OH Density Research Articles

Hydroxyl Spectroscopy of Laboratory Air Laser-Ignition

This work investigates spatial and temporal distributions of hydroxyl, OH, in laser-plasma in laboratory air at standard ambient temperature and pressure. Of interest are determination of temperature and density of OH and establishment of a correlation of molecular OH emission spectra with shadow graphs for time delays of 50 to 100 μs, analogous to previous work on shadow graph and emission spectroscopy correlation for cyanide, CN, in gas mixtures and for time delays of the order of 1 μs. Wavelength- and sensitivity-corrected spatiotemporal data analysis focuses on temperature inferences using molecular OH emission spectroscopy. Near-IR radiation from a Q-switched laser device initiates optical breakdown in laboratory air. The laser device provides 6 ns, up to 850 milli Joule, pulses at a wavelength of 1064 nm, and focal irradiance in the range of 1 to 10 terawatt per centimeter-squared. Frequency doubled beams are utilized for capturing shadow graphs for visualization of the breakdown kernel at time delays in the range of 0.1 to 100 μs. OH emission spectra of the laser plasma, spatially resolved along the slit dimension, are recorded in the wavelength range of 298 nm to 321 nm, and with gate widths adjusted to 10 μs for the intensified charge-coupled device that is mounted at the exit plane of a 0.64 m Czerny-Turner configuration spectrometer. Diatomic OH signals occur due to recombination of the plasma and are clearly distinguishable for time delays larger than 50 μs, but are masked by spectra of N2 early in the plasma decay.

Generation of High-Density Pulsed Gas–Liquid Discharge Plasma Using Floating Electrode Configuration at Atmospheric Pressure

In this paper, a high-density gas–liquid discharge plasma is obtained combined with nanosecond pulse voltage and a floating electrode. The discharge images, the waveforms of pulse voltage and discharge current, and the optical emission spectra are recorded. Gas temperature and electron density are calculated by the optical emission spectra of N2 (C3Πu → B3Πg) and the Stark broadening of Hα, respectively. The emission intensities of N2 (C3Πu → B3Πg), N2+ (B2Σ → X2Π), OH (A2Σ → X2Π), O (3p5P → 3s5S0), He (3d3D → 3p3P20), gas temperature, and electron density are acquired by optical emission spectra to discuss plasma characteristics varying with spatial distribution, discharge gap, and gas flow rate. The spatial distributions of discharge characteristics, including gas temperature, electron density, and emission intensities of N2 (C3Πu → B3Πg), N2+ (B2Σ → X2Π), OH (A2Σ → X2Π), O (3p5P → 3s5S0), and He (3d3D → 3p3P20), are presented. It is found that a high-density discharge plasma with the electron density of 2.2 × 1015 cm−3 and low gas temperature close to room temperature is generated. While setting the discharge gap distance at 10 mm, the discharge area over liquid surface has the largest diameter of 20 mm; under the same conditions, electron density is in the order of 1015 cm−3, and gas temperature is approximately 330 K. In addition, the discharge plasma characteristics are not kept consistent in the axial direction, in which the emission intensities of N2+ (B2Σ → X2Π), N2 (C3Πu → B3Πg), OH (A2Σ → X2Π), and gas temperature increased near the liquid surface. As the discharge gap is enlarged, the gas temperature increases, whereas the electron density remains almost constant. Moreover, as the gas flow rate was turned up, the electron density increased and the gas temperature was kept constant at 320 K.

Characterization of OH species in kHz He/H2O atmospheric pressure dielectric barrier discharges

This work investigates experimentally and numerically the chemical kinetics of OH species generated in kHz helium atmospheric-pressure dielectric barrier discharges with different H2O concentrations. The OH densities of cases are determined from measurements of the ultraviolet absorption spectroscopy (UVAS) system with transition parameters evaluated by LIFBASE. The 1D plasma fluid model (PFM) with compiled chemistry covering chemical kinetics of helium, H2O, and interaction reactions of reactive species including the formation of charged cluster ions is employed to capture the discharge dynamics for analyzing mechanisms of OH species properly. The simulated current densities of cases with 0.8% and 2.0% H2O concentrations increase from 29 to 51 A m−2, agreeing with experimental measurements. The simulated OH density increases from 1.7 × 1019 m−3 to 2.4 × 1019 m−3 as the H2O concentration increases from 0.4% to 2.0%, agreeing with OH densities interpreted from the UVAS system for cases with various H2O concentrations. In general, the 1D PFM developed captures the discharge behavior and predicts the OH densities of cases with different H2O concentrations. The simulated results reveal that the electron and H2O+ are essential species for OH production through dissociative reactions (e + H2O → e + OH + H) and (H2O+ + H2O → H+(H2O) + OH) contributing to 61% and 23% of OH production, respectively. It is observed that the OH self-recombination reaction (OH + OH + He → H2O2 + He) contributes to 32% of OH consumption as the dominant reaction. Moreover, the stepwise recombination reactions (OH + H2O2 → H2O + HO2 and OH + HO2 → O2 + H2O) initiated by OH species contribute to the overall 27% of OH consumption as another dominant mechanism. The Penning ionization reactions involving water molecules are the dominant reactions for electron production in the present discharge with H2O up to 0.8%, then the electron-impact ionization becomes the dominant reaction for electron production in cases with higher H2O concentrations. As the essential species contributing to the OH consumption, H species is produced majorly from the dissociation reaction (e + H2O → e + OH + H). As one of the dominant OH production reactions, the dissociative attachment (e + H2O → H− + OH) is the dominant electron consumption reaction, resulting in the reactive H− species as the source of negative cluster ions. The high average reaction rates of dissociative attachment reactions consume most of the electrons after the breakdown, which leads to the variation of discharge current density. The saturation of OH species, which is observed experimentally and numerically, is attributed to the transition of electron chemical kinetics and the recombination reactions of OH species.

Absolute OH density measurements in a CO2–H2O glow discharge by laser-induced fluorescence spectroscopy

Time-resolved absolute OH density measurements in a millisecond-pulsed DC glow discharge from carbon dioxide with water admixture, by laser-induced fluorescence (LIF) spectroscopy, are presented. For this purpose, a novel technique is suggested to calibrate the OH LIF setup by means of two-photon absorption LIF spectroscopy measurements on a CO-filled gas cell. OH densities in the order of 1 × 1018 m−3 are reported at a pressure of 6.67 mbar with a water admixture of 20% and a discharge current of 50 mA. Furthermore, the time evolution of the CO2 conversion is determined from collision energy transfer LIF and validated against the literature. The time-dependent rotational temperature of OH is obtained with LIF thermometry.

Plasma dynamics, instabilities and OH generation in a pulsed atmospheric pressure plasma with liquid cathode: a diagnostic study

While plasma–liquid interactions have been an important focus in the plasma research community, the impact of the strong coupling between plasma and liquid on plasma properties and processes remains not fully understood. In this work, we report on the impact of the applied voltage, pulse width and liquid conductivity on the plasma morphology and the OH generation for a positive pulsed DC atmospheric pressure plasma jet with He–0.1% H2O mixture interacting with a liquid cathode. We adopted diagnostic techniques of fast imaging, 2D laser induced fluorescence of OH and Thomson scattering spectroscopy. We show that plasma instabilities and enhanced evaporation occur and have a significant impact on the OH generation. At elevated plasma energies, it is found that the plasma contracts due to a thermal instability through Ohmic heating and the contraction coincides with a depletion in the OH density in the core due to electron impact dissociation. For lower plasma energies, the instability is suppressed/delayed by the equivalent series resistor of the liquid electrode. An estimation of the energy flux from the plasma to the liquid shows that the energy flux of the ions released into the liquid by positive ion hydration is dominant, and significantly larger than the energy needed to evaporate sufficient amount of water to account for the measured H2O concentration increase near the plasma–liquid interface.

Reaction dynamic process of low pressure discharge plasma in microwave resonant cavity

Low-pressure discharge is an important problem that restricts the development of microwave components of spacecraft toward high-power and miniaturization. To clarify the mechanism of low-pressure discharge of microwave component in spacecraft, we build an emission spectroscopy diagnostic platform for studying the low-pressure radio frequency (RF) discharge plasma, and investigate the plasma reaction dynamics of low-pressure RF discharge of microwave cavity resonator and the damage effect of discharge on microwave component. The emission spectra of the plasma inside the resonator under different gas pressure conditions are obtained, and it is found that the density of hydroxyl OH (A-X), excited nitrogen molecules N<sub>2</sub> (C-B) and oxygen atoms O (3p<sup>5</sup>P→3s<sup>5</sup>S<sup>0</sup>) in the plasma each show a first-increasing and then decreasing trend with the increase of gas pressure. The kinetic mechanism of the plasma reaction behind this phenomenon is analyzed, and it is found that the gas pressure can influence the concentration magnitude of each species in the plasma by changing the species production and consumption paths as well as the average electron temperature of the plasma. The variation law of plasma emission spectrum with the input power is studied, and the trends of linear increase of particle concentration with the increase of input power at different air pressures are found. This study provides a reference for investigating the mechanism of low-pressure RF discharge and the reliable design of spacecraft microwave components.

Influence of different O2/H2O ratios on He atmospheric pressure plasma jet impinging on a dielectric surface

A two-dimensional (2D) axisymmetric fluid model is built to investigate the effect of different O2 and H2O admixture on the plasma dynamics and the distribution of reactive species in He atmospheric pressure plasma jet (APPJ). The increase of O2:H2O ratio slows down both the intensity and the propagation speed of the ionization wave. Due to the decrease of both H2O ionization rate and H2O Penning ionization as well as the stronger electronegativity of O2, the increase of O2:H2O ratio results in a significant reduction of electron density in the APPJ, which restricts the occurrence of electron collision ionization reactions and inhibits the propagation of plasma. The excitation energy loss of O2 is not the reason for the weakening of the plasma ionization wave. The densities of O2 +, O− and O2 − increase with the rise of O2 admixture while H2O+ decreases due to the decrease of electron density and H2O concentration. OH− density is affected by both the increase of O− and the decrease of H2O so it shows a peak in the case of O2:H2O = 7:3. O is mainly produced by the excitation reactions and the electron recombination reaction (e + O2 + → 2O), which is directly related to the O2 concentration. OH is mainly produced by e + H2O → e + H + OH so the OH density decreases due to the decrease of electron density and H2O concentration with the increase of O2:H2O ratio. On the dielectric surface, when the propagation of the streamer extinguishes, O flux shows an upward trend while the OH flux decreases, and the propagation distance of O and OH decreases with the increase of O2:H2O ratio.

A hybrid computational framework for the simulation of atmospheric pressure plasma jets: the importance of the gas flow model

A hybrid computational framework, consisting of a detailed turbulence flow model, a global model, and a model for the calculation of the electron energy probability function, is developed to predict the density of plasma generated species along the axial direction of plasma jets. The framework is applied to an Ar/O2 plasma in a kINPen 09 device without a shielding gas. A reaction set of 764 reactions and 84 species is considered. The effect of different turbulence flow models, namely the detailed and high cost large eddy simulation (LES) model and the simple and low cost realizable k–ε model, on the densities of plasma generated species is investigated at different values of absorbed power. The effect is not severe on the density of the majority of the species, justified by the small differences in the inputs of the global model, i.e. the volume averaged axial velocity and density of air species (coming from the turbulence flow model). Nevertheless, the differences in the densities of O2(1Σg), O−, O2 −, O(1D), O, H, H2(r), H−, N2O(v), H7O3 +, H9O4 +, H15O7 + and OH− are remarkably affected by the choice of the turbulence flow model and may reach an order of magnitude. The detailed LES model is a proper choice for Ar jets and this is reinforced by the comparison of the results of the framework with atomic oxygen experimental measurements along the axial direction of the jet: the use of the LES model leads to atomic oxygen density closer to the measured one compared to (the use of) the realizable k–ε model. Finally, an evaluation of the assumptions required for the use of global models in plasma jets is performed, demonstrating their validity for the case studied.

Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds

Abstract We present high-sensitivity CH 9 cm ON/OFF observations toward 18 extragalactic continuum sources that have been detected with OH 18 cm absorption in the Millennium survey with the Arecibo telescope. CH emission was detected toward 6 of the 18 sources. The excitation temperature of CH has been derived directly through analyzing all detected ON and OFF velocity components. The excitation temperature of CH 3335 MHz transition ranges from −54.5 to −0.4 K and roughly follows a log-normal distribution peaking within [−5, 0] K, which implies overestimation by 20% to more than 10 times during calculating CH column density by assuming the conventional value of −60 or −10 K. Furthermore, the column density of CH would be underestimated by a factor of 1.32 ± 0.03 when adopting local thermal equilibrium assumption instead of using the CH three hyperfine transitions. We found a correlation between the column density of CH and OH following log N(CH) = (1.80 ± 0.49) log N(OH) −11.59 ± 6.87. The linear correlation between the column density of CH and H2 is consistent with that derived from visible wavelengths studies, confirming that CH is one of the best tracers of H2 components in diffuse molecular gas.

Effect of fluorine doping on the network pore structure of non-porous organosilica bis(triethoxysilyl)propane (BTESP) membranes for use in molecular separation

Long-chain organosilica bis(triethoxysilyl)propane (BTESP) membranes typically have a flexible non-porous structure. Fluorine was used to tune the network pore structure of BTESP membranes in an effort to improve the gas permeation properties. The network pore size was enlarged and the effect of calcination temperature on the network structure was evaluated based on gel and membrane characterizations. Fluorine-doped BTESP membranes calcined at 350 °C and 650 °C have shown H2 permeance on the orders of 1.2 × 10−6 mol m−2 s−1 Pa−1 and 1.5 × 10−6 mol m−2 s−1 Pa−1 with H2/N2 selectivities of 8 and 6, respectively, which indicates similar pore sizes with lower condensation effect at high temperature of 650 °C, that was suppressed due to the presence of Si–F and C–F bonds. Undoped BTESP membranes, on the other hand, showed H2/N2 selectivity that was significantly lower—from 24 to 11 at 650 °C. FT-IR and N2 adsorption isotherms clearly indicated that fluorine significantly decreased the Si–OH density and increased the surface area and micropore volume. Further water adsorption analysis revealed that fluorine significantly increased the hydrophobicity of the BTESP network structure. Overall, the results of this study endorse the effectiveness of fluorine to control the network pore structure in both wet and dry molecular separation systems.

Plasma-droplet interaction study to assess transport limitations and the role of ⋅OH, O⋅,H⋅,O2(a 1Δg),O3, He(23 S) and Ar(1s 5) in formate decomposition

Plasmas interacting with liquid microdroplets are gaining momentum due to their ability to significantly enhance the reactivity transfer from the gas phase plasma to the liquid. This is, for example, critically important for efficiently decomposing organic pollutants in water. In this contribution, the role of ⋅OH as well as non-⋅OH-driven chemistry initiated by the activation of small water microdroplets in a controlled environment by diffuse RF glow discharge in He with different gas admixtures (Ar, O2 and humidified He) at atmospheric pressure is quantified. The effect of short-lived radicals such as O⋅ and H⋅ atoms, singlet delta oxygen (O2(a 1Δg)), O3 and metastable atoms of He and Ar, besides ⋅OH radicals, on the decomposition of formate dissolved in droplets was analyzed using detailed plasma diagnostics, droplet characterization and ex situ chemical analysis of the treated droplets. The formate decomposition increased with increasing droplet residence time in the plasma, with ∼70% decomposition occurring within ∼15 ms of the plasma treatment time. The formate oxidation in the droplets is shown to be limited by the gas phase ⋅OH flux at lower H2O concentrations with a significant enhancement in the formate decomposition at the lowest water concentration, attributed to e−/ion-induced reactions. However, the oxidation is diffusion limited in the liquid phase at higher gaseous ⋅OH concentrations. The formate decomposition in He/O2 plasma was similar, although with an order of magnitude higher O⋅ radical density than the ⋅OH density in the corresponding He/H2O plasma. Using a one-dimensional reaction–diffusion model, we showed that O2(a 1Δg) and O3 did not play a significant role and the decomposition was due to O⋅, and possibly ⋅OH generated in the vapor containing droplet-plasma boundary layer.

Following O and OH in He/O2 and He/H2O gas mixtures—from the gas phase through the liquid phase to modifications on a biological sample

Applied cold atmospheric plasma allows for the controlled delivery of reactive oxygen and nitrogen species tailored for specific applications. Through the manipulation of the plasma parameters, feed gases, and careful consideration of the environment surrounding the treatment target, selective chemistries that preferentially influence the target can be produced and delivered. To demonstrate this, the COST reference microscale atmospheric pressure plasma jet is used to study the generation and transport of O and OH from the gas phase through the liquid to the biological model target cysteine. Relative and absolute species densities of OH and O are measured in the gas phase through laser induced fluorescence (LIF) and two-photon absorption LIF respectively. The transport of these species is followed into the liquid phase by hydrogen peroxide quantification and visualized by a fluorescence assay. Modifications to the model biological sample cysteine exposed to OH and H2O2 dominated chemistry (He/H2O (0.25%)) and O dominated chemistry (He/O2 (0.6%)) is measured by FTIR spectroscopy. The origin of these species that modify cysteine is considered through the use of heavy water (HO) and mass spectrometry. It is found that the reaction pathways differ significantly for He/O2 and He/H2O. Hydrogen peroxide is formed mainly in the liquid phase in the presence of a substrate for He/O2 whereas for He/H2O it forms in the gas phase. The liquid chemistry resulting from the He/O2 admixture mainly targets the sulfur moiety of cysteine for oxidation up to irreversible oxidation states, while He/H2O treatment leads preferentially to reversible oxidation products. The more O or OH/H2O2 dominated chemistry produced by the two gas admixtures studied offers the possibility to select species for target modification.

Absolute OH density and gas temperature measurements by laser induced fluorescence in a microsecond pulsed discharge generated in a conductive NaCl solution

Electrical discharges in bubbles in liquid water have been extensively investigated for many applications including water treatment and often the OH radical is considered to be a key enabler of many applications. Nonetheless, to date, no OH density measurements have been reported for discharges in bubbles or in liquids in general. In this paper, we report an investigation of discharges in bubbles by laser-induced fluorescence (LIF). The plasma was generated by applying a microsecond voltage pulse to a needle electrode submerged in an aqueous NaCl solution in a Hele-Shaw cell. To determine the gas temperature and OH radical density, a LIF model resolving the energy transfer at the rotational level was implemented. Gas compositions at different gas temperature were obtained with a 0-D kinetics model. The LIF model also enabled us to explain the non-equilibrium rotational distribution of the OH ground state by LIF. Gas temperatures and OH radical densities up to 4900 ± 800 K and 3.2 ± 0.5 × 1017 cm−3 are consistent with conditions favoring thermally driven reactions in the plasma-filled vapor bubble.

Chemical kinetics and density measurements of OH in an atmospheric pressure He + O2 + H2O radiofrequency plasma

This work presents experiments and modelling of OH densities in a radio-frequency driven atmospheric-pressure plasma in a plane-parallel geometry, operated in helium with small admixtures of oxygen and water vapour (He + O2 + H2O). The density of OH is measured under a wide range of conditions by absorption spectroscopy, using an ultra-stable laser-driven broad-band light source. These measurements are compared with 0D plasma chemical kinetics simulations adapted for high levels of O2 (1%). Without O2 admixture, the measured density of OH increases from 1.0 × 1014 to 4.0 × 1014 cm−3 for H2O admixtures from 0.05% to 1%. The density of atomic oxygen is about 1 × 1013 cm−3 and grows with humidity content. With O2 admixture, the OH density stays relatively constant, showing only a small maximum at 0.1% O2. The simulations predict that the atomic oxygen density is strongly increased by O2 addition. It reaches ∼1015 cm−3 without humidity, but is limited to ∼1014 cm−3 beyond 0.05% water content. The addition of O2 has a weak effect on the OH density because, while atomic oxygen becomes a dominant precursor for the formation of OH, it makes a nearly equal contribution to the loss processes of OH. The small increase in the density of OH with the addition of O2 is instead due to reaction pathways involving increased production of HO2 and O3. The simulations show that the densities of OH, O and O3 can be tailored relatively independently over a wide range of conditions. The densities of O and O3 are strongly affected by the presence of small quantities (0.05%) of water vapour, but further water addition has little effect. Therefore, a greater range and control of the reactive species mix from the plasma can be obtained by the use of well-controlled multiple gas admixtures, instead of relying on ambient air mixing.

Quantum study of reaction O(3 P) + H2 (v, j) → OH + H: OH formation in strongly UV-irradiated gas.

The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the dependence of the reaction rate coefficient on the rotational and vibrational state of H2 and evaluate the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons, where H2 can be efficiently pumped to excited vibrational states. We use a recently calculated potential energy surface and carry out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O(3 P) + H2 (v, j) → OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2, although even for high vibrational states of H2 (v = 4-5) for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to A V = 1.3-2.3, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, of about 60%. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.

Investigation on the Behavior of Electrons in the Bullet as Well as Its Dependence on Applied Voltage and H2O Concentration in the Atmospheric-Pressure Ar/H2O Plasma Jets

This article presents a systematic investigation on the behavior of electrons in the bullet in the Ar/H2O plasma jet in increasing the applied voltage and water vapor concentration based on a needle-plate discharge configuration and a 1-D particle-in-cell Monte Carlo method. The scattering angle of the electron in the bullet is described using the angle between the electron scattering direction and the bullet propagation direction. The following results are obtained. The evolution characteristic of the angle spectrum of electrons in the bullet in increasing the applied voltage is the same as that in increasing water vapor concentration, namely, the electrons scattered toward the target get increasingly less. When increasing the applied voltage, the OH density, the electron density, and the average electron energy $E_{\mathrm {ave}}$ increase. However, the increase in water vapor concentration leads to an evidently increasing OH density and a decrease in both the electron density and $E_{\mathrm {ave}}$ . In the range of electron scattering angles below 90° in the bullet, the percentage of electrons with energy above 3.27 eV (the threshold of inducing dissociative electron attachment to water molecule) increases with increasing applied voltage but presents a slight decrease in increasing water vapor concentration. The majority of electrons to most probably interact with the target are of energies higher than 3.27 eV. Finally, a notable effect of H2O admixture is that it gives rise to the decrease in not only $E_{\mathrm {ave}}$ but also the electron density in the bullet, especially the latter decreases evidently.

Measurement of the density and rotational temperature of OH in a saturated water vapor slot-excited microwave plasma

OH density and rotational temperature were measured in a saturated water vapor slot-excited microwave plasma using spatio-temporally resolved laser-induced fluorescence. The microwave power was 20–100 W under continuous wave mode, whereas 40–200 W peak under pulse-modulated mode (30% duty cycle, 100 Hz). The water vapor pressure was 2.3 kPa. An approximately 2 mm thick flat-shaped plasma was generated on the surface of a slot antenna. The OH density and rotational temperature in the plasma ranged from 1 × 1014 cm−3 to 1 × 1015 cm−3 and from 1100 K to 3000 K, respectively. OH was produced via various routes originating from electron collisions (e + H2O) and was primarily lost through recombination (OH+OH → H2O+O) and diffusion. The production rate (k p ) of OH per H2O molecule, which was calculated based on a simplified reaction model using the measured OH density and temperature, showed a relation of , where P represents the microwave power. This relationship implies a two-step production process of OH via the vibrational excitation of H2O. Although the OH density increased with increasing microwave power, the density became saturated at high power values.

Measuring plasma-generated ·OH and O atoms in liquid using EPR spectroscopy and the non-selectivity of the HTA assay

Plasma-generated hydroxyl radicals (·OH) and oxygen atoms (O) produced by the COST reference plasma jet, a micro-scaled atmospheric pressure plasma jet, were investigated using a variety of experimental techniques. Several gas admixtures were studied to distinguish the contributions of the two reactive oxygen species. Large discrepancies between inferred aqueous ·OH densities were noted when using a 2-hydroxyterephthalic acid (HTA) fluorescence assay and electron paramagnetic resonance (EPR) measurements with the spin trap 5,5-dimethyl-1-pyrroline N-oxide—especially when oxygen was present in the feed gas. A series of follow-up experiments including optical emission spectroscopy, H2O2 quantification, and EPR measurements of atomic oxygen using the spin trap 2,2,6,6-tetramethylpiperidine, revealed that the inconsistencies between the measured aqueous ·OH were likely due to the propensity of atomic oxygen to hydroxylate TA in a manner indistinguishable from ·OH. This renders the HTA assay non-selective when both ·OH radicals and atomic oxygen are present, which we report for all three gas admixtures in our experiments. Additionally, considerable degradation of both HTA and the spin adducts measured using EPR spectroscopy was apparent, meaning actual radical densities in the plasma-treated liquid may be considerably higher than implied. Degradation rates compared favorably to previously measured gas phase densities of atomic oxygen in the predecessor of the COST jet and reported degradation of other chemical probes. These results show the prolific role of atomic oxygen in plasma-induced liquid chemistry and caution against diagnostic techniques that are unable to account for it.

Simultaneous Retrievals of Nighttime O(3P) and Total OH Densities From Satellite Observations of Meinel Band Emissions

AbstractRetrieving the total number density [OH], which is used in the chemical balance equations and is the sum of both ground and excited vibrational state populations, is a challenging problem to such a degree there exist no such estimates from recent space observations. We present a novel retrieval approach to simultaneously and self‐consistently derive both [O(3P)] and total [OH] in the nighttime mesosphere and lower thermosphere which operates with the Meinel band volume emission rates and their ratios. Its application to the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) observations shows a good agreement of the retrieved [O(3P)] taking into account the measurement uncertainties and variation of model inputs used. However, retrieved [OH] show a large discrepancy, mainly above 90 km. SCIAMACHY [OH] is generally in good agreement with WACCM [OH] after accounting for uncertainties, while SABER OH is larger than WACCM [OH] by up to a factor of 2.

The distribution of OH radicals in a pulsed DC He + H2O plasma jet with the presence of a dielectric material

The distribution and density of OH radicals in a pulsed DC He + H2O plasma jet are studied by a two-dimensional (2D) plasma jet model. This article shows the effect of adding H2O to the pure He plasma jet on the OH density distribution and the changes of the OH density under different H2O contents. The process of streamer propagation toward the dielectric surface and development of the streamer on the dielectric surface are discussed in this article. The total OH radicals are generated from two parts in a He + H2O atmospheric pressure plasma jet (APPJ). The first part is produced with the water in ambient air outside the tube, and the other part is generated with the water in the helium flow. At the time when the streamer approaches the dielectric surface, it is found that electron-impact dissociation of H2O and electron neutralization of H2O+ are the main reactions to generate OH species and three-body combination of OH and OH with He is the main reaction to consume OH species. The OH density shows a donut shape in a pure He APPJ, while the density distribution of OH radicals under the nozzle gradually becomes uniform with the increase in the H2O content added to the helium flow, but the maximum value is at the both ends of the streamer outside the tube. The electron density initially increases with the increase in the proportion of H2O added to the He plasma jet and then gradually decreases with the H2O ratio greater than 0.5%. The enhancement of Penning ionization with the increasing H2O content is the main reason for the increase in electron density, while the increasing consumption of electrons by the excitation energy loss causes the decrease in the electron density with the H2O ratio more than 0.5%. The increase in the H2O content will bring both positive and negative effects. On the one hand, the increase in the H2O content added to the He gas flow will promote the generation of OH species. On the other hand, the decrease in the electron density caused by the increasing excitation energy loss slows the rapid growth of the reaction rates of the two main reactions related to the production of OH species. Eventually, OH density reaches saturation with the H2O ratio at 1.5% under the combined effect mentioned above.