OH Layer Research Articles

Convection: the likely source of the medium-scale gravity waves observed in the OH airglow layer near Brasilia, Brazil, during the SpreadFEx campaign

Abstract. Six medium-scale gravity waves (GWs) with horizontal wavelengths of λH=60–160 km were detected on four nights by Taylor et al. (2009) in the OH airglow layer near Brasilia, at 15° S, 47° W, during the Spread F Experiment (SpreadFEx) in Brazil in 2005. We reverse and forward ray trace these GWs to the tropopause and into the thermosphere using a ray trace model which includes thermospheric dissipation. We identify the convective plumes, convective clusters, and convective regions which may have generated these GWs. We find that deep convection is the highly likely source of four of these GWs. We pinpoint the specific deep convective plumes which likely excited two of these GWs on the nights of 30 September and 1 October. On these nights, the source location/time uncertainties were small and deep convection was sporadic near the modeled source locations. We locate the regions containing deep convective plumes and clusters which likely excited the other two GWs. The last 2 GWs were probably also excited from deep convection; however, they must have been ducted ~500–700 km if so. Two of the GWs were likely downwards-propagating initially (after which they reflected upwards from the Earth's surface), while one of the GWs was likely upwards-propagating initially from the convective plume/cluster. We also estimate the amplitudes and vertical scales of these waves at the tropopause, and compare their scales with those from a simple, linear convection model. Finally, we calculate each GW's dissipation altitude, location, and amplitude. We find that the dissipation altitude depends sensitively on the winds at and above the OH layer. We also find that several of these GWs may have penetrated to high enough altitudes to potentially seed equatorial spread F (ESF) if located somewhat farther from the magnetic equator.

Spatial scales of extinction and dissipation in the near field of non-premixed turbulent jet flames

Simultaneous high-resolution Rayleigh scattering imaging and planar laser-induced fluorescence (PLIF) of OH are combined to measure the dissipative scales associated with thermal mixing and the structure and scales of extinguished regions of the reaction zone. Measurements are performed throughout the near field ( x/ d = 5, 10, 15, 20) of two turbulent, non-premixed methane/hydrogen/nitrogen jet flames with Re = 15,200 and 22,800 (flames DLR-A and DLR-B of the TNF workshop). Locally extinguished regions are identified by discontinuities in the OH layers, and the extinction hole sizes are measured. For each flame, the probability density function of the hole sizes is very similar throughout the entire near field, with the most likely hole size being 1.9 mm in DLR-A and 1.1 mm in DLR-B. Extinction events are equally probable at all measurement locations in DLR-A. In the DLR-B flame, there is a progression from frequent extinction close to the nozzle to more continuous reaction zones further downstream. The approximate instantaneous location of the stoichiometric contour is determined using the OH-PLIF images, enabling statistical analysis of dissipative scales conditioned on rich and lean conditions. The widths of the thin, elongated structures that dominate the thermal dissipation field are measured. Statistics of this microscale are qualitatively similar in both flames, with the higher Reynolds number producing smaller scales throughout the flow field. For dissipation layers in rich regions, the layer widths increase significantly with increasing temperature, while on the lean side the layer widths decrease with increasing temperature.

Extinction and interruption of diffusion flame interacting with a large scale vortex

Instantaneous and simultaneous measurements of two-dimensional temperature and OH LIF profiles by combining Rayleigh scattering with laser-induced fluorescence (LIF) were demonstrated in nitrogen-diluted hydrogen (H2 30%+N2 70%) diffusion flame interacting with a large scale vortex induced by the continuous acoustic excitation on the fuel side. It is investigated to analyze dynamic behavior of flame extinction and interruption during the flame–vortex interaction processes. The results obtained are described as follows. (1) The reaction zone of the closed dome type is formed due to fuel jet and remaining air at vortex front. The upper OH layer above this region is disintegrated near the central axis. The temperature and the OH LIF at the reaction zone become significantly lowered at the convex and circumferential part of the vortex where temperature layer of the vortex exists and the thinning of the OH LIF and temperature layer and the consequent flame extinction are induced. (2) By curling up motion of the vortex, the burnt gas is pulled up from the upstream reaction zone and makes axial high-temperature region between the vortex and fuel jet. The burnt gas reaches the circumferential reaction zone and the low temperature layer between circumferential reaction zone and the burnt gas dissipates locally with time. The dissipated burnt gas reduces the supply of fuel into the circumferential reaction zone and interruption of the reaction zone is induced.

Modifying the adsorption characteristics of water on Ru{0001} with preadsorbed oxygen

The coadsorption of water and preadsorbed oxygen on Ru{0001) was studied by synchrotron-based high-resolution x-ray photoelectron spectroscopy. A dramatic change was observed in the interaction of water with oxygen between low and high oxygen precoverages. Low oxygen coverages below 0.18 ML induce partial dissociation, which leads to an adsorbed layer of H2O and OH. Around half the oxygen atoms take part in this reaction. All OH recombines upon heating to 200 K and desorbs together with H2O. Oxygen coverages between 0.20 and 0.50 ML inhibit dissociation, instead a highly stable intact water species is observed, which desorbs at 220 K. This species is significantly more stable than intact water on the clean surface. The stabilization is most likely due to the formation of hydrogen bonds with neighboring oxygen atoms. For intermediate oxygen coverages around 0.18 ML, the dissociation behavior depends on the preparation conditions, which points toward possible mechanisms and pathways for partial dissociation of water on Ru{0001}.

OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles – a comparison with OH*(3-1) temperatures from Maynooth (53.2° N, 6.4° W)

Abstract. OH-equivalent temperatures were derived from all of the temperature profiles retrieved in 2004 and 2005 by the ACE-FTS instrument in a 5 degree band of latitude centred on a ground-based observing station at Maynooth. A globally averaged OH volume emission rate (VER) profile obtained from WINDII data was employed as a weighting function to compute the equivalent temperatures. The annual cycle of temperature thus produced was compared with the annual cycle of temperatures recorded at the ground-based station more than a decade earlier from the OH*(3-1) Meinel band. Both data sets showed excellent agreement in the absolute value of the temperature minimum (~162 K) and in its time of occurrence in the annual cycle at summer solstice. Away from mid-summer, however, the temperatures diverged and reach a maximum disagreement of more than 20 K in mid-winter. Comparison of the Maynooth ground-based data with the corresponding results from two nearby stations in the same time-period indicated that the Maynooth data are consistent with other ground stations. The temperature difference between the satellite and ground-based datasets in winter was reduced to 14–15 K by lowering the peak altitude of the weighting function to 84 km. An unrealistically low peak altitude would be required, however, to bring temperatures derived from the satellite into agreement with the ground-based data. OH equivalent temperatures derived from the SABER instrument using the same weighting function produced results that agreed well with ACE-FTS. When the OH 1.6 μm VER profile measured by SABER was used as the weighting function, the OH equivalent temperatures increased in winter as expected but the summer temperatures were reduced resulting in an approximately constant offset of 8.6±0.8 K between ground and satellite values with the ground values higher. Variability in both the altitude and width of the OH layer within a discernable seasonal variation were responsible for the changes introduced. The higher temperatures in winter were due to primarily to the lower altitude of the OH layer, while the colder summer temperatures were due to a thinner summer OH layer. We are not aware of previous reports of the effect of the layer width on ground-based temperatures. Comparison of OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles with OH*(3-1) temperatures from Wuppertal at 51.3° N which were measured during the same period showed a similar pattern to the Maynooth data from a decade earlier, but the warm offset of the ground values was lower at 4.5±0.5 K. This discrepancy between temperatures derived from ground-based instruments recording hydroxyl spectra and satellite borne instruments has been observed by other observers. Further work will be required by both the satellite and ground-based communities to identify the exact cause of this difference.

Strong Fano resonance of oxygen-hydrogen bonds on oblique angle deposited Mg nanoblades

Magnesium hydroxide [Mg(OH)2] thin layers were formed by a chemical reaction between Mg nanoblades and water. Infrared (IR) spectroscopy showed a OH bond-stretching vibrational mode at 3699cm−1. The assignment was verified by reactions with heavy water, producing OD bonds with the expected isotopic frequency shift. An asymmetric Fano line shape was observed for the OH layer on metallic Mg, while a symmetric Lorentzian (or Bright–Wigner) profile was observed for the OH layer on insulating MgH2. The results indicate that the OH layer on the Mg nanoblades is so thin that the vibrational mode couples to the free-electron continuum of Mg metal.

Investigation of local flame structures and statistics in partially premixed turbulent jet flames using simultaneous single-shot CH and OH planar laser-induced fluorescence imaging

We report on the application of simultaneous single-shot imaging of CH and OH radicals using planar laser-induced fluorescence (PLIF) to investigate partially premixed turbulent jet flames. Various flames have been stabilized on a coaxial jet flame burner consisting of an outer and an inner tube of diameter 22 and 2.2 mm, respectively. From the outer tube a rich methane/air mixture was supplied at a relatively low flow velocity, while a jet of pure air was introduced from the inner one, resulting in a turbulent jet flame on top of a laminar pilot flame. The turbulence intensity was controlled by varying the inner jet flow speed from 0 up to 120 m/s, corresponding to a maximal Reynolds number of the inner jet airflow of 13,200. The CH/OH PLIF imaging clearly revealed the local structure of the studied flames. In the proximity of the burner, a two-layer reaction zone structure was identified where an inner zone characterized by strong CH signals has a typical structure of rich premixed flames. An outer reaction zone characterized by strong OH signals has a typical structure of a diffusion flame that oxidizes the intermediate fuels formed in the inner rich premixed flame. In the moderate-turbulence flow, the CH layers were very thin closed surfaces in the entire flame, whereas the OH layers were much thicker. In the high-intensity-turbulence flame, the CH layer remained thin until it vanished in the upper part of the flame, showing local extinction and reignition behavior of the flame. The single-shot PLIF images have been utilized to determine the flame surface density (FSD). In low and moderate turbulence intensity cases the FSDs determined from CH and OH agreed with each other, while in the highly turbulent case a locally broken CH layer was observed, leading to a significant difference in the FSD results determined via the OH and CH radicals. Furthermore, the means and the standard deviations of CH and OH radicals were obtained to provide statistical information about the flames that may be used for validation of numerical calculations.

Temperature dependence of nitrogen mineralization and microbial status in OH horizon of a temperate forest ecosystem

It was hypothesized that increasing air and/or soil temperature would increase rates of microbial processes including litter decomposition and net N mineralization, resulting in greater sequestration of carbon and nitrogen in humus, and consequently development in OH horizon (humus horizon). To quantify the effect of temperature on biochemical processes controlling the rate of OH layer development three adjacent forest floors under beech, Norway spruce and mixed species stands were investigated at Solling forest, Germany by an incubation experiment of OH layer for three months. Comparing the fitted curves for temperature sensitivity of OH layers in relation to net N mineralization revealed positive correlation across all sites. For the whole data set of all stands, a Q10 (temperature sensitivity index) value of 2.35–2.44 dependent on the measured units was found to be adequate for describing the temperature dependency of net N mineralization at experimental site. Species-specific differences of substrate quality did not result in changes in biochemical properties of OH horizon of the forest floors. Temperature elevation increased net N mineralization without significant changes in microbial status in the range of 1 to 15°C. A low Cmic/Corg (microbial carbon/organic carbon) ratio at 20°C indicated that the resource availability for decomposers has been restricted as reflected in significant decrease of microbial biomass.

Catalytic and electrocatalytic activity of Pt-Ru/C electrode for hydrogen oxidation in alkaline and acid aqueous electrolytes: EMSI and EPOC effects

The kinetics of the oxidation of H2 on PtRu/C gas-diffusion electrode was studied by interfacing the electrode with aqueous electrolytes at different pH values. The conducting electrolytes were KOH and HClO4 aqueous solutions with different concentrations. It is shown that the nature of the aqueous electrolyte plays the role of an active catalyst support for the PtRu/C electrode which drastically affects its catalytic properties. During the aforementioned interaction, termed electrochemical metal support interaction (EMSI), the electrochemical potential of the electrons at the catalyst Fermi level is equalized with the electrochemical potential of the solvated electron in the aqueous electrolyte. The electrochemical experiments carried out at various pH values showed that the electrochemical promotion catalysis (EPOC) is more intense when the catalyst-electrode is interfaced with electrolytes with high pH values where the OH ionic conduction prevails. It was concluded that similar to the solid state electrochemical systems EPOC proceeds through the formation of a polar adsorbed promoting layer of OH ? -, electrochemically supplied by the OH species, at the three phase boundaries of the gas exposed gas diffusion catalyst-electrode surface.

Selective MOCVD of titanium oxide and zirconium oxide thin films using single molecular precursors on Si(1 0 0) substrates

Titanium oxide (TiO 2) and zirconium oxide (ZrO 2) thin films have been deposited on modified Si(1 0 0) substrates selectively by metal-organic chemical vapor deposition (MOCVD) method using new single molecular precursor of [M(O i Pr) 2(tbaoac) 2] (M=Ti, Zr; tbaoac=tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (μCP) method was adapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has –CH 3 terminal group. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) by modifying standard synthetic procedures. Selective depositions of TiO 2 and ZrO 2 were achieved in a home-built horizontal MOCVD reactor in the temperature range of 300–500 °C and deposition pressure of 1×10 −3–3×10 −2 Torr. N 2 gas (5 sccm) was used as a carrier gas during film depositions. TiO 2 and ZrO 2 thin films were able to deposit on the hydrophilic area selectively. The difference in surface characteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO 2 or Si–OH layer on the Si(1 0 0) substrate led to the site-selectivity of oxide thin film growth.

O(1S), OH, and O2(b) airglow layer perturbations due to AGWs and their implied effects on the atmosphere

The O(1S) (green line) night airglow emission in response to atmospheric gravity wave (AGW) perturbations was simulated with a linear, one‐dimensional model. The results were combined with previously modeled O2(b, 0–1) atmospheric band and OH Meinel band emission response (Liu and Swenson, 2003) to derive amplitude and phase relations among multiple airglow layers in response to gravity waves with various intrinsic parameters and damping rates (β). The simulations show that the vertical profile of the standard deviation of the perturbed green line volume emission rate (VER) has a centroid altitude that is 3 km lower and a full‐width‐half‐maximum 2.1 km smaller than the unperturbed VER profile, similar to findings for the OH and O2(b) band layers. Relative phase differences and amplitudes of vertically propagating waves can be deduced from zenith observations of the layers. Airglow weighted responses to waves are related through a cancellation factor (CF) for both layer intensity and temperature. The vertical wavelength can be deduced from relative phase information of three airglow layers separated in altitude. The vertical flux of horizontal momentum associated with gravity waves is deduced from intrinsic wave parameters. Wave damping versus altitude is used to deduce the flux divergence and local accelerations resulting from dissipative waves. The simulations are useful in calculating wave information and wave effects on the atmosphere from multiwavelength, zenith airglow observations.

Nanostructure formation by reactions of H 2O with pre-adsorbed O on a Ag(1 1 0) surface

We present results of an STM investigation of water interaction with an oxygen covered Ag(1 1 0) in the case of the O(4 × 1) reconstructed surface. Regarding the formation of one-layer-thick silver nanostructures previously demonstrated, they point to the key role of the surface temperature at which the water dosing is made. Indeed we measure silver nanostructuring for dosing temperatures lower than 235 K. We follow, in real time during the water dosing, the modifications induced at the surface for two temperatures of 200 K and 240 K. Drastic differences are exhibited. At 200 K, after an initial stage of formation of molecular assembly strips along the [0 0 1], the reactive process leading to the conversion to an OH layer occurs clearly going along with the appearance and development of quasi-rectangular silver nanostructures. At 240 K, no such initial phase is evidenced. The complete conversion to an OH row structure of the scanned area occurs with no concomitant silver nanostructure formation. The dynamical behaviour of the reaction front allows the unravelling of the key role of the developing OH row ends intersecting the remaining Ag–O rows as particular reactive adsorption sites for the completion of the OH layer.

Hydroxyl-Induced Wetting of Metals by Water at Near-Ambient Conditions

We report different wetting properties of Cu(110) and Cu(111) at near-ambient conditions using in situ photoemission spectroscopy. At near-ambient conditions of pressure (1 Torr) and temperature (295 K), the Cu(110) surface is covered with a mixed OH and H2O layer, whereas the Cu(111) surface remains clean and adsorbate-free. We show that wetting is controlled by the presence of OH groups on the surface, acting as anchors for water adsorption. Hydroxylation of the Cu(110) surface is facilitated by a lower activation barrier for water dissociation compared to Cu(111).

OH Planar Laser-Induced Fluorescence and Emission Imaging in High Pressure LOx/Methane Flames

The application of planar laser-induced fluorescence of OH to high-pressure liquid oxygen/gaseous methane flames is investigated in this article. As pressure is increased, the maximum level of OH fluorescence decreases while an interfering light intensity increases. It is shown that suitable data can only be obtained by properly tuning the detection scheme. Narrowband filtering of OH fluorescence is required to reduce the level of interfering signals. An analysis of the interfering light indicates that it is associated with polycyclic aromatic hydrocarbon fluorescence originating from a region surrounding the flame. OH and polycyclic aromatic hydrocarbon fluorescence signal amplitudes become comparable at a pressure of 2.5 MPa which constitutes an upper bound for standard imaging. Below that limit the flame is well characterized and features thin, wrinkled OH layers developing in the vicinity of the liquid oxygen jet. The initial flame sheet is continuous but it becomes highly corrugated further downstream when the liquid oxygen jet breaks down. The flame edge standoff distance is greater than a few LOx post lip sizes indicating that stabilization is less well achieved than in the case of liquid oxygen/hydrogen flames where combustion typically begins at less than one lip size from the injector.

HYDROXYL TIME-SCALE CORRELATIONS IN TURBULENT COUNTERFLOW NONPREMIXED FLAMES

Time-series measurements of OH concentration have been obtained in turbulent counterflow nonpremixed H2/CH4/air flames to study the influence of turbulence parameters on the dynamics of such flames by independently varying the Reynolds number (Re) and global strain rate (SR). The autocorrelation functions from the measured time series are found to be self-similar and can be fully characterized by their integral time scales. The integral time scale displays a power law dependence on both Re and SR. At lower strain rates, fluctuations in the OH layer are nearly independent of Reynolds number. An increase in SR changes the spatial displacement of OH, paving the way for faster fluctuations of the OH layer at higher Reynolds numbers. Both the bulk strain rate and Reynolds number scale linearly with the jet exit velocity (U). Furthermore, the combined effect of SR and Re on the integral time scale behaves approximately as U−1.35 in these counterflow nonpremixed flames. Hence, the Re−1.4 dependency previously observed for the OH integral time scale in jet diffusion flames is likely due to a combined effect of jet exit velocity on both strain rate and Reynolds number. Finally, stochastic time-series simulations are used to examine the relationship between time scales for OH and mixture fraction in turbulent nonpremixed flames. We thus demonstrate that integral time scale for mixture fraction is considerably greater for opposed-jet as compared to free-jet flames.

Oxygen plasma treated epitaxial ZnO thin films for Schottky ultraviolet detection

The effects of oxygen plasma treatment on epitaxial ZnO thin films grown by molecular beam epitaxy were studied. The Au–ZnO–In junction exhibiting ohmic behaviour before the treatment gradually changes to a Schottky junction with the increase in oxygen plasma treatment time. The crystallinity and the surface microstructure did not change to any great extent after the treatment. However, the x-ray photoelectron spectroscopy studies show the removal of conductive OH layer from the surface of ZnO films and the current–voltage characteristics of Au–ZnO–In junction exhibit the rectifying behaviour after oxygen plasma treatment. The fabricated Au–ZnO–Au ultraviolet (UV) detector was successfully tested and was observed to be sensitive to the two UV sources used. The photoresponsivities of the UV detector for the irradiation of two different power densities 350 (λ = 356 nm) and 420 µW cm−2 (λ = 254 nm) are 13.5 A W−1, 15.3 A W−1 at 5 V and 128.9 A W−1, 160 A W−1 at 10 V, respectively.

Height-dependent meteor temperatures and comparisons with lidar and OH measurements

A new method is introduced that allows meteor radars to potentially produce height-dependent temperatures, rather than simply averages over the meteor region. The method is applied to data from the Clovar radar, near London, Ontario, and then a three-way comparison between Rayleigh lidar temperatures, hydroxyl temperatures, and meteor temperatures is undertaken. The three methods prove to be complementary. The OH measurements have good accuracy, but suffer slightly from lack of precise knowledge about their height and the fact that they are effectively integrated over the depth of the OH layer. The lidar temperatures are measured at well-defined altitudes and have better accuracy than the meteor method. The meteor temperatures have the largest errors, but still provide sufficient accuracy for many types of atmospheric studies, and have the advantage that these measurements can be made 24 h a day and in all sky conditions (including during cloud and strong sunlight and moonlight). The measurements from these instruments are complementary in that they are useful for studying the temperature on different time and altitude scales. PACS No.: 94.10.Dy

STM study of hydroxyl formation atO∕Ag(110)

We report here on a low temperature scanning tunneling microscopy study of the hydroxylation of oxygen precovered Ag(110) at a crystal temperature of $250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. For water doses comparable with the oxygen coverage, we observe complete removal of the O-Ag added rows and the formation of an OH-induced superstructure in the perpendicular $(⟨1\overline{1}0⟩)$ direction, in accord with previous results. OH sits in the channels of the unreconstructed surface, in agreement with the adsorption at threefold hollow sites proposed by K. Bange et al. [Surf. Sci. 183, 334 (1987)]. The OH layer disorders upon localized voltage pulses. The damage extends along the $⟨1\overline{1}0⟩$ direction, suggesting a strong interaction among neighboring OH groups.

Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow

This paper reports measurements in turbulent nonpremixed CH 4/H 2 jet flames issuing into a heated and highly diluted coflow. These conditions emulate those of moderate or intense low-oxygen-dilution (MILD) combustion. The spatial distribution of the hydroxyl radical (OH), formaldehyde (H 2CO), and temperature, imaged using planar laser-induced fluorescence and Rayleigh scattering laser diagnostic techniques, are measured and presented. Comparisons are made between three jet Reynolds numbers and two coflow O 2 levels. Measurements are taken at two downstream locations. The burner used in this work facilitates the additional study on the effects of the entrainment of surrounding air on the flame structure at downstream locations. Reducing the coflow oxygen level is shown to lead to a suppression of OH as a result of the reduced temperatures in the reaction zone. Decreasing the oxygen level of the coflow also results in a broadening of the OH distribution. At downstream locations, the surrounding air mixes with the jet and coflow. The subsequent drop in the temperature of the oxidant stream can lead to a rupture of the OH layer. Localized extinction allows premixing of the fuel with the surrounding air. The result is an increase in the reaction rate, highlighting the need for homogeneous mixing to maintain MILD combustion conditions.

Observation of night OH in the mesosphere

[1] Satellite measurements from the Aura MLS instrument show a layer of OH near 82 km in the night. This layer confirms earlier measurements by ground-based LIDAR. The MLS and LIDAR observations measure OH in the lowest vibrational state and are distinct, but related chemically, from vibrationally-excited emission from the OH Meinel bands in the near infrared. The Caltech 1-D model has been extended to include vibrational dependence of OH reactions and shows good agreement with MLS OH data and with observations of the Meinel bands. The model shows a chemical lifetime of HOx that increases from less than a day at 80 km to over a month at 87 km. Above this altitude transport processes become an important part of HOx chemistry. The model predicts that ground state OH represents 99% of the total OH up to 84 km. Citation: Pickett, H. M., W. G. Read, K. K. Lee, and Y. L. Yung (2006), Observation of night OH in the mesosphere, Geophys. Res. Lett., 33, L19808, doi:10.1029/2006GL026910.