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NASA NTRS1994

Effects of the Mount Pinatubo eruption on the radiative and chemical processes in the troposphere and stratosphere

The Lawrence Livermore National Laboratory two-dimensional zonally-averaged chemical-radiative-transport model of the global atmosphere was used to study the effects of the 15 June 1991 eruption of the Mt. Pinatubo volcano on stratospheric processes. SAGE 2 time-dependent aerosol surface area density and optical extinction data were used as input into the model. By 22 December 1991, a maximum equatorial change of -1.8 percent in column ozone was derived from heterogeneous chemical processes that convert NO(x) into HNO3 on sulfuric acid aerosols. Radiative feedbacks from increased aerosol optical thickness independently changes column ozone by approximately -3.5 percent for the same period. This occurs from increasing the net heating of the lower stratosphere, which indirectly increases chemical reaction rates via their temperature dependence and from changes in actinic fluxes, which directly modify photodissociation rates. Including both heterogeneous and radiative effects changes column ozone by -5.5 percent. The model-derived change overestimates the decrease in column ozone relative to the TOMS instrument on the Nimbus 7 satellite. Maximum local ozone decreases of 12 percent were derived in the equatorial region, at 25 km. Model-derived column NO2 peaked (-14 percent) at 30 deg S in October 1991. The timing of the NO2 peak is consistent with observation, but the model underestimates the magnitude of the decrease. Local concentrations of NO(x) (NO + NO2), ClO(x) (Cl + ClO), and HO(x) (OH + HO2), in the lower stratosphere between 30 deg S and 30 deg N, were calculated to have changed by -40 percent, +100 to +160 percent, and +120 to +140 percent respectively.

Kinnison, Douglas E.↗
NASA NTRS1991

Human operator tracking performance with a vibrotactile display

Vibrotactile displays have been designed and used as a sensory aid for the blind. In the present work the same 6 x 24 'Optacon' type vibrotactile display (VTD) was used to characterize human operator (HO) tracking performance in pursuit and compensatory tasks. The VTD was connected via a microprocessor to a one-dimensional joy stick manipulator. Various display schemes were tested on the VDT, and were also compared to visual tracking performance using a specially constructed photo diode matrix display comparable to the VTD.

Inbar, Gideon F.↗
NASA NTRS1993

Multiple steady states in atmospheric chemistry

The equations describing the distributions and concentrations of trace species are nonlinear and may thus possess more than one solution. This paper develops methods for searching for multiple physical solutions to chemical continuity equations and applies these to subsets of equations describing tropospheric chemistry. The calculations are carried out with a box model and use two basic strategies. The first strategy is a 'search' method. This involves fixing model parameters at specified values, choosing a wide range of initial guesses at a solution, and using a Newton-Raphson technique to determine if different initial points converge to different solutions. The second strategy involves a set of techniques known as homotopy methods. These do not require an initial guess, are globally convergent, and are guaranteed, in principle, to find all solutions of the continuity equations. The first method is efficient but essentially 'hit or miss' in the sense that it cannot guarantee that all solutions which may exist will be found. The second method is computationally burdensome but can, in principle, determine all the solutions of a photochemical system. Multiple solutions have been found for models that contain a basic complement of photochemical reactions involving O(x), HO(x), NO(x), and CH4. In the present calculations, transitions occur between stable branches of a multiple solution set as a control parameter is varied. These transitions are manifestations of hysteresis phenomena in the photochemical system and may be triggered by increasing the NO flux or decreasing the CH4 flux from current mean tropospheric levels.

Stewart, Richard W.↗
NASA NTRS1994

OSSE observations of the Crab pulsar

We present results of the Compton Gamma-Ray Observatory (CGRO) Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Crab pulsar, made during MJD 48373-48406 (1991 April 27 - 1991 May 30) and MJD 48798-48804 (1992 June 25 - 1992 July 1). Pulsar light curves and spectra over the approximately 0.05 to 10 MeV range are presented. The arrival time of the gamma-ray peak and the radio peak agree to within 30 microsec which is better than the approximately 300 microsec accuracy of the measurements. The overall pulse phase averaged spectrum in the 0.1 - 10 MeV range is well-fit by a power law of the form 0.05 x (E/0.13 MeV)exp(-(1.99 +/- 0.03)) photons/sq cm/s. The outer-gap model (with gap parameter equal to 0.46) provided to us by Ho agrees with the data to better than 20%. The spectra of the bridge and second peak are slightly harder than the first peak as measured by the hardness ratio (approximately 110 - 220 keV)/(approximately 50 - 105 keV): P1 = 0.54 +/- 0.01, P2 = 0.63 +/- 0.01, bridge = 0.68 +/- 0.03. The phase of the two peaks in the light curve is constant over the 50 - 550 keV range to within the accuracy of the measurements (better than 0.02 in phase). No evidence was found for variability of the light curve on timescales from 2 minutes (less than a factor of 1.8) to 1 year (less than a factor of 1.06), where these are 3 sigma upper limits. However, when we examine the historical database, we find, in agreement with Nolan et al. (1993), that there is evidence for a 13 year variation in the ratio of the intensity of peak 2 to peak 1. We show that if this is interpreted as being due to precession (which changes the relative view of the intrinsic gamma-ray pulse as seen on earth), the variation is consistent with models of neutron star structure. The optical data may be in conflict with the interpretation however. We found no statistically significant lines in the 50 - 550 keV range in the spectrum. The average 3 sigma upper limits in 10(exp -3) photons/sq cm/s for lines at 0.073, 0.078, 0.4, 0.44, 0.511, and 0.545 MeV are 0.3, 0.5, 0.6, 0.5, and 0.1.

Ulmer, M. P.↗
NASA NTRS1995

Sensitivity of two-dimensional model predictions of ozone response to stratospheric aircraft: An update

The Goddard Space Flight Center (GSFC) two-dimensional model of stratospheric photochemistry and dynamics has been used to calculate the O3 response to stratospheric aircraft (high-speed civil transport (HSCT)) emissions. The sensitivity of the model O3 response was examined for systematic variations of five parameters and two reaction rates over a wide range, expanding on calculations by various modeling groups for the NASA High Speed Research Program and the World Meteorological Organization. In all, 448 model runs were required to test the effects of variations in the latitude, altitude, and magnetitude of the aircraft emissions perturbation, the background chlorine levels, the background sulfate aerosol surface area densities, and the rates of two key reactions. No deviation from previous conclusions concerning the response of O3 to HSCTs was found in this more exhaustive exploration of parameter space. Maximum O3 depletions occur for high-altitude, low altitude HSCT perturbations. Small increases in global total O3 can occur for low-altitude, high-altitude injections. Decreasing aerosol surface area densities and background chlorine levels increases the sensitivity of model O3 to the HSCT perturbations. The location of the aircraft emissions is the most important determinant of the model response. Response to the location of the HSCT emissions is not changed qualitatively by changes in background chlorine and aerosol loading. The response is also not very sensitive to changes in the rates of the reactions NO + HO2 yields NO2 + OH and HO2 + O3 yields OH + 2O2 over the limits of their respective uncertainties. Finally, levels of lower stratospheric HO(sub x) generally decrease when the HSCT perturbation is included, even though there are large increases in H2O due to the perturbation.

Considine, David B.↗
NASA NTRS1994

The diurnal variation of hydrogen, nitrogen, and chlorine radicals: Implications for the heterogeneous production of HNO2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower statosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heteogeneous hydrolysis of N2O5 on sulfate aerosols agree with measured concentrations of NO, NO2, and ClO throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of (OH) and (HO2) coincides with the rise of (NO) from photolysis of NO2, suggesting a new source of HO(x) that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allow for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of (OH) and (HO2).

Salawitch, R. J.↗
NASA NTRS1994

A new numerical model of the middle atmosphere. 2: Ozone and related species

A new two-dimensional model with detailed photochemistry is presented. The model includes descriptions of planetary wave and gravity wave propagation and dissipation to characterize the wave forcing and associated mixing in the stratosphere and mesosphere. Such a representation allows for explicit calculation of the regions of strong mixing in the middle atmosphere required for accurate simulation of trace gas transport. The new model also includes a detailed description of photochemical processes in the stratosphere and mesosphere. The downward transport of H2, H2O, and NO(y) from the mesosphere to the stratosphere is examined, and it is shown that mesospheric processes can influence the distributions of these chemical species in polar regions. For HNO3 we also find that small concentrations of liquid aerosols above 30 km could play a major role in determining the abundance in polar winter at high latitudes. The model is also used to examine the chemical budget of ozone in the midlatitude stratosphere and to set constraints on the effectiveness of bromine relative to chlorine for ozone loss and the role of the HO2 + BrO reaction. Recent laboratory data used in this modeling study suggest that this process greatly enhances the effectiveness of bromine for ozone destruction, making bromine-catalyzed chemistry second only to HO(x)-catalyzed ozone destruction in the contemporary stratosphere at midlatitudes below about 18 km. The calculated vertical distribution of ozone in the lower stratosphere agrees well with observations, as does the total column ozone during most seasons and latitudes, with the important exception of southern hemisphere winter and spring.

Garcia, Rolando R.↗
NASA NTRS1994

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmposphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller that observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constraints for the reaction CO + OH and OH + HO2 within their specified uncertainties. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. We also consider the possiblity that HO(x) radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity.

Nair, Hari↗
NASA NTRS1995

Quaternary borocarbides: New class of intermetallic superconductors

Our recent discovery of superconductivity (SC) in the four-element multiphase Y-Ni-B-C system at an elevated temperature (TC approximately 12 K) has opened up great possibilities of identifying new superconducting materials and generating new physics. Superconductivity with Tc (greater than 20 K) higher than that known so far in bulk intermetallics has been observed in multiphase Y-Pd-B-C and Th-Pd-B-C systems and a family of single phase materials RENi2B2C (RE= Y, rare earth) have been found. Our investigations show YNi2B2C to be a strong coupling hard type-II SC. HC2(T) exhibits an unconventional temperature dependence. Specific heat and magnetization studies reveal coexistence of SC and magnetism in RNi2B2C (R = Ho, Er, Tm) with magnetic ordering temperatures (Tc approximately 8 K, 10.5 K, 11 K and Tm approximately 5 K, approximately 7K, approximately 4 K respectively) that are remarkably higher than those in known magnetic superconductors . Mu-SR studies suggest the possibility of Ni atoms carrying a moment in TmNi2B2C. Resistivity results suggests a double re-entrant transition (SC-normal-SC) in HoNi2B2C. RENi2B2C (RE = Ce, Nd, Gd) do not show SC down to 4.2 K. The Nd- and Gd-compounds order magnetically at approximately 4.5 K and approximately 19.5 K, respectively. Two SC transitions are observed in Y-Pd-B-C (Tc approximately 22 K, approximately 10 K) and in Th-Pd-B-C (Tc approximately 20 K, approximately 14 K) systems, which indicate that there are at least two structures which support SC in these borocarbides. In our multiphase ThNi2B2C we observe SC at approximately 6 K. No SC was seen in multiphase UNi2B2C, UPd2B2C, UOs2Ge2C and UPd5B3C(0.35) down to 4.2 K. Tc in YNi2B2C is depressed by substitutions (Gd, Th and U at Y-sites and Fe, Co at Ni-sites).

Nagarajan, R.↗
NASA NTRS1996

Airborne Instruments for the In Situ Detection of ClONO2, NO2, ClO, and BrO in the Stratosphere

Design and construction of an in situ sensor for the detection of stratospheric ClONO2, ClO, BrO, and NO2, was conceived as a two-year program. The experiment has two novel components: a resistive silicon thermal dissociation heater used to fragment ClONO2 into ClO and NO2 and a laser-induced fluorescence sensor for N02. These two new components are integrated into an experiment that uses technology developed in our labs for the ER-2 ClO and ER-2 HO(X) instruments. During the first year we reconstructed our laboratory prototypes for ClONO2 and NO2 detection and made substantial improvements in the calibration apparatus. Results from these laboratory experiments have been used to refine the design of the flight instrument. During this year we began the design of all of the long-lead items required to produce a flight instrument: including the design and fabrication of the air flow system used to direct stratospheric air to our halogen sensors, design and prototyping of an aircraft-compatible thermal dissociation heater, and development and test of a new high powered laser system. Finally, we have designed and released for fabrication several subsystems.

Anderson, James G.↗
NASA NTRS1996

Laboratory Studies of Chemical and Photochemical Processes Relevant to Stratospheric Ozone

The purpose of this project is to reduce the uncertainty in several key gas-phase kinetic processes which impact our understanding of stratospheric ozone. The main emphasis of this work is on measuring rate coefficients and product channels for reactions of HOx and NOx species in the temperature range 200 K to 240 K relevant to the lower stratosphere. Other areas of study have included infrared spectroscopic studies of the HO radical, measurements of OH radical reactions with alternative fluorocarbons, and determination of the vapor pressures of nitric acid hydrates under stratospheric conditions. The results of these studies will improve models of stratospheric ozone chemistry and predictions of perturbations due to human influences.

Zahniser, Mark S.↗
NASA NTRS1996

Ozone Production and Loss Rate Measurements in the Middle Stratosphere

The first simultaneous measurements of HO(x), NO(x), and Cl(x) radicals in the middle stratosphere show that NO(x) catalytic cycles dominate loss of ozone (O3) for altitudes between 24 and 38 km; Cl(x) catalytic cycles are measured to be less effective than previously expected; and there is no 'ozone deficit' in the photochemically dominated altitude range from 31 and 38 km, contrary to some previous theoretical studies.

Jucks, Kenneth W.↗
NASA NTRS1996

Review of recent thermophotovoltaic (TPV) research at Lewis Research Center

Thermophotovoltaic (TPV) research at NASA Lewis Research Center that began in the late 1980's is reviewed. This work has been concentrated on low bandgap indium gallium arsenide (InGaAs) PV cells and rare earth yttrium aluminum garnet (YAG) thin film selective emitters, as well as, TPV system studies. An emittance theory has been developed for the thin film emitters. Experimental spectral emittance results for erbium Er-YAG and holmium Ho-YAG show excellent emittance (greater than or equal to .7) within the emission bands. The .75 eV InGaAs PV cells fabricated at Lewis have excellent quantum efficiency. An efficiency of 13% has been measured for this cell coupled to an Er-YAG selective emitter and a short pass IR filter.

Chubb, Donald L.↗
NASA NTRS1997

Laboratory Studies of Chemical and Photochemical Processes Relevant to Stratospheric Ozone

The purpose of this project is to reduce the uncertainty in several key gas-phase kinetic processes which impact our understanding of stratospheric ozone. The main emphasis of this work is on measuring rate coefficients and product channels for reactions of HO(x) and NO(x) species in the temperature range 200 K to 240 K relevant to the lower stratosphere. The results of these studies will improve models of stratospheric ozone chemistry and predictions of perturbations due to human influences. The second year's effort has focussed the design and construction of the proposed high pressure flow reactor on three separate areas: (1) the construction of the high pressure flow reactor; (2) characterization of the turbulent flow profile; and (3) demonstration of the instrument by measuring HO2 + NO2 and HO2 + NO reaction rate coefficients.

Villalta, Peter W.↗
NASA NTRS1994

The Diurnal Variation of Hydrogen, Nitrogen, and Chlorine Radicals: Implications for the Heterogeneous Production of HNO2

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N205 on sulfate aerosols agree with measured concentrations of NO, NO2, and ClO throughout the day, but fail to account for high concentrations of OH and H02 observed near sunrise and sunset. The morning burst of [OH] and [HO2] coincides with the rise of [NO] from photolysis of N02, suggesting a new source of HO, that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HN02 results in an excellent simulation of the diurnal variations of [OH] and [HO2].

Salawitch, R. J.↗
NASA NTRS1995

The Heat of Formation of HNO

The HNO molecule is of interest in both combustion and atmospheric chemistry. For example, Guadagnini et al. have recently presented ab initio potential energy surfaces for the three lowest lying electronic states of HNO and then used these in examining several chemical reactions that take place in the combustion of nitrogen containing fuels and in the oxidation of atmospheric nitrogen. We have previously studied the ground state potential energy surface (i.e., stationary points along the HNO rev. reaction HON path), vibrational spectrum (using an accurate quartic force field), zero-point energy, and bonding of HNO using coupled-cluster ab initio methods. HNO is also very interesting because of the unique nature of its bonding characteristics. That is, the potential energy surface is very flat along the H-N bonding coordinate thereby giving unusual harmonic and fundamental vibrational frequencies, and the H-N bond energy is rather weak in comparison to other H-N bond energies. In fact, using experimental heats of formation for HO, H, and NO, the H- bond energy is computed to be only 49.9 kcal/ mol (298 K). However, ab initio calculations of isodesmic reaction energies involving HNO, FNO, ClNO, and several other molecules have shown that there is an inconsistency in the experimental heats of formation of the XNO (X=H, F, and Cl) species. Hence the motivation for this study was to determine a very accurate(DELTA)H(sup o)(sub f) value for HNO using state of-the-art ab initio methods. Based on many recent studies it is evident that the singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), in conjunction with large one-particle basis sets should be reliable to better than +0.8 kcal/mol for this quantity. The computational methodology is described in the next section followed by our results and discussion. Conclusions are presented in the final section.

Lee, Timothy J.↗
NASA NTRS1998

Modeling Macro- and Micro-Scale Turbulent Mixing and Chemistry in Engine Exhaust Plumes

Simulation of turbulent mixing and chemical processes in the near-field plume and plume-vortex regimes has been successfully carried out recently using a reduced gas phase kinetics mechanism which substantially decreased the computational cost. A detailed mechanism including gas phase HOx, NOx, and SOx chemistry between the aircraft exhaust and the ambient air in near-field aircraft plumes is compiled. A reduced mechanism capturing the major chemical pathways is developed. Predictions by the reduced mechanism are found to be in good agreement with those by the detailed mechanism. With the reduced chemistry, the computer CPU time is saved by a factor of more than 3.5 for the near-field plume modeling. Distributions of major chemical species are obtained and analyzed. The computed sensitivities of major species with respect to reaction step are deduced for identification of the dominant gas phase kinetic reaction pathways in the jet plume. Both the near field plume and the plume-vortex regimes were investigated using advanced mixing models. In the near field, a stand-alone mixing model was used to investigate the impact of turbulent mixing on the micro- and macro-scale mixing processes using a reduced reaction kinetics model. The plume-vortex regime was simulated using a large-eddy simulation model. Vortex plume behind Boeing 737 and 747 aircraft was simulated along with relevant kinetics. Many features of the computed flow field show reasonable agreement with data. The entrainment of the engine plumes into the wing tip vortices and also the partial detrainment of the plume were numerically captured. The impact of fluid mechanics on the chemical processes was also studied. Results show that there are significant differences between spatial and temporal simulations especially in the predicted SO3 concentrations. This has important implications for the prediction of sulfuric acid aerosols in the wake and may partly explain the discrepancy between past numerical studies (that employed parabolic or temporal approximations) and the measured data. Finally to address the major uncertainty in the near-field plume modeling related to the plume processing of sulfur compounds and advanced model was developed to evaluate its impact on the chemical processes in the near wake. A comprehensive aerosol model is developed and it is coupled with chemical kinetics and the axisymmetric turbulent jet flow models. The integrated model is used to simulate microphysical processes in the near-field jet plume, including sulfuric acid and water binary homogeneous nucleation, coagulation, non-equilibrium heteromolecular condensation, and sulfur-induced soot activation. The formation and evolution of aerosols are computed and analyzed. The computed results show that a large number of ultra-fine (0.3--0.6 nm in radius) volatile HSO4 - HO embryos are generated in the near-field plume. These embryos further grow in size by self coagulation and condensation. Soot particles can be activated by both heterogeneous nucleation and scavenging of H2SO4-H2O aerosols. These activated soot particles can serve as water condensation nuclei for contrail formation. Conditions under which ice contrails can form behind aircrafts are studied. The sensitivities of the threshold temperature for contrail formation with respect to aircraft propulsion efficiency, relative humidity, and ambient pressure are evaluated. The computed aerosol properties for different extent of fuel sulfur conversion to S(VI) (SO3 and H2SO4) in engine are examined and the results are found to be sensitive to this conversion fraction.

Menon, Suresh↗
NASA NTRS1997

Distribution and Seasonality of Selected Hydrocarbons and Halocarbons over the Western Pacific Basin During PEM-West A and PEM-West B

Nonmethane hydrocarbons (NMHCs) and halocarbons were measured in the troposphere over the northwestern Pacific as part of the airborne component of NASA's Pacific Exploratory Mission-West Phase B (PEM-West B). This study took place in late winter of 1994, a period characterized by maximum outflow from the Asian continent. The results are compared to those from Pacific Exploratory Mission-West Phase A (PEM-West A), which was flown in the same region during late summer of 1991, when flow from the subtropical western Pacific dominated the lower troposphere. Mixing ratios of NMHCs, tetrachloroethene (C2Cl4), and methyl bromide (CH3Br) were significantly higher during PEM-West B than during PEM-West A, particularly at latitudes north of 25 deg N and altitudes lower than 6 km. The primary reasons for these higher ambient concentrations were the seasonal increase in the atmospheric lifetimes of trace gases controlled by HO radical reactions, and the more frequent input of continental air masses. During PEM-West B, air masses of continental origin observed north of 25 deg N latitude were augmented with urban signature gases such as C2Cl4. By contrast, more southerly continental outflow had characteristics associated with combustion sources such as biomass burning, including wood fuel burning. During the summer PEM-West A period, the spatial distribution of methyl iodide (CH3I) was consistent with effective oceanic sources at all latitudes, being especially strong in tropical and subtropical regions. At low latitudes, PEM-West B CH3I mixing ratios in the lower troposphere were similar to PEM-West A, but at latitudes greater than about 25 deg N PEM-West B concentrations were significantly reduced. Equatorial regions exhibited enhanced CH3I mixing ratios extending into the upper tropical troposphere, consistent with fast vertical transport of air from the tropical marine boundary layer.

Blake, Nicola J.↗