Engineering topics

Allen, Mark

Publications and source records attributed to Allen, Mark.

At least 37 records · Page 2

NASA NTRS1994

(abstract) Odd Hydrogen in the Atmospheres of Earth and Mars

The Martian atmosphere has many features in common with the terrestrial mesosphere. Both share similar pressure and temperature ranges, and much of the same chemistry operates in each. For example, the radical species H, OH, and H(sub 2)O, which comprise the odd hydrogen family, are of central importance in the catalytic destruction of CO and O(sub 3) in both atmospheres. The inclusion of recent chemical kinetics data, specifically temperature dependent CO(sub 2) absorption cross-sections, into our one dimensional photochemical model of the Martial atmosphere shows that oxidation of CO by odd hydrogen is too efficient. The incorporation of smaller cross sections for CO(sub 2) leads to an enhanced photolysis rate of water vapor, increasing odd hydrogen to the point where the predicted mixing ratio of CO in our model is substantially less than the observed value of 6.5 x 10(sup -4). Interestingly, most photochemical models of the terrestrial mesosphere underestimate the CO and O(sub 3) densities using currently accepted photodissociation and kinetic rate coefficients. This has also been attributed to an overabundance of odd hydrogen in the models. We shall show that agreement between models and observations of CO in the Martian atmosphere as well as of CO and O(sub 3) in the terrestrial mesosphere can be achieved by revising the rate constants for the reactions OH + HO(sub 2) and CO + OH within their published uncertainties. The fact that similar revisions alleviate discrepancies in both the terrestrial and Martian atmospheres warrants a re-evaluation of these key rate constants at the appropriate temperatures and pressures.

dioxide ozone odd hydrogen family pressure tempera↗

NASA NTRS1993

A global analysis of the ozone deficit in the upper stratosphere and lower mesosphere

The global measurements of temperature, ozone, water vapor, and nitrogen dioxide acquired by the Limb Infrared Monitor of the Stratosphere (LIMS), supplemented by a precomputed distribution of chlorine monoxide, are used to test the balance between odd oxygen production and loss in the upper stratosphere and lower mesosphere. An efficient photochemical equilibrium model, whose validity is ascertained by comparison with the results from a fully time-dependent one-dimensional model at selected latitudes, is used in the calculations. The computed ozone abundances are systematically lower than observations for May 1-7, 1979, which suggests, contrary to the conclusions of other recent studies, a real problem in model simulations of stratospheric ozone.

Eluszkiewicz, Janusz↗

NASA NTRS1993

A distributed system for visualizing and analyzing multivariate and multidisciplinary data

THe Linked Windows Interactive Data System (LinkWinds) is being developed with NASA support. The objective of this proposal is to adapt and apply that system in a complex network environment containing elements to be found by scientists working multidisciplinary teams on very large scale and distributed data sets. The proposed three year program will develop specific visualization and analysis tools, to be exercised locally and remotely in the LinkWinds environment, to demonstrate visual data analysis, interdisciplinary data analysis and cooperative and interactive televisualization and analysis of data by geographically separated science teams. These demonstrators will involve at least two science disciplines with the aim of producing publishable results.

Jacobson, Allan S.↗

NASA NTRS1992

The relative abundance of ethane to acetylene in the Jovian stratosphere

The inclusion of the results of laboratory kinetics studies on the reaction of C2H3 and H2 to yield C2H4, which is suggestive of an efficient chemical mechanism for the hydrogenation of C2H2 to C2H6, can be included in a comprehensive model of the Jupiter atmosphere hydrocarbon photochemistry to explain the observed altitude variation of the C2H6/C2H2 ratio. The sensitivity of these results to uncertainties in key low-temperature rate constants is demonstrated. These key reaction-rate constants decrease with falling temperature.

Allen, Mark↗

NASA NTRS1992

Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere

The present theoretical analysis of particle formation mechanisms at conditions relevant to the troposphere and stratosphere of Neptune indicates that the hydrocarbon-nucleation process in the lower stratosphere is very inefficient, because saturation ratios much greater than unity are required in the cases of aerosol formation by homogeneous, heterogeneous, and ion-induced nucleation. The latter two of these, however, should be possible on Neptune for most of the complex hydrocarbon species in question. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the species in question, condensable-species abundance, supersaturation temperatures, and the number and type of available condensation ions or nuclei.

Moses, Julianne I.↗

NASA NTRS1992

Solar control of the upper atmosphere of Triton

If the upper atmosphere and ionosphere of Triton are controlled by precipitation of electrons from Neptune's magnetosphere as previously proposed, Triton could have the only ionosphere in the solar system not controlled by solar radiation. However, a new model of Triton's atmosphere, in which only solar radiation is present, predicts a large column of carbon atoms. With an assumed, but reasonable, rate of charge transfer between N2(+) and C, a peak C(+) abundance results that is close to the peak electron densities measured by Voyager in Triton's ionosphere. These results suggest that Triton's upper atmospheric chemistry may thus be solar-controlled. Measurement of key reaction rate constants, currently unknown or highly uncertain at Triton's low temperatures, would help to clarify the chemical and physical processes occurring in Triton's atmosphere.

Lyons, James R.↗

NASA NTRS1992

Enhancement of atmospheric radiation by an aerosol layer

The presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer. Such haze layers result from the injection of aerosol-forming material into the stratosphere by volcanic eruptions. Simple heuristic arguments show that the increase in flux below the haze layer, relative to a clear sky case, is a consequence of 'photon trapping'. The magnitude of these flux perturbations, as a function of aerosol properties and illumination conditions, is explored with a new radiative transfer model that can accurately compute fluxes in an inhomogeneous atmosphere with nonconservative scatterers having arbitrary phase function. One calculated consequence of the El Chichon volcanic eruption is an increase in the midday surface actinic flux at 20 deg N latitude, summer, by as much as 45 percent at 2900 A. This increase in flux in the UV-B wavelength range was caused entirely by aerosol scattering, without any reduction in the overhead ozone column.

Michelangeli, Diane V.↗

NASA NTRS1992

Martian atmospheric chemistry during the time of low water abundance

The importance of odd hydrogen (or HO(x)) radicals in the catalytic recombination of carbon monoxide and oxygen in the Martian atmosphere is a well known fact. The inclusion of recent chemical kinetics data, specifically temperature-dependent CO2 absorption cross sections, into our one dimensional photochemical model shows that HO(x) is too efficient in this regard. The absorption cross sections of CO2 are smaller than previously assumed; this leads to a reduction in the photolysis rate of CO2 while the photolysis rate of H2O has increased. As a consequence the predicted mixing ratio of CO in our models is substantially less than the observed value of 6.5(10)(exp -4). Simultaneous measurements of water, ozone, and carbon monoxide were obtained in the Martian atmosphere in early Dec. 1990 (L(sub s) for Mars was 344 deg.).

Nair, Hari↗

NASA NTRS1992

A distributed system for visualizing and analyzing multivariate and multidisciplinary data

The Linked Windows Interactive Data System (Link Winds) is being developed with NASA support. The objective of this proposal is to adapt and apply that system in a complex network environment containing elements to be found by scientists working multidisciplinary teams on very large scale and distributed data sets. The proposed three year program will develop specific visualization and analysis tools, to be exercised locally and remotely in the Link Winds environment, to demonstrate visual data analysis, interdisciplinary data analysis and cooperative and interactive televisualization and analysis of data by geographically separated science teams. These demonstrations will involve at least two science disciplines with the aim of producing publishable results.

Jacobson, Allan S.↗

NASA NTRS1992

Smart materials fabrication and materials for micro-electro-mechanical systems; Symposium Proceedings, San Francisco, CA, Apr. 28-30, 1992

A conference on the rapidly developing fields of `smart materials' and micro-electro-mechanical systems produced papers in the areas of fabrication and characterization of ferroelectric thin films; polycrystalline silicon; optical, chemical, and biological sensors; thin film shape memory alloys; materials characterization; and alternative materials and applications.

Jardine, A. Peter↗

NASA NTRS1991

Carbon suboxide in Comet Halley?

The extremely dark nucleus of Comet Halley suggests that its volatile ices contain a few percent of carbonaceous material in the form of graphitic or amorphous carbon. The very high abundance of light elements in the coma dust and the emission feature near 3.4 microns further suggest the presence of a significant organic component, although the identified carbon-containing materials' parent species cannot account for all of such a component. It is presently proposed that an additional contribution from carbon suboxide can account for these observational data, assuming a production rate about 0.03-0.04 times that of water.

Huntress, Wesley T., Jr.↗

NASA NTRS1991

A test of odd-oxygen photochemistry using Spacelab 3 atmospheric trace molecule spectroscopy observations

The ozone profile calculated using a 1D time-dependent photochemical model, with recommended absorption cross sections and reaction rate constants, is systematically less than Spacelab 3 (May 1985) atmospheric trace molecule spectroscopy (ATMOS) observations at sunset, 30 deg N latitude, throughout the upper stratosphere and mesosphere. Model results are about 9 percent smaller than the nominal ATMOS O3 value at 40 km and 50 percent less at 76 km. Given the uncertainties associated with the ATMOS profile, the model 'deficit' may be as large as 15 percent at 36 km, increasing to about 70 percent at 76 km. A similar model O3 deficit exists at sunrise, 47 deg S latitude, in the stratosphere: nominal differences of not more than 25 percent, increasing to not more than 45 percent when the ATMOS uncertainties are considered. Model results closely approximate ATMOS observations in both the upper stratosphere and mesosphere upon enhancing odd-oxygen production by increasing the O2 Schumann-Runge band and Herzberg-continuum cross section values used in the calculations.

Allen, Mark↗

NASA NTRS1991

Heterogeneous reactions with NaCl in the El Chichon volcanic aerosols

Previous investigations of the effects of the 1982 eruption of the El Chichon volcano could not explain all the observations of changes in O3, HCl, NO and NO2 simultaneously without proposing unproven chemical reactions. Since reactions between solid NaCl and gaseous ClNO3 and N2O5 rapidly produce photochemically active chlorine species and solid NaNO3 in laboratory experiments, it is suggested that these reactions could have occurred with the NaCl observed to be present in the El Chichon sulfuric acid areosols. As a consequence, it is predicted that HCl should increase substantially, while NO(x) should decrease, in agreement with the measurements after the eruption. Ozone should be only slightly affected by these reactions. Reactions between solid NaCl and the acids H2SO4 and HNO3 might prove to be important, but sufficient evidence regarding their efficiency and the pressure of HNO3 in the aerosols is lacking.

Michelangeli, Diane V.↗

NASA NTRS1991

Inferring the abundances of ClO and HO2 from Spacelab 3 atmospheric trace molecule spectroscopy observations

The vertical distributions of the important highly reactive stratospheric species, ClO and HO2, have been inferred from Spacelab 3 (May 1985) Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements of more detectable radical and reservoir species. A simple steady state algebraic expression for ClO, utilizing the observed ClONO2/NO2 abundance ratio, approximates the ClO results of a time-dependent photochemical model at sunset (30 deg N). Balloon measurements of ClO and comparisons of time-dependent photochemical model calculations of ClONO2 and HCl with the corresponding ATMOS profiles suggest that the actual ClO values are less than the time-dependent model profiles for ClO at sunset and sunrise (47 deg S). Errors in the current model simulation of the partitioning among the principal free chlorine species (HCl, ClONO2, and ClO) are indicated. The accuracy of the time-dependent model calculations of HO2 is suggested by the agreement between model results and ATMOS observations for HO2NO2 and balloon measurements of HO2. This confirms for the first time the procedure suggested previously by a number of authors of deriving HO(x) abundances from observed fields of O3 and H2O.

Allen, Mark↗

NASA NTRS1991

Troposphere-stratosphere interactions in a one-dimensional model of Jovian photochemistry

The case of a chemically unreactive species flowing downward through the stratosphere and troposphere with a constant flux is presently treated by a one-dimensional Jovian atmosphere model which encompasses the coupling between a rapidly mixed troposphere and a stagnant stratosphere, so that the contrast between the peak stratosphere and tropopause concentrations is reflective of the variation between the lower stratosphere and upper troposphere eddy diffusion coefficients. Numerical simulations of unreactive CO and C2H6 species indicate that upper troposphere abundances may possess a substantial photochemical contribution. The implications of these modeling results for higher spectral resolution observations are noted.

Landry, Bridget↗

NASA NTRS1990

Hot and cold gas toward young stellar objects

High-resolution M band spectra are presented for the seven embedded IR sources W3 IRS 5, S140 IRS1, NGC 7538 IRS 1, NGC 7538 IRS 9, GL 2136, LkH-alpha 101, and MWC 349A, and the data are combined with previously published work for W33A and GL 2591. Cold CO is seen toward all nine sources, with temperatures from 11 K to 66 K. Column densities of cold CO are presented. Hot gas is seen toward eight of the nine objects with temperatures from 120 K to 1010 K. New lower limits to the hot gas density are obtained. The hot gas toward GL 2591, GL 2136, W3 IRS 5, and S140 IRS 1 is probably very near the central source and heated via gas-grain collisions. The optical depth in the silicate feature is strongly correlated with the (C-13)O column density, confirming that silicate optical depth is a useful measure of gas column density. The ratio of solid-to-gaseous CO is obtained for seven sources.

Mitchell, George F.↗

NASA NTRS1990

Stratospheric NO, NO2, and N2O5 - A comparison of model results with Spacelab 3 Atmospheric Trace Molecule Spectroscopy (ATMOS) measurements

The Spacelab 3 Atmospheric Trace Molecule Spectroscopy measurements in 1985 of stratospheric NO/NO2 abundance ratios were compared with abundance ratios calculated using a time-dependent photochemical model. A good agreement between calculated and observed values of the NO/NO2 abundance ratio was found through much of the stratosphere, indicating that the temperature-dependent rate constant K3 (NO + O3) used in the model is properly parameterized.

Allen, Mark↗

NASA NTRS1990

Spatial variation of ozone depletion rates in the springtime Antarctic polar vortex

An area-mapping technique, designed to filter out synoptic perturbations of the Antarctic polar vortex such as distortion or displacement away from the pole, was applied to the Nimbus-7 TOMS (Total Ozone Mapping Spectrometer) data. This procedure reveals the detailed morphology of the temporal evolution of column O3. The results for the austral spring of 1987 suggest the existence of a relatively stable collar region enclosing an interior that is undergoing large variations. A simplified photochemical model of O3 loss and the temporal evolution of the area-mapped polar O3 are used to constrain the chlorine monoxide (ClO) concentrations in the springtime Antarctic vortex. The O3 loss rates could be larger than deduced here because of underestimates of total O3 by TOMS near the terminator.

Yung, Yuk L.↗