. The presence of active nitrogen, formation of nitrogen-based reactive species, and acidification are responsible for the better bactericidal activity in the air plasma indirect treatment. Cell membrane damage and accumulation of intracellular reactive oxygen species result in inactivation of S. aureus. Selective and more efficient formation of reactive species is attractive to industrial processes such as waste water treatment.

Applicability of pulsed corona discharge treatment for the degradation of chloroform

Degradation of chloroform was investigated using plasma corona discharge by a needle-plate reactor. Chloroform concentration of 200 mg/L was completely degraded in 10 min of treatment. Almost complete dechlorination happened in 10 min and 91% total organic carbon (TOC) removal occurred in 12 min of treatment. Several important parameters such as input voltage, initial concentration, pH, conductivity, alkalinity, presence of OH radical and aqueous electron scavengers were evaluated to understand their effect on rate of chloroform degradation. The degradation rate of chloroform increased with increase in pH, conductivity and alkalinity of the solution. The maximum concentrations of OH radical, H2O2 and superoxide produced after 10 min of corona discharge were estimated to be 0.260 mM, 0.353 mM and 0.046 mM respectively. Various intermediates such as dichloromethane, dichloroethane, trichloroethane, trichloroethylene (TCE), tetrachloroethylene (PCE), trichloronitromethane and acetaldehyde were identified using GC-MS purge and trap technique. Based on the identified intermediates, reductive and oxidative degradation pathway of chloroform is proposed.

Enhancement of photocatalytic activity of Bi2MoO6 by fluorine substitution

The fluorine substitution on Bi2MoO6 (F-Bi2MoO6) can be easily synthesized by a hydrothermal method. Based on the XRD result, it could be speculated that fluorine ion can substitute oxygen ion in the crystal lattice of Bi2MoO6. The photodegradation activity of F-Bi2MoO6 almost is 1.9 times as high as the single Bi2MoO6 for degradation of RhB and 1.7 times for decomposition of 2, 4-CDP under the visible light. The enhanced catalytic activity of F-Bi2MoO6 should be attributed to the wider band gap, the more positive VB potential and the higher concentration of OH radicals due to the F doping. Additionally, F doping formed the donor state below the Fermi energy, which can trap the electrons and increase the efficiency of charge separation remarkably.

Low-level summertime isoprene observed at a forested mountaintop site in southern China: implications for strong regional atmospheric oxidative capacity

Abstract. To investigate the atmospheric oxidative capacity (AOC) in forested high mountain areas adjacent to the photochemistry-active Pearl River Delta (PRD) region in southern China, one-month online observations of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were conducted at a national background station in Nanling Mountains in summer 2016. The results showed that the observed daytime isoprene levels (377±46 pptv) were significantly lower in comparison with other forest sites within and outside China, although the sampling site was surrounded with subtropical evergreen broad-leaved trees which are strong isoprene emitters. Also, high daytime (MVK + MACR) ∕ isoprene ratios (1.9±0.5) were observed. Based on the observations, we hypothesized that the lower isoprene levels in the study forest might be attributable to a strong AOC in relation to the elevated regional complex air pollution. In further data analyses, high site-level concentrations of daytime OH (7.3±0.5×106 molecules cm−3) and nighttime NO3 radicals (6.0±0.5×108 molecules cm−3) were estimated by using a photochemical box model incorporating the master chemical mechanism (PBM-MCM), and high regional mixing ratios of OH (19.7±2.3×106 molecules cm−3) during 09:00–15:00 LT were also obtained by applying a parameterization method with measured aromatic hydrocarbons. And besides, high initial mixing ratios (1213±108 pptv) and short atmospheric reaction time (0.27 h) of isoprene during the day were derived by a sequential reaction approach. All these indicate that isoprene was rapidly and highly oxidized in this forest, which supports our hypothesis. The study suggests that the complex air pollution in the PRD may have significantly elevated the background AOC of the adjacent forests, and probably affects the regional air quality and ecological environment in the long term. The feedback of forest ecosystems to the increasing AOC in southern China warrants further studies.

Tropospheric OH and stratospheric OH and Cl concentrations determined from CH4, CH3Cl, and SF6 measurements

The hydroxyl (OH) radical is the key oxidant in the global atmosphere as it controls the concentrations of toxic gases like carbon monoxide and climate relevant gases like methane. In some regions, oxidation by chlorine (Cl) radical is also important, and in the stratosphere both OH and Cl radicals impact ozone. An empirical method is presented to determine effective OH concentrations in the troposphere and lower stratosphere, based on CH4, CH3Cl, and SF6 data from aircraft measurements (IAGOS-CARIBIC) and a ground-based station (NOAA). Tropospheric OH average values of 10.9 × 105 (σ = 9.6 × 105) molecules cm−3 and stratospheric OH average values of 1.1 × 105 (σ = 0.8 × 105) molecules cm−3 were derived over mean ages derived from SF6. Using CH4 led to higher OH estimates due to the temperature dependence of the CH4 + OH reaction in the troposphere and due to the presence of Cl in the stratosphere. Exploiting the difference in effective OH calculated from CH3Cl and CH4 we determine the main altitude for tropospheric CH4 oxidation to be 4.5 ~ 10.5 km and the average Cl radical concentration in the lower stratosphere to be 1.1 × 104 (σ = 0.6 × 104) molecules cm−3 (with a 35% measurement uncertainty). Furthermore, the data are used to examine the temporal trend in annual average stratospheric OH and Cl radical concentrations between 2010 and 2015. The year 2013 showed highest stratospheric OH and lowest Cl but no clear temporal trend was observed in the data in this period. These data serve as a baseline for future studies of stratospheric circulation changes.

Computational investigation of the atmospheric oxidation reactions of polybrominated dibenzo-p-dioxins initiated by OH radicals

Density functional theory (DFT) method was used to study OH-initiated atmospheric oxidative reactions of some polybrominated dibenzo-p-dioxins (PBDDs). B3LYP functional and 6–311++G (2df,p) basis set were applied to optimize molecular structures of all stationary points involved in the investigated reactions. The rate constants for key elementary reactions were estimated by means of transition state theory. The computational results demonstrate that all addition reactions of PBDDs with OH radicals can occur spontaneously at standard conditions, however, the OH addition reactions are very slow due to low atmospheric concentration of OH radicals. Addition reactions occurring at γ-C position dominate in OH addition of all PBDDs. With the number of bromine atoms substituting at α-sites increases, the overall rate coefficients of OH addition decrease. The succeeding addition reactions of PBDD-OH adducts with O2 take place hardly both thermodynamically and kinetically. Abstraction reaction of H atoms by O2 is a governing route for PBDD-α(β)-adducts without bromine atoms at the same site, while the fused-ring CO bond fission is a main reaction channel for PBDD-γ-adducts, which will produce substituted phenoxy radicals.

Evaluation of OH and HO<sub>2</sub> concentrations and their budgets during photooxidation of 2-methyl-3-butene-2-ol (MBO) in the atmospheric simulation chamber SAPHIR

Abstract. Several previous field studies have reported unexpectedly large concentrations of hydroxyl and hydroperoxyl radicals (OH and HO2, respectively) in forested environments that could not be explained by the traditional oxidation mechanisms that largely underestimated the observations. These environments were characterized by large concentrations of biogenic volatile organic compounds (BVOC) and low nitrogen oxide concentration. In isoprene-dominated environments, models developed to simulate atmospheric photochemistry generally underestimated the observed OH radical concentrations. In contrast, HO2 radical concentration showed large discrepancies with model simulations mainly in non-isoprene-dominated forested environments. An abundant BVOC emitted by lodgepole and ponderosa pines is 2-methyl-3-butene-2-ol (MBO), observed in large concentrations for studies where the HO2 concentration was poorly described by model simulations. In this work, the photooxidation of MBO by OH was investigated for NO concentrations lower than 200 pptv in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. Measurements of OH and HO2 radicals, OH reactivity (kOH), MBO, OH precursors, and organic products (acetone and formaldehyde) were used to test our current understanding of the OH-oxidation mechanisms for MBO by comparing measurements with model calculations. All the measured trace gases agreed well with the model results (within 15 %) indicating a well understood mechanism for the MBO oxidation by OH. Therefore, the oxidation of MBO cannot contribute to reconciling the unexplained high OH and HO2 radical concentrations found in previous field studies.

Investigation of wall chemical effect using PLIF measurement of OH radical generated by pulsed electric discharge

Near-wall distribution of OH radical concentration provides a measure of the wall chemical effect near a solid surface. However, it is not a straightforward process to isolate the wall chemical effect from the effect of gas-phase reactions because the OH radical distribution can be influenced by both the wall chemical effect and the effect of heat and radicals released by gas-phase reactions. In the present study, instead of the flame, OH radicals generated by a pulsed electric discharge is used in order to eliminate the interference from the gas-phase reaction. OH field with comparable concentration to a methane/air premixed flame has been achieved by tuning the input electric power of pulsed electric discharge. The wall chemical effect is investigated by comparing OH distributions near the quartz wall and the quartz wall with 100-nm-thick alumina coating, while the wall thermal boundary condition is kept identical. High-resolution near-wall measurement of OH distribution was carried out by microscopic planar laser-induced fluorescence (PLIF), and the result was analyzed with the aid of numerical simulations with a surface reaction mechanism. It is found that the initial sticking coefficients estimated on the quartz/alumina surfaces are almost the same with the results in our previous methane/air flame experiment (Saiki and Suzuki, 2013). In the present OH field with electric discharge, it is easier to investigate the radical quenching effect, as the wall chemical effect on OH is decoupled from the gas-phase reaction. The chemical action defined as the wall-normal OH concentration gradient divided by the local OH concentration, which is an index of the wall chemical effect, increases with increasing wall temperature in the OH field generated by pulsed electric discharge.

Scavenging effects of ascorbic acid and mannitol on hydroxyl radicals generated inside water by an atmospheric pressure plasma jet

In this study, we have studied the scavenging effects of radical scavengers (ascorbic acid and mannitol) on •OH radicals generated inside water by an atmospheric pressure plasma jet. Plasma was generated by using a sinusoidal power supply (applied voltage: 2.4 kV, frequency: 40 kHz) at a constant argon gas flow rate of 400 sccm and it was characterized by electrical and optical measurements. The technique of ultra-violet (UV) absorption spectroscopy was applied to investigate the effects of scavenging at 3, 6 and 9 mm depths below the water surface by using terephthalic acid as a •OH radical sensitive probe. Also, the effects of scavenging were studied by changing the concentration of radical scavengers and plasma exposure time. For both radical scavengers, the reduced concentrations of •OH radicals were found to decrease with increasing depths as their production would also be reduced by decreasing intensities of plasma-initiated UVs with increasing depths. The effectiveness of ascorbic acid on the scavenging of •OH radicals appeared much stronger than mannitol at all depths inside water since the deprotonated form of ascorbic acid consumes two •OH radicals whereas mannitol consumes only one. Also, the reduced concentration of OH radicals with addition of scavengers was found to be increased under longer plasma exposure time even though the generation of OH radicals were also increased. The detailed mechanism on the formation of •OH radicals inside water has been discussed along with the causes of scavenging. These results can be an important milestone in the applications of non-thermal plasma sources requiring the reduction of •OH radicals.

Modelling of ammonia/air non-premixed turbulent swirling flames in a gas turbine-like combustor at various pressures

This study simulated bluff body stabilised non-premixed turbulent ammonia (NH3)/air flames using swirling flows. Although the space and time-averaged emission (STAE) of NO was found to decrease with increase in the global equivalence ratio (global ) under stoichiometric and rich conditions, similar to the behaviour of premixed flames, the local NO concentration distribution within the combustor was heterogeneous, unlike the almost uniform NO distribution in premixed flames. Zones of high NO concentration were identified near the combustor wall boundaries, whereas there was almost nil NO in the combustor centre, irrespective of the global . The localised NO concentrations in non-premixed flames were shown to depend on the local within the combustor and the model reproduced the NO emission characteristics of premixed flames in terms of . This study also found that the effect of pressure on NO emissions is significant irrespective of the combustion type, because reducing the OH radical concentration in NH3/air flame at high pressure limits the NO generation. However, the STAE of NO of non-premixed flames were slightly higher than those of premixed flames, especially under rich conditions, possibly owing to zones of elevated NO concentrations near the wall boundaries. The introduction of an additional NH3 stream external to the air stream (by splitting the original NH3 flow) may mitigate this effect by creating an almost uniform distribution within the combustor and thus a homogeneous NO distribution. The STAE of NO was minimised (and equalled that of a premixed flame) by applying a volumetric flow ratio between the innermost and outermost NH3 flows of 0.6:0.4.