Search NASA⌕ Search

SEARCH · Search NASA

Results for “theoretical photochemistry”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

The photochemistry of planetary atmospheres

Recent theoretical and observational investigations of photochemical processes in the atmospheres of the planets and their satellites are reviewed. Particular attention is given to the CO2-dominated atmospheres of Mars and Venus, the hydrogen-dominated atmospheres of the Jovian planets, the SO2 atmosphere of Io, and the massive atmospheres of Titan and Triton. The principal reaction paths involved are listed and briefly characterized, and numerical data on atmospheric compositions are given in tables.

Dalgarno, A.↗
NASA NTRS1986

Theoretical analysis of isotope effects on ozone formation in oxygen photochemistry

In situ measurements of stratospheric ozone and laboratory studies of ozone production in electric discharge through oxygen have shown previously that ozone containing heavy isotopes of oxygen (O-17, O-18) may be formed preferentially. In order to assess the relevance of thee latter experiment to the stratospheric measurements, detailed understanding of the effect of isotopic substitution on the O3 formation reaction O + O2 + M yields O3 + M and on the O atom exchange reaction O + O2 + O yields O2 + O is necessary. In this work, an estimate of the effect of isotopic substitution on the recombination rate is made by us of approximate dynamical theories and statistical mechanics. The results indicate the possibility of isotope effects on the O + O2 recombination rate of the order of several percent at stratospheric temperatures. In general, recombination reactions involving heavy (mass 49, 50) O3 formation are found to be slower than the reaction leading to normal (mass 48) O3 formation. The calculated isotope effects are sufficiently small that the uncertainties in the model input and the approximations in the dynamical theories will probably make the quantitative nature of these results subject to considerable uncertainty. This isotope effect should not be observable in the atmosphere given the precision of the current measurements but may be crucial in the understanding of the laboratory experiments, where observed enhancements are only of the order of several percent. Possible reasons for this discrepancy between the observed enhancement and predicted depletion are presented.

Kaye, J. A.↗
NASA NTRS1978

The sensitivity of a model of the ambient stratosphere to empirical inputs

The usefulness of a general application of sensitivity studies to the modeling of the ambient stratosphere using the GSFC steady-state photochemistry model is examined. The comparison of theoretical model results with field measurements of trace species is used to assess the applicability of the model to the stratosphere, to help determine the eddy diffusion profile, and to validate mechanisms within the photochemistry of the model. Specific examples of each are discussed and the results of a comprehensive sensitivity study used to elucidate certain points about the comparison are analyzed.

Butler, D. M.↗
NASA NTRS1987

Theoretical constraints on oxygen and carbon dioxide concentrations in the Precambrian atmosphere

Simple (one-dimensional) climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity. Such models provide only lower bounds on CO2, so it is possible that CO2 levels were substantially higher than this and that the Archean climate was much warmer than today. Periods of extensive glaciation during the early and late Proterozoic, on the other hand, indicate that the climate at these times was relatively cool. To be consistent with climate models CO2 partial pressures must have declined from approximately 0.03 to 0.3 bar around 2.5 Ga ago to between 10(-3) and 10(-2) bar at 0.8 Ga ago. This steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data, which implies that pCO2 did not change appreciably during that time. Oxygen was essentially absent from the Earth's atmosphere and oceans prior to the emergence of a photosynthetic source, probably during the late Archean. During the early Proterozoic the atmosphere and surface ocean were apparently oxidizing, while the deep ocean remained reducing. An upper limit of 6 x 10(-3) bar for pO2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean. The establishment of oxidizing conditions in the deep ocean, marked by the disappearance of banded iron formations approximately 1.7 Ga ago, permitted atmospheric oxygen to climb to its present level. O2 concentrations may have remained substantially lower than today, however, until well into the Phanerozoic.

Review, Tutorial↗
NASA NTRS1992

Overview for the reanalysis of Mariner 9 UV spectrometer data for ozone, cloud, and dust abundances, and their interaction over climate timescales

Mariner 9 UV spectrometer data were reinverted for the ozone abundance, cloud abundance, dust abundance, and polar-cap albedo. The original reduction of the spectra ignored the presence of atmospheric dust and clouds, even though their abundance is substantial and can mask appreciable amounts of ozone if not accounted for (Lindner, 1988). The Mariner 9 ozone data has been used as a benchmark in all theoretical models of atmospheric composition, escape, and photochemistry. A second objective is to examine the data for the interrelationship of the ozone cycle, dust cycle, and cloud cycle, on an annual, inter-annual, and climatic basis, testing predictions by Lindner (1988). This also has implications for many terrestrial ozone studies, such as the ozone hole, acid rain, and ozone-smog. A third objective is to evaluate the efficacy of the reflectance spectroscopy technique at retrieving the ozone abundance on Mars. This would be useful for planning ozone observations on future Mars missions or the terrestrial troposphere.

Lindner, Bernhard Lee↗
NASA NTRS1979

Possible existence and role of excited ozone precursors in the stratosphere

Laboratory kinetic experiments, quantum theoretical studies and certain recently discovered infrared emissions from the stratosphere together imply the possible existence of a metastable, energetic, and reactive form of ozone in the upper atmosphere. This species can potentially affect the upper atmospheric photochemistry and heat balance. For these reasons there is a need for further quantum theoretical and laboratory chemical kinetic studies addressing some of the relevant properties of the internally excited ozone precursors.

Prasad, S. S.↗
NASA NTRS1987

Transport and photochemical modeling. Studies of atmospheric species

A program of research studies related to the photochemistry, radiative transfer, and dynamics of the stratosphere is described. Investigations were conducted in two broad areas: (1) studies of the stratospheric processes and their response to external perturbations, and (2) analysis of satellite measurements in conjunction with theoretical models. Contemporary one dimensional photochemical, radiative-convective model was used to assess the impact of perturbations such as solar flux variability, increases in atmospheric carbon dioxide, chlorofluoromethanes and other greenhouse gases. Data from satellite experiments such as LIMS and SBUV, were used along with theoretical models to develop a climatology of trace species in the stratosphere. The consistency of contemporary ozone photochemistry was examined in the light of LIMS data. Research work also includes analysis of stratospheric nitrogen dioxide distributions from different satellite experiments, investigation of the wintertime latitudinal gradients in NO2, estimation of the stratospheric odd nitrogen level and its variability, and studies related to the changes in ozone in the Antarctic, and mid latitude Southern Hemisphere.

Natarajan, M.↗
NASA NTRS1986

Planetary Fabry-Perot spectroscopy

Uranus and Jupiter HD/H2 spectra determine bulk D/H ratios which are significantly different from theoretical expectations, with the Uranus D/H about half the Jovian value. Feasibility of Mars photochemistry study with a PEPSIOS spectrometer was demonstrated with visible band O2, H2O, and CO2 spectra; and near infrared (1.57 micron) spectra of CO and CO2; and NASA/IRTF observations have been proposed in support of the Mars Observer mission objectives. Analysis of monochromatic images clearly resolve the azimuthal and radial structure in the Jovian nebula (ion partitioning, temperature structure, ion/neutral interactions), adding a unique and important post-Voyager perspective and providing a baseline for forthcoming Galileo experiments. Simultaneous ground based and IUE observations of Jovian nebula are under analysis. Doppler resolved image sequences (data cubes) of Io sodium neutrals clearly isolate the high velocity charge exchange jet, a sensitive ground-based probe of the tenuous neutral atmosphere of Io.

Trauger, J. T.↗
NASA NTRS1979

Organic chemistry on Titan

Features taken from various models of Titan's atmosphere are combined in a working composite model that provides environmental constraints within which different pathways for organic chemical synthesis are determined. Experimental results and theoretical modeling suggest that the organic chemistry of the satellite is dominated by two processes: photochemistry and energetic particle bombardment. Photochemical reactions of CH4 in the upper atmosphere can account for the presence of C2 hydrocarbons. Reactions initiated at various levels of the atmosphere by cosmic rays, Saturn 'wind', and solar wind particle bombardment of a CH4-N2 atmospheric mixture can account for the UV-visible absorbing stratospheric haze, the reddish appearance of the satellite, and some of the C2 hydrocarbons. In the lower atmosphere photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. It is concluded that the surface of Titan may contain ancient or recent organic matter (or both) produced in the atmosphere.

Chang, S.↗
NASA NTRS1995

Numerical simulation of the general circulation of the atmosphere of Titan

The atmospheric circulation of Titan is investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Meteorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature. Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m sec-1 near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, we suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze. We present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.

NASA Discipline Exobiology↗
NASA NTRS1979

Minor constituents in the stratosphere and mesosphere

A review of the research in minor constituents in the stratosphere and mesosphere, carried out between 1975 and 1978, is presented. Much of the theoretical research was done with the aid of one-dimensional models. Different aspects of these models are discussed. Measurements of the chlorofluoromethanes, hydrochloric acid, nitric oxide, nitrous oxide, and hydrogen oxide were conducted. It is noted that the hydrogen oxides are now assuming a larger role in stratospheric photochemistry than have been postulated before. The effect of water vapor and the hydrogen oxides on the overall chemistry of the stratosphere was investigated theoretically, along with the possible relationship between solar activity and atmospheric ozone. The mesosphere study included ozone, water vapor, nitric oxide, and odd nitrogen investigations.

Hudson, R. D.↗
NASA NTRS1984

Results of a comprehensive study of the photochemistry of N2(+) in the ionosphere

The improved match that can be obtained between the Atmospheric Explorer (AE) data on the ionospheric F layer N2(+) abundance and theoretical predictions by changing the dissociative recombination rate coefficient is demonstrated. Historically, models have overestimated the N2(+) concentration. It is shown that the calculated enhancement is due to charge exchange between 0(+)(2D) with N2. Increasing alpha by a factor of 2-3, at least in the orbits examined, can augment the destruction of vibrationally excited N2(+) ions. However, the validity of the correction is dependent on the availability of further laboratory data.

Abdou, W. A.↗
NASA NTRS1985

The response of ozone to solar proton events during solar cycle 21 - A theoretical interpretation

The effects associated with the solar proton events (SPE) during solar cycle 21 are examined. The only SPE documented to cause an ozone depletion below 45 km (approximately 2 mbar) was the event which occurred on Aug. 4, 1972. The ozone decreases associated with this event follow approximately the theoretically predicted behavior discussed by Heath et al. (1977) and Reagan et al. (1981). Ozone decreases during the July 13, 1982 and the Dec. 8, 1982, SPEs also follow approximately their theoretically predicted behavior between 60 and 85 km. Attention is given to the energy deposition by solar protons and electrons, photochemistry, model descriptions, model results, the solar zenith angle effect, three solar proton events during solar cycle 20, sensitivity studies, and temperature effects.

Jackman, C. H.↗
NASA NTRS2000

Solar UV Radiation and the Origin of Life on Earth

We have started a comprehensive, interdisciplinary study of the influence of solar ultraviolet radiation on the atmosphere of of the early Earth. We plan to model the chemistry of the Earth atmosphere during its evolution, using observed UV flux distributions of early solar analogs as boundary conditions in photochemical models of the Earth's atmosphere. The study has four distinct but interlinked parts: (1) Establishing the radiation of the early Sun; (2) Determining the photochemistry of the early Earth's atmosphere; (3) Estimating the rates of H2 loss from the atmosphere; and (4) Ascertaining how sensitive is the photochemistry to the metallicity of the Sun. We are currently using STIS and EUVE to obtain high-quality far-UV and extreme-UV observations of three early-solar analogs. We will perform a detailed non-LTE study of each stars, and construct theoretical model photosphere, and an empirical model chromospheres, which can be used to extrapolate the continuum to the Lyman continuum region. Given a realistic flux distribution of the early Sun, we will perform photochemical modeling of weakly reducing primitive atmospheres to determine the lifetime and photochemistry of CH4. In particular, we will make estimates of the amount of CH4 present in the prebiotic atmosphere, and estimate the atmospheric CH4 concentration during the Late Archean (2.5-3.0 b.y. ago) and determine whether it would have been sufficiently abundant to help offset reduced solar luminosity at that time. Having obtained a photochemical model, we will solve for the concentrations of greenhouse gasses and important pre-biotic molecules, and perform a detailed radiative transfer calculations to compute the UV flux reaching the surface.

Heap, Sara R.↗
NASA NTRS1978

Organic chemistry on Titan

Observations of nonequilibrium phenomena on the Saturn satellite Titan indicate the occurrence of organic chemical evolution. Greenhouse and thermal inversion models of Titan's atmosphere provide environmental constraints within which various pathways for organic chemical synthesis are assessed. Experimental results and theoretical modeling studies suggest that the organic chemistry of the satellite may be dominated by two atmospheric processes: energetic-particle bombardment and photochemistry. Reactions initiated in various levels of the atmosphere by cosmic ray, Saturn wind, and solar wind particle bombardment of a CH4 - N2 atmospheric mixture can account for the C2-hydrocarbons, the UV-visible-absorbing stratospheric haze, and the reddish color of the satellite. Photochemical reactions of CH4 can also account for the presence of C2-hydrocarbons. In the lower Titan atmosphere, photochemical processes will be important if surface temperatures are sufficiently high for gaseous NH3 to exist. Hot H-atom reactions initiated by photo-dissociation of NH3 can couple the chemical reactions of NH3 and CH4 and produce organic matter.

Chang, S.↗