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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 55 records · Page 3
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NASA NTRS1999

Antarctic Photochemistry: Uncertainty Analysis

Understanding the photochemistry of the Antarctic region is important for several reasons. Analysis of ice cores provides historical information on several species such as hydrogen peroxide and sulfur-bearing compounds. The former can potentially provide information on the history of oxidants in the troposphere and the latter may shed light on DMS-climate relationships. Extracting such information requires that we be able to model the photochemistry of the Antarctic troposphere and relate atmospheric concentrations to deposition rates and sequestration in the polar ice. This paper deals with one aspect of the uncertainty inherent in photochemical models of the high latitude troposphere: that arising from imprecision in the kinetic data used in the calculations. Such uncertainties in Antarctic models tend to be larger than those in models of mid to low latitude clean air. One reason is the lower temperatures which result in increased imprecision in kinetic data, assumed to be best characterized at 298K. Another is the inclusion of a DMS oxidation scheme in the present model. Many of the rates in this scheme are less precisely known than are rates in the standard chemistry used in many stratospheric and tropospheric models.

Stewart, Richard W.↗
NASA NTRS2000

Photochemistry of Saturn's Atmosphere: Hydrocarbon Chemistry and Comparisons with ISO Observations - 1

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column 12 densities of CH3, CH3C2H, and C4H2 above 10 mbar are 4 (sup +2) (sub -1.5) x 10 (exp 13) cm (sup -2), (1.1 plus or minus 0.3) x 10 (exp 15) cm (exp -2), and (1.2 plus or minus 0.3) x 10 (exp 14) cm (sup -2), respectively. The observed ISO emission features also indicate C2H2 mixing ratios of 1.2 (sup +0.9) (sub -0.6) x 10 (exp -6) at 0.3 mbar and (2.7 plus or minus 0.8) x 10 (exp -7) at 1.4 mbar, and a C2H6 mixing ratio of (9 plus or minus 2.5) x 10 (exp -6) at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2 sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than approximately 3 x 10 (exp 4) sq cm s (sup -1) at pressures greater than 1 mbar in order to reproduce the C2H6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH3 radicals if the low-temperature chemistry of CH3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere-diacetylene, butane, and water condense between approximately 1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.

Moses, Julianne I.↗
NASA NTRS2000

Photochemistry Saturn's Atmosphere: Effects of an Influx of External Oxygen - 2

We use a one-dimensional diurnally averaged model of photochemistry and diffusion in Saturn's stratosphere to investigate the influence of extraplanetary debris on atmospheric chemistry. In particular, we consider the effects of an influx of oxygen from micrometeoroid ablation or from ring-particle diffusion; the contribution from cometary impacts, satellite debris, or ring vapor is deemed to be less important. The photochemical model results are compared directly with Infrared Space Observatory (ISO) observations to constrain the influx of extraplanetary oxygen to Saturn. From the ISO observations, we determine that the column densities of CO2 and H2O above 10 mbar in Saturn's atmosphere are (6.3 +/- 1) x 10(exp 14) and (1.4 +/- 0.4) x 10(exp 15)/ square cm, respectively; our models indicate that a globally averaged oxygen influx of (4+/-2) x 10(exp 6) O atoms /sq cm/s is required to explain these observations. Models with a locally enhanced influx of H20 operating over a small fraction of the projected area do not provide as good a fit to the ISO H2O observations. If volatile oxygen compounds comprise one-third to one-half of the exogenic source by mass, then Saturn is currently being bombarded with (3 +/- 2) x 10(exp -16) g/square cm/s of extraplanetary material. To reproduce the observed CO2/H2O ratio in Saturn's stratosphere, some of the exogenic oxygen must arrive in the form of a carbon-oxygen bonded species such as CO or CO2. An influx consistent with the composition of cometary ices fails to reproduce the high observed CO2/H2O ratio, suggesting that (i) the material has ices that are slightly more carbon-rich than is typical for comets, (ii) a contribution from an organic-rich component is required, or (iii) some of the hydrogen-oxygen bonded material is converted to carbon-oxygen bonded material without photochemistry (e.g., during the ablation process). We have also reanalyzed the 5-micron CO observations of Noll and Larson and determine that the CO lines are most sensitive to the 100- to 400-mbar column density for which we derive a range of (0.7-1.5) x 10(exp 17)/square cm; the CO observations do not allow us to distinguish between an external or internal source of CO on Saturn. If we assume that all the extraplanetary oxygen derives from a micrometeoroid source, then the unfocused dust flux at 9.5 AU must be (i) (1 +/- 0.7) x 10(exp -16) g/square cm/s if interstellar grains are the source of the external oxygen on Saturn, (ii) (4 +/- 3) x 10 (exp -17) g/ sq. cm./s if IDPs on randomly inclined, highly eccentric orbits (e.g., Halley-type comet grains) are the source of the external oxygen, or (iii) (2+/- 1.4) x 10(exp -18)/g/sq cm/s if IDPs on low inclination, low eccentricity orbits (e.g., Kuiper-belt grains) are the source of the external oxygen. These estimates can be used in combination with future Cassini dust detection data to determine the ultimate source of the dust at Saturn's distance from the Sun.

Moses, Julianne I.↗
NASA NTRS2015

Uncertainties in Isoprene Photochemistry and Emissions: Implications for the Oxidative Capacity of Past and Present Atmospheres and for Climate Forcing Agents

Current understanding of the factors controlling biogenic isoprene emissions and of the fate of isoprene oxidation products in the atmosphere has been evolving rapidly. We use a climate-biosphere-chemistry modeling framework to evaluate the sensitivity of estimates of the tropospheric oxidative capacity to uncertainties in isoprene emissions and photochemistry. Our work focuses on trends across two time horizons: from the Last Glacial Maximum (LGM, 21 000 years BP) to the preindustrial (1770s); and from the preindustrial to the present day (1990s). We find that different oxidants have different sensitivities to the uncertainties tested in this study, with OH being the most sensitive: changes in the global mean OH levels for the LGM-to-preindustrial transition range between -29 and +7, and those for the preindustrial-to-present day transition range between -8 and +17, across our simulations. Our results suggest that the observed glacial-interglacial variability in atmospheric methane concentrations is predominantly driven by changes in methane sources as opposed to changes in OH, the primary methane sink. However, the magnitudes of change are subject to uncertainties in the past isoprene global burdens, as are estimates of the change in the global burden of secondary organic aerosol (SOA) relative to the preindustrial. We show that the linear relationship between tropospheric mean OH and tropospheric mean ozone photolysis rates, water vapor, and total emissions of NOx and reactive carbon first reported in Murray et al. (2014) does not hold across all periods with the new isoprene photochemistry mechanism. Our results demonstrate that inadequacies in our understanding of present-day OH and its controlling factors must be addressed in order to improve model estimates of the oxidative capacity of past and present atmospheres.

methane↗
NASA NTRS2016

Solid-State Photochemistry as a Formation Mechanism for Titan's Stratospheric C4N2 Ice Clouds

We propose that C4N2 ice clouds observed in Titan's springtime polar stratosphere arise due to solid-state photochemistry occurring within extant ice cloud particles of HCN-HC3N mixtures. This formation process resembles the halogen-induced ice particle surface chemistry that leads to condensed nitric acid trihydrate (NAT) particles and ozone depletion in Earth's polar stratosphere. As our analysis of the Cassini Composite Infrared Spectrometer 478 per centimeter ice emission feature demonstrates, this solid-state photochemistry mechanism eliminates the need for the relatively high C4N2 saturation vapor pressures required (even though they are not observed) when the ice is produced through the usual procedure of direct condensation from the vapor.

Titan's stratosphere↗
NASA NTRS2018

The Photochemistry of Purine in Ice Analogs Relevant to Dense Interstellar Clouds

The aromatic nitrogen heterocyclic compound purine is the core structural framework of many important biomolecules, particularly nucleobases. Purine and purine derivatives have been observed in carbonaceous chondrites, and it has been hypothesized that the exogenous delivery of these compounds, along with many other biologically relevant compounds, may have played a role in the emergence of life. Numerous experiments in our laboratory have demonstrated that the nucleobases used by life to encode genetic material could have been produced abiotically under astrophysically relevant conditions. Specically, the UV photoprocessing of pyrimidine and purine in simple ices of astrophysical interest has resulted in the production of all five biological nucleobases, namely, uracil (RNA), cytosine (RNA and DNA), thymine (DNA), adenine (RNA and DNA), and guanine (RNA and DNA). Additionally, follow-up work has examined the photochemistry of pyrimidine in more complex astrophysical ice mixtures to better understand the formation of these compounds under realistic conditions. In this work, we examine the photochemistry of purine in more complex ices of astrophysical interest and compare our results with those from simpler ice mixtures. We also examine the effects of competing parallel synthesis of organic compounds in the ices (unrelated to purine). Finally, we discuss the astrophysical and astrobiological implications of our findings.

Molecular Processes↗
NASA NTRS2018

The Photochemistry of Purine in Ice Analogs Relevant to Dense Interstellar Clouds

The aromatic nitrogen heterocyclic compound purine is the core structural framework of many important biomolecules, particularly nucleobases. Purine and purine derivatives have been observed in carbonaceous chondrites, and it has been hypothesized that the exogenous delivery of these compounds, along with many other biologically relevant compounds, may have played a role in the emergence of life. Numerous experiments in our laboratory have demonstrated that the nucleobases used by life to encode genetic material could have been produced abiotically under astrophysically relevant conditions. Specifically, the UV photoprocessing of pyrimidine and purine in simple ices of astrophysical interest has resulted in the production of all five biological nucleobases, namely, uracil (RNA), cytosine (RNA and DNA), thymine (DNA), adenine (RNA and DNA), and guanine (RNA and DNA). Additionally, follow-up work has examined the photochemistry of pyrimidine in more complex astrophysical ice mixtures to better understand the formation of these compounds under realistic conditions. In this work, we examine the photochemistry of purine in more complex ices of astrophysical interest and compare our results with those from simpler ice mixtures. We also examine the effects of competing parallel synthesis of organic compounds in the ices (unrelated to purine). Finally, we discuss the astrophysical and astrobiological implications of our findings.

Astrobiology↗
NASA NTRS1997

The 2140 cm-1 (4.673 microns) solid CO band: the case for interstellar O2 and N2 and the photochemistry of nonpolar interstellar ice analogs

The infrared spectra of CO frozen in nonpolar ices containing N2, CO2, O2, and H2O and the UV photochemistry of these interstellar/precometary ice analogs are reported. The spectra are used to test the hypothesis that the narrow 2140 cm-1 (4.673 microns) interstellar absorption feature attributed to solid CO might be produced by CO frozen in ices containing nonpolar species such as N2 and O2. It is shown that mixed molecular ices containing CO, N2, O2, and CO2 provide a good match to the interstellar band at all temperatures between 12 and 30 K both before and after photolysis. The optical constants (real and imaginary parts of the index of refraction) in the region of the solid CO feature are reported for several of these ices. The N2 and O2 absorptions at 2328 cm-1 (4.296 microns) and 1549 cm-1 (6.456 microns), respectively, are also shown. The best matches between the narrow interstellar band and the feature in the laboratory spectra of nonpolar ices are for samples which contain comparable amounts of N2, O2, CO2, and CO. Co-adding the CO band from an N2:O2:CO2:CO = 1:5:1/2:1 ice with that of an H2O:CO = 20:1 ice provides an excellent fit across the entire interstellar CO feature. The four-component, nonpolar ice accounts for the narrow 2140 cm-1 portion of the feature which is associated with quiescent regions of dense molecular clouds. Using this mixture, and applying the most recent cosmic abundance values, we derive that between 15% and 70% of the available interstellar N is in the form of frozen N2 along several lines of sight toward background stars. This is reduced to a range of 1%-30% for embedded objects with lines of sight more dominated by warmer grains. The cosmic abundance of O tied up in frozen O2 lies in the 10%-45% range toward background sources, and it is between 1% and 20% toward embedded objects. The amount of oxygen tied up in CO and CO2 frozen in nonpolar ices can be as much as 2%-10% toward background sources and on the order of 0.2%-5% for embedded objects. Similarly 3%-13% of the carbon is tied up in CO and CO2 frozen in nonpolar ices toward field stars, and 0.2%-6% toward embedded objects. These numbers imply that most of the N is in N2, and a significant fraction of the available O is in O2 in the most quiescent regions of dense clouds. Ultraviolet photolysis of these ices produces a variety of photoproducts including CO2, N2O, O3, CO3, HCO, H2CO, and possibly NO and NO2. XCN is not produced in these experiments, placing important constraints on the origin of the enigmatic interstellar XCN feature. N2O and CO3 have not been previously considered as interstellar ice components.

NASA Discipline Exobiology↗
NASA NTRS1987

Determination of O2(a1 delta g) and O2(b1 sigma+ g) yields in the reaction O + ClO --> Cl + O2: implications for photochemistry in the atmosphere of Venus

A discharge flow apparatus with chemiluminescence detector has been used to study the reaction O + ClO --> Cl + O2, where O2 = O2(a1 delta g) or O2(b1 sigma+ g). The measured quantum yields for producing O2(a1 delta g) and O2(b1 sigma+ g) in the above reaction are less than 2.5 x 10(-2) and equal to (4.4 +/- 1.1) x 10(-4), respectively. The observed O2(a1 delta g) airglow of Venus cannot be explained in the context of standard photochemistry using our experimental results and those reported in recent literature. The possibility of an alternative source of O atoms derived from SO2 photolysis in the mesosphere of Venus is suggested.

Non-NASA Center↗
NASA NTRS1996

Mechanism of cyanoacetylene photochemistry at 185 and 254 nm

The role of cyanoacetylene (HC3N) in the atmospheric photochemistry of Titan and its relevance to polymer formation are discussed. Investigation of the relative light absorption of HC3N, acetylene (C2H2), and diacetylene (C4H2) revealed that HC3N is an important absorber of UV light in the 205- to 225-nanometer wavelength region in Titan's polar regions. Laboratory studies established that photolysis of C2H2 initiates the polymerization of HC3N even though the HC3N is not absorbing the UV light. Quantum yield measurements establish that HC3N is 2-5 times as reactive as C2H2 for polymer formation. Photolysis of HC3N with 185-nanometer light in the presence of N2, H2, Ar, or CF4 results in a decrease in the yield of 1,3,5-tricyanobenzene (1,3,5-tcb), while photolysis in the presence of CH4, C2H6, or n-C4H10 results in an increase in 1,3,5-tcb. The rate of loss of HC3N is increased by all gases except H2, where it is unchanged. It was not possible to detect 1,3,5-tcb as a photoproduct when the partial pressure of HC3N was decreased to 1 torr. Photolysis of HC3N with 254-nanometer light in the presence of H2 or N2 results in the formation of 1,2,4-tcb, while photolysis in the presence of CH4, C2H6, or n-C4H10 results in the formation of increasing amounts of 1,3,5-tcb. Mechanisms for the formation of polymers are presented.

Non-NASA Center↗
NASA NTRS1996

Hydrocarbon photochemistry in the upper atmosphere of Jupiter

The hydrocarbon photochemistry in the upper atmosphere of Jupiter is investigated using a one-dimensional, photochemical-diffusive, and diurnally averaged model. The important chemical cycles and pathways among the major species are outlined and a standard model for the North Equatorial Belt region is examined in detail. It is found that several traditionally dominant chemical pathways among the C and C2 species are replaced in importance by cycles involving C-C4 species. The pressure and altitude profiles of mixing ratios for several observable hydrocarbon species are compared with available ultraviolet- and infrared-derived abundances. The results of sensitivity studies on the standard model with respect to variations in eddy diffusion profile, solar flux, atomic hydrogen influx, latitude, temperature, and important chemical reaction rates are presented. Measured and calculated airglow emissions of He at 584 angstroms and H at 1216 angstroms are also used to provide some constraints on the range of model parameters. The relevance of the model results to the upcoming Galileo mission is briefly discussed. The model is subject to considerable improvement; there is a great need for laboratory measurements of basic reaction rates and photodissociation quantum yields, even for such simple species as methylacetylene and allene. Until such laboratory measurements exist there will be considerable uncertainty in the understanding of the C3 and higher hydrocarbons in the atmospheres of the jovian planets.

NASA Discipline Number 52-20↗
NASA NTRS1985

A model for the origin of photosynthesis--III. The ultraviolet photochemistry of uroporphyrinogen

The photochemical ramifications of the high ultraviolet flux on the primordial earth prior to the formation of the ozone layer have been considered in a study of the ultraviolet photochemistry of uroporphyrinogen (urohexahydroporphyrin), a colorless compound which absorbs strongly at wavelengths less than 220 nanometers. Urohexahydroporphyrin was investigated since it is the first macrocycle formed on the biosynthetic pathway of chlorophyll and can be used to test the hypothesis that the biosynthetic pathway to chlorophyll recapitulates the evolutionary history of photosynthesis. When urohexahydroporphyrin is illuminated in aqueous anaerobic solution, hydrogen gas is produced. More hydrogen gas is produced in the presence of a colloidal platinum catalyst. The products of the photooxidation of urohexahydroporphyrin are urotetrahydroporphyrin (uroporphomethene) and uroporphyrin. This research shows how the oxidation of uroporphyrinogen to uroporphyrin, the first biogenetic porphyrin, could have occurred anaerobically and abiotically on the primordial earth.

Evolution, Chemical↗
NASA NTRS2011

Condensed-Phase Photochemical Processes in Titan's Aerosols and Surface: The Role of Longer Wavelength Photochemistry

We will discuss photochemical properties of Titan's organic molecules in the condensed phase as solid aerosols or surface material, from small linear polyyenes (polyacetylenes and polycyanoacetylenes) such as C2H2, C4N2, HC5N, etc. In particular we will focus on photochemistry caused by longer wavelength UV-VIS photons (greater than 250 nm) photons that make it through Titan's atmosphere to the haze region (approximately 100 km) and on to the surface of Titan.

Surface area↗
DOE OSTI2021

Nitrene Photochemistry of Manganese N ‐Haloamides**

Abstract Manganese complexes supported by macrocyclic tetrapyrrole ligands represent an important platform for nitrene transfer catalysis and have been applied to both C−H amination and olefin aziridination catalysis. The reactivity of the transient high‐valent Mn nitrenoids that mediate these processes renders characterization of these species challenging. Here we report the synthesis and nitrene transfer photochemistry of a family of Mn III N ‐haloamide complexes. The S= 2 N ‐haloamide complexes are characterized by 1 H NMR, UV‐vis, IR, high‐frequency and ‐field EPR (HFEPR) spectroscopies, and single‐crystal X‐ray diffraction. Photolysis of these complexes results in the formal transfer of a nitrene equivalent to both C−H bonds, such as the α‐C−H bonds of tetrahydrofuran, and olefinic substrates, such as styrene, to afford aminated and aziridinated products, respectively. Low‐temperature spectroscopy and analysis of kinetic isotope effects for C−H amination indicate halogen‐dependent photoreactivity: Photolysis of N ‐chloroamides proceeds via initial cleavage of the Mn−N bond to generate Mn II and amidyl radical intermediates; in contrast, photolysis of N ‐iodoamides proceeds via N−I cleavage to generate a Mn IV nitrenoid (i.e., {MnNR} 7 species). These results establish N ‐haloamide ligands as viable precursors in the photosynthesis of metal nitrenes and highlight the power of ligand design to provide access to reactive intermediates in group‐transfer catalysis.

Van Trieste, III, Gerard P.↗