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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

On the Role of Trend and Variability of Hydroxyl Radical (OH) in the Global Methane Budget

Decadal trends and interannual variations in the hydroxyl radical (OH), while poorly constrained at present, are critical for understanding the observed evolution of atmospheric methane (CH4). Through analyzing the OH fields simulated by the model ensemble of the Chemistry-Climate Model Initiative (CCMI), we find (1) the negative OH anomalies during the El Niño years mainly corresponding to the enhanced carbon monoxide (CO) emissions from biomass burning and (2) a positive OH trend during 1980-2010 dominated by the elevated primary production and the reduced loss of OH due to decreasing CO after 2000. Both two-box model inversions and variational 4D inversions suggest that ignoring the negative anomaly of OH during the El Niño years leads to a large overestimation of the increase in global CH4 emissions by up to10±3Tg yr-1to match the observed CH4 increase over these years. Not accounting for the increasing OH trends given by the CCMI models leads to an underestimation of the CH4 emission increase by 23±9Tg yr-1from 1986 to 2010. The variational inversion estimated CH4 emissions show that the tropical regions contribute most to the uncertainties related to OH. This study highlights the significant impact of climate and chemical feedbacks related to OH on the top-down estimates of the global CH4 budget.

Hydroxyl radical (OH)↗
NASA NTRS1991

Photodissociation of the hydroxyl radical (OH) at 157 nm

The photodissociation of the OH radical was studied at 157 nm via the detection of the product H atoms with the resonance fluorescence technique. OH radicals were produced in a fast-flow cell from the reaction between H and NO2 and subsequently photodissociated by an excimer laser operating on the F2 emission. The quantum yield for photodissociation of OH was measured to be 1.10 + or - 0.28. The photodissociation cross section was calculated to be 6.6 x 10 to the -18th sq cm (+ or - 25 percent).

Cody, R. J.↗
NASA NTRS2017

Quantifying the Causes of Differences in Tropospheric OH Within Global Models

The hydroxyl radical (OH) is the primary daytime oxidant in the troposphere and provides the main loss mechanism for many pollutants and greenhouse gases, including methane (CH4). Global mean tropospheric OH differs by as much as 80% among various global models, for reasons that are not well understood. We use neural networks (NNs), trained using archived output from eight chemical transport models (CTMs) that participated in the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols and Transport Model Intercomparison Project (POLMIP), to quantify the factors responsible for differences in tropospheric OH and resulting CH4 lifetime (Tau CH4) between these models. Annual average Tau CH4, for loss by OH only, ranges from 8.0 to 11.6 years for the eight POLMIP CTMs. The factors driving these differences were quantified by inputting 3-D chemical fields from one CTM into the trained NN of another CTM. Across all CTMs, the largest mean differences in Tau CH4 (Delta Tau CH4) result from variations in chemical mechanisms (Delta Tau CH4 = 0.46 years), the photolysis frequency (J) of O3 yields O(D-1) (0.31 years), local O3 (0.30 years), and CO (0.23 years). The Delta Tau CH4 due to CTM differences in NO(x) (NO + NO2) is relatively low (0.17 years), although large regional variation in OH between the CTMs is attributed to NO(x). Differences in isoprene and J(NO2) have negligible overall effect on globally averaged tropospheric OH, although the extent of OH variations due to each factor depends on the model being examined. This study demonstrates that NNs can serve as a useful tool for quantifying why tropospheric OH varies between global models, provided that essential chemical fields are archived.

Surface Measurements and Models↗
DOE OSTI2024

Absolute Reactive Oxygen Species Densities in the Effluent of the COST Reference Source and Plasma-generated Atomic Oxygen Density Measurements in Liquid using TALIF

Reactive oxygen species are one of the most important species produced in a plasma. In particular the role of atomic oxygen and the hydroxyl radical OH has attracted many studies ranging from plasma medicine to plasma-driven biocatalysis and water treatment. Atomic oxygen and the hydroxyl radical OH are very reactive and react unselectively with many different substrates. The quantification of atomic oxygen and OH is challenging because of their reactivity and unknown reactions and rate constants.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1994

Aircraft-borne, laser-induced fluorescence instrument for the in situ detection of hydroxyl and hydroperoxyl radicals

The odd-hydrogen radicals OH and HO2 are central to most of the gas-phase chemical transformations that occur in the atmosphere. Of particular interest is the role that these species play in controlling the concentration of stratospheric ozone. This paper describes an instrument that measures both of these species at volume mixing ratios below one part in 10(exp 14) in the upper troposphere and lower stratosphere. The hydroxyl radical (OH) is measured by laser induced fluorescence at 309 nm. Tunable UV light is used to pump OH to the first electric state near 282 nm. the laser light is produced by a high-repetition rate pulsed dye-laser powered with all solid-state pump lasers. HO2 is measured as OH after gas-phase titration with nitric oxide. Measurements aboard a NASA ER-2 aircraft demonstrate the capability of this instrument to perform reliably with very high signal-to-noise ratios (greater than 30) achieved in short integration times (less than 20 sec).

Wennberg, P. O.↗
NASA NTRS1994

High-resolution Fourier transform spectroscopy of the Meinel system of OH

The infrared spectrum of the hydroxyl radical OH, between 1850 and 9000/cm has been measured with a Fourier transform spectrometer. The source, a hydrogen-ozone diffusion flame, was designed to study the excitation of rotation-vibration levels of the OH Meinel bands under conditions similar to those in the upper atmosphere which produce the nighttime OH airglow emission. Twenty-three bands were observed: nine bands in the Delta upsilon = 1 sequence, nine bands in the Delta upsilon = 2 sequence, and five bands in the Delta upsilon = 3 sequence. A global nonlinear least-squares fit of 1696 lines yielded molecular parameters with a standard deviation of 0.003/cm. Term values are computed, and transition frequencies in the Delta upsilon = 3, 4, 5, 6 sequences in the near-infrared are predicted.

Abrams, Mark C.↗
NASA NTRS2022

First Observations of Mesospheric OH Emission Profiles from OMPS/LP

The hydroxyl radical (OH) is one of the most reactive trace species and plays several important roles in the photochemical equilibrium and energy balance in the mesosphere. Global observations of OH from satellite instruments have a role to play in the study of OH and water vapor variations. This study describes an advanced algorithm to detect mesospheric OH emission profiles from the Suomi NPP satellite Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP). A triplet technique has been adapted to the OMPS/LP radiance measurements for determining OH emission signatures and OH index (OHI) from the OH A2Σ+-X2Π 0-0 band near the 308.8-nm wavelength. The derived mesospheric profiles provide an overall picture of the vertical distribution of OHI between 55 and 84 km and seasonal and latitudinal variability of the strength and height of the OHI. The observed annual cycle is correlated with the water vapor cycle and anticorrelated with the mesospheric temperature cycle. The data show that the relationships persist during the period of April 2012–December 2020. The seasonal behavior of OHI may be associated with variations in solar illumination or mesospheric water vapor abundance. The influence of solar illumination is dominant in the midlatitudes, while the OHI pattern is dominated by water vapor photolysis and other influences in the tropics.

Observations of Mesospheric OH Emission↗
DOE OSTI2025

Two-dimensional mapping of absolute OH densities in an atmospheric pressure plasma effluent via planar laser-induced fluorescence: effects of He/H 2 O and He/O 2 mixtures in N 2 and air, with and without solid targets

Planar laser-induced fluorescence (LIF) was employed to measure the absolute density of hydroxyl radicals (OH) in the effluent of the COST Reference Microplasma Jet for two feed gas mixtures: He/H 2 O and He/O 2 . Experiments were conducted with the effluent propagating into air and N 2 environments. For the He/H 2 O case, measurements were also performed with the effluent impinging on a solid target at varying distances from the jet nozzle. Calibration of the OH-LIF signal from the COST-Jet was achieved by comparing it to a reference signal generated by the photofragmentation of H 2 O 2 . Results demonstrated that OH densities were sustained longer when the effluent propagates in a nitrogen environment compared to air, particularly with water added to the feed gas. The broader OH distribution in N 2 suggests slower consumption due to the absence of oxygen, which accelerates OH depletion in air via reactions involving O 2 and HO 2 . Even when water was not added to the feed, as in the He/O 2 case, appreciable OH densities were observed, due to gas impurities and reactive species interactions with atmospheric humidity, forming reaction fronts that delineate the gas flow. Two-dimensional fluid dynamics simulations elucidated the influence of atmospheric gas entrainment and solid targets on the OH distribution. Experimental trends were further compared with a zero-dimensional chemistry model to explore OH production and consumption mechanisms in air and nitrogen environments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2014

Changes in Global Tropospheric OH Expected as a Result of Climate Change Over the Last Several Decades

The oxidizing capacity of the troposphere is controlled primarily by the abundance of hydroxyl radical (OH). The global mean concentration of OH, [OH]_GLOBAL, inferred from measurements of methyl chloroform, has remained relatively constant during the past several decades, despite rising levels of CH4 that should have led to a steady decline. Here we examine other factors that may have affected [OH]_GLOBAL, such as the changing overhead burden of stratospheric O3 using observations from OMI, TOMS and SBUV; rising tropospheric H2O using observations from AIRS and reanalysis fields from MERRA; and widening of the climatological tropics using widening rate estimates from the literature and global OH fields from the GEOS Chemistry-Climate Model. Our analysis suggests these factors may have contributed a positive trend to [OH]_GLOBAL large enough to counter the decrease due to CH4.

Climate Change↗
NASA NTRS2020

Missing OH Reactivity in the Global Marine Boundary Layer

The hydroxyl radical (OH) reacts with thousands of chemical species in the atmosphere, initiating their removal and the chemical reaction sequences that produce ozone, secondary aerosols, and gas-phase acids. OH reactivity, which is the inverse of OH lifetime,influences the OH abundance and the ability of OH to cleanse the atmosphere. The NASA Atmospheric Tomography (ATom) campaign used instruments on the NASA DC-8 aircraft to measure OH reactivity and more than 100 trace chemical species. ATom presented a unique opportunity to test the completeness of the OH reactivity calculated from the chemical species measurements by comparing it to the measured OH reactivity over two oceans across four seasons. Although, throughout much of the free troposphere, the calculated OH reactivity was below the limit-of-detection for the ATom instrument used to measure OH reactivity, the instrument was able to measure the OH reactivity in and just above the marine boundary layer. The mean measured value of OH reactivity in the marine boundary layer across all latitudes and all ATom deployments was 1.9 s-1, which is 0.5 s-1larger than the mean calculated OH reactivity. The missing OH reactivity, the difference between the measured and calculated OH reactivity, varied between 0 s-1to 3.5 s-1, with the highest values over the Northern Hemisphere Pacific Ocean. Correlations of missing OH reactivity with formaldehyde, dimethyl sulfide, butanal, and sea surface temperature suggest the presence of unmeasured or unknown volatile organic compounds or oxygenated volatile organic compounds associated with ocean emissions.

NASA Atmospheric Tomography (ATom)↗
NASA NTRSDate not supplied

Linking OH Variability to Observable Variables, Meteorology and Transport

The hydroxyl radical (OH) plays a vital role in tropospheric chemistry, as it provides the dominant sink for a multitude of pollutants and climate-relevant gases such as methane. Observational constraints on the global distribution and temporal variability of OH are limited, and models simulate a wide range of OH distributions. While OH itself has a short atmospheric lifetime, OH is photochemically coupled to longer-lived species that undergo atmospheric transport. Here, we investigate how much of the OH variability within and between models can be explained by differences in observable species to develop diagnostics for OH differences. We find that NO 2 and water vapor together explain much of the spatial and temporal variability in simulated OH, and we use satellite observations to identify biases in these variables. The OH response to ENSO also differs between models, and we investigate potential causes of these differences such as differences in convection or lightning NOx. We also explore the potential of idealized tracers to represent the OH distribution. Within a single model, meteorological variables such as humidity and idealized tracers of transport can explain a significant portion of the OH spatial variability. We use a Gradient Boosted Regression Trees, a type of machine learning, to account for non-linear relationships between OH and the input variables.

Meteorology↗
NASA NTRS1993

An aircraft instrument design for in situ tropospheric OH measurements by laser induced fluorescence at low pressures

The hydroxyl radical (OH) is important for many processes involved in tropospheric chemistry. For instance, it initiates the photochemical degradation of gases that cause global climate change, such as methane and the chlorofluorocarbon substitutes (HCFCs). Because of its reactivity, its abundances are less than 0.1 pptv. Thus, OH has been very difficult to measure accurately, despite its importance. Techniques have evolved, however, so that good measurements of tropospheric OH abundances are now possible. One of these techniques that is adaptable to aircraft measurements is the laser induced fluorescence detection of the OH radical in a detection chamber at low pressures. The current ground-based instrument, which can be readily adapted to aircraft, can detect OH abundances of 1.4 x 10 exp 5 OH molecules/cu cm with S/N = 2 in 30 sec, and 5 x 10 exp 4/cu cm in 5 min.

Brune, William H.↗
NASA NTRS2012

OH PLIF Visualization of the UVa Supersonic Combustion Experiment: Configuration A

Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were performed in the University of Virginia s dual-mode scramjet experiment. The test section was set up in configuration A, which includes a Mach 2 nozzle, combustor, and extender section. Hydrogen fuel was injected through an unswept compression ramp at two different equivalence ratios. Through the translation of the optical system and the use of two separate camera views, the entire optical range of the combustor was accessed. Single-shot, average, and standard deviation images of the OH PLIF signal are presented at several streamwise locations. The results show the development of a highly turbulent flame structure and provide an experimental database to be used for numerical model assessment.

Johansen, Craig T.↗
NASA NTRS2013

OH PLIF Visualization of the UVa Supersonic Combustion Experiment: Configuration A

Hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were performed in the University of Virginia supersonic combustion experiment. The test section was set up in configuration A, which includes a Mach 2 nozzle, combustor, and extender section. Hydrogen fuel was injected through an unswept compression ramp at two different equivalence ratios. Through the translation of the optical system and the use of two separate camera views, the entire optically accessible range of the combustor was imaged. Single-shot, average, and standard deviation images of the OH PLIF signal are presented at several streamwise locations. The results show the development of a highly turbulent flame structure and provide an experimental database to be used for numerical model assessment.

Johansen, Craig T.↗
NASA NTRS1994

Measurement of tropospheric OH and HO2 by laser-induced fluorescence at low pressure

The hydroxyl radical (OH) is the primary oxidant in the atmosphere, responsible for many photochemical reactions that affect both regional air quality and global climate change. Because of its high reactivity, abundances of OH in the troposphere are less than 1 part per trillion by volume (pptv) and thus difficult to measure accurately. This paper describes an instrument for the sensitive detection of OH in the troposphere using low-pressure laser-induced fluorescence. Ambient air is expanded into a low pressure detection chamber, and OH is both excited and detected using the A(sup 2) Epsilon(+)(v prime = 0) yields X(sup 2)Pi(v double prime = 0) transition near 308 nm. An injector upstream of the detection axis allows for the addition of reagent NO to convert ambient HO2 to OH using the fast reaction HO2 + NO yields OH + NO2. Using recent advances in laser and detector technologies, this prototype instrument is able to detect less than 1 x 10(exp 5) molecules/cu cm (0.004 pptv) of OH with an integration time of 30 s with negligible interferences.

Stevens, P. S.↗
NASA NTRS2013

OH PLIF Visualization of the UVa Supersonic Combustion Experiment: Configuration C

Non-intrusive hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) measurements were obtained in configuration C of the University of Virginia supersonic combustion experiment. The combustion of hydrogen fuel injected through an unswept compression ramp into a supersonic cross-flow was imaged over a range of streamwise positions. Images were corrected for optical distortion, variations in the laser sheet profile, and different camera views. Results indicate an effect of fuel equivalence ratio on combustion zone shape and local turbulence length scale. The streamwise location of the reaction zone relative to the fuel injector was also found to be sensitive to the fuel equivalence ratio. The flow boundary conditions in the combustor section, which are sensitive to the fuel flow rate, are believed to have caused this effect. A combination of laser absorption and radiative trapping effects are proposed to have caused asymmetry observed in the images. The results complement previously published OH PLIF data obtained for configuration A along with other non-intrusive measurements to form a database for computational fluid dynamics (CFD) model validation.

McRae, Colin D.↗
NASA NTRS2016

OH PLIF Visualization of a Premixed Ethylene-fueled Dual-Mode Scramjet Combustor

Hydroxyl radical (OH) planar induced laser fluorescence (PLIF) measurements have been performed in a small-scale scramjet combustor at the University of Virginia Aerospace Research Laboratory at nominal simulated Mach 5 enthalpy. OH lines were carefully chosen to have fluorescent signal that is independent of pressure and temperature but linear with mole fraction. The OH PLIF signal was imaged in planes orthogonal to and parallel to the freestream flow at different equivalence ratios. Flameout limits were tested and identified. Instantaneous planar images were recorded and analyzed to compare the results with width increased dual-pump enhanced coherent anti-Stokes Raman spectroscopy (WIDECARS) measurements in the same facility and large eddy simulation/Reynolds average Navier-Stokes (LES/RANS) numerical simulation. The flame angle was found to be approximately 10 degrees for several different conditions, which is in agreement with numerical predictions and measurements using WIDECARS. Finally, a comparison between NO PLIF non-combustion cases and OH PLIF combustion cases is provided: the comparison reveals that the dominant effect of flame propagation is freestream turbulence rather than heat release and concentration gradients.

Cantu, Luca M. L.↗
NASA NTRSDate not supplied

Airborne observations in support of a satellite observation-based OH product

This presentation outlines work to date analyzing airborne observations to improve a satellite observation-based hydroxyl radical (OH) column product by interrogating model chemistry processes. Anderson et al., 2023, established a machine learning method to combine satellite observations of O3, CO, NO2, HCHO, H2O, and aerosol optical depth, along with analyzed sea surface temperatures, for the prediction of tropospheric column OH (TCOH). Since the TCOH machine learning model was trained on output from the MERRA-2 GMI model simulation, we seek to identify if any model deficiencies may yield errors in the TCOH prediction. By evaluating F0AM box model simulations and neural networks trained to reproduce in situ OH concentrations, together with output reaction rates and interpretability metrics, respectively, we validate the Anderson et al. TCOH model and assess the largest contributors to uncertainties.

Hydroxyl, oxidizing capacity, troposphere, airborn↗