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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 109 records · Page 6
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NASA NTRS2007

Aerosol Absorption and Radiative Forcing

We present a comprehensive examination of aerosol absorption with a focus on evaluating the sensitivity of the global distribution of aerosol absorption to key uncertainties in the process representation. For this purpose we extended the comprehensive aerosol-climate model ECHAM5-HAM by effective medium approximations for the calculation of aerosol effective refractive indices, updated black carbon refractive indices, new cloud radiative properties considering the effect of aerosol inclusions, as well as by modules for the calculation of long-wave aerosol radiative properties and instantaneous aerosol forcing. The evaluation of the simulated aerosol absorption optical depth with the AERONET sun-photometer network shows a good agreement in the large scale global patterns. On a regional basis it becomes evident that the update of the BC refractive indices to Bond and Bergstrom (2006) significantly improves the previous underestimation of the aerosol absorption optical depth. In the global annual-mean, absorption acts to reduce the shortwave anthropogenic aerosol top-of-atmosphere (TOA) radiative forcing clear-sky from -0.79 to -0.53 W m(sup -2) (33%) and all-sky from -0.47 to -0.13W m(sup -2 (72%). Our results confirm that basic assumptions about the BC refractive index play a key role for aerosol absorption and radiative forcing. The effect of the usage of more accurate effective medium approximations is comparably small. We demonstrate that the diversity in the AeroCom land-surface albedo fields contributes to the uncertainty in the simulated anthropogenic aerosol radiative forcings: the usage of an upper versus lower bound of the AeroCom land albedos introduces a global annual-mean TOA forcing range of 0.19W m(sup -2) (36%) clear-sky and of 0.12W m(sup -2) (92%) all-sky. The consideration of black carbon inclusions on cloud radiative properties results in a small global annual-mean all-sky absorption of 0.05W m(sup -2) and a positive TOA forcing perturbation of 0.02W m(sup -2). The long-wave aerosol radiative effects are small for anthropogenic aerosols but become of relevance for the larger natural dust and sea-salt aerosols.

Stier, Philip↗
NASA NTRS2013

Absorption Properties of Mediterranean Aerosols Obtained from Multi-year Ground-based and Satellite Remote Sensing Observations

Aerosol absorption properties are of high importance to assess aerosol impact on regional climate. This study presents an analysis of aerosol absorption products obtained over the Mediterranean Basin or land stations in the region from multi-year ground-based AERONET and satellite observations with a focus on the Absorbing Aerosol Optical Depth (AAOD), Single Scattering Albedo (SSA) and their spectral dependence. The AAOD and Absorption Angstrom Exponent (AAE) data set is composed of daily averaged AERONET level 2 data from a total of 22 Mediterranean stations having long time series, mainly under the influence of urban-industrial aerosols and/or soil dust. This data set covers the 17 yr period 1996-2012 with most data being from 2003-2011 (approximately 89 percent of level-2 AAOD data). Since AERONET level-2 absorption products require a high aerosol load (AOD at 440 nm greater than 0.4), which is most often related to the presence of desert dust, we also consider level-1.5 SSA data, despite their higher uncertainty, and filter out data with an Angstrom exponent less than 1.0 in order to study absorption by carbonaceous aerosols. The SSA data set includes both AERONET level-2 and satellite level-3 products. Satellite-derived SSA data considered are monthly level 3 products mapped at the regional scale for the spring and summer seasons that exhibit the largest aerosol loads. The satellite SSA dataset includes the following products: (i) Multi-angle Imaging SpectroRadiometer (MISR) over 2000-2011, (ii) Ozone Monitoring Instrument (OMI) near-UV algorithm over 2004-2010, and (iii) MODerate resolution Imaging Spectroradiometer (MODIS) Deep-Blue algorithm over 2005-2011, derived only over land in dusty conditions. Sun-photometer observations show that values of AAOD at 440 nm vary between 0.024 +/- 0.01 (resp. 0.040 +/- 0.01) and 0.050 +/- 0.01 (0.055 +/- 0.01) for urban (dusty) sites. Analysis shows that the Mediterranean urban-industrial aerosols appear "moderately" absorbing with values of SSA close to approximately 0.94-0.95 +/- 0.04 (at 440 nm) in most cases except over the large cities of Rome and Athens, where aerosol appears more absorbing (SSA approximately 0.89-0.90 +/- 0.04). The aerosol Absorption Angstrom Exponent (AAE, estimated using 440 and 870 nm) is found to be larger than 1 for most sites over the Mediterranean, a manifestation of mineral dust (iron) and/or brown carbon producing the observed absorption. AERONET level-2 sun-photometer data indicate the existence of a moderate East-West gradient, with higher values over the eastern basin (AAEEast. = 1.39/AAEWest. = 1.33) due to the influence of desert dust. The North-South AAE gradient is more pronounced, especially over the western basin. Our additional analysis of AERONET level-1.5 data also shows that organic absorbing aerosols significantly affect some Mediterranean sites. These results indicate that current climate models treating organics as nonabsorbing over the Mediterranean certainly underestimate the warming effect due to carbonaceous aerosols. A~comparative analysis of the regional SSA variability has been attempted using satellite data. OMI and MODIS data show an absorbing zone (SSA approximately 0.90 at 470-500 nm) over Northeastern Africa that does not appear in the MISR retrievals. In contrast, MISR seems able to observe the East-West SSA gradient during summer, as also detected by AERONET. Also, the analysis of SSA provided by satellites indicates that the aerosol over the Mediterranean Sea appears less absorbing during spring (MAM) than summer (JJA).

satellite remote sensing observations↗
NASA NTRSDate not supplied

Absorption Laser Differential Interferometry for Simultaneous Colinear Flow Property and Fluctuation Measurements

An absorption laser differential interferometer (A-LDI) was constructed and tested in an effort to simultaneously and colinearly measure flow properties (pressure, temperature, concentration) and flow fluctuations (changes in optical path length). Whereas typical LDI and focused LDI (FLDI) systems use fixed wavelength lasers as the light source (often either green at approximately 532 nm or red at approximately 630 nm), a tunable 760 nm diode laser was used to probe the A-band absorption spectrum of molecular oxygen. The A-LDI system was used to measure O2 absorption and flow fluctuations in a sub-atmospheric pressure-controlled test cell, with absorption profiles from direct absorption and wavelength modulation absorption matching simulated results, while simultaneously being sensitive to high frequency fluctuations from jets of either compressed air or 100% O2 from a gas bottle. The A-LDI system was also installed at the NASA Langley Research Center Probe Calibration Tunnel to assess the freestream flow state in a Mach 3.5 flow. LDI measurements of the freestream indicated increasing amplitudes of flow fluctuations with increasing stagnation pressure at a fixed stagnation temperature, while absorption results demonstrated sensitivity to freestream pressure changes.

Joshua M. Weisberger↗
NASA NTRS2017

Synthetic Absorption Lines for a Clumpy Medium: A Spectral Signature for Cloud Acceleration in AGN?

There is increasing evidence that the highly ionized multiphase components of AGN disc winds may be due to thermal instability. The ions responsible for forming the observed X-ray absorption lines may only exist in relatively cool clumps that can be identified with the so-called warm absorbers. Here we calculate synthetic absorption lines for such warm absorbers from first principles by combining 2D hydrodynamic solutions of a two-phase medium with a dense grid of photoionization models to determine the detailed ionization structure of the gas. Our calculations reveal that cloud disruption, which leads to a highly complicated velocity field (i.e. a clumpy flow), will only mildly affect line shapes and strengths when the warm gas becomes highly mixed but not depleted. Prior to complete disruption, clouds that are optically thin to the driving UV resonance lines will cause absorption at an increasingly blueshifted line-of-sight velocity as they are accelerated. This behavior will imprint an identifiable signature on the line profile if warm absorbers are enshrouded in an even broader absorption line produced by a high column of intercloud gas. Interestingly, we show that it is possible to develop a spectral diagnostic for cloud acceleration by differencing the absorption components of a doublet line, a result that can be qualitatively understood using a simple partial covering model. Our calculations also permit us to comment on the spectral differences between cloud disruption and ionization changes driven by flux variability. Notably, cloud disruption offers another possibility for explaining absorption line variability.

line: formation – galaxies: nuclei – quasars: ↗
NASA NTRS2024

Estimation of End-of-Life Solar Absorptivity for Complex, Multi-Phased Space Missions

Solar absorptivity degradation over time is an important contributor to the thermal performance of a spacecraft at the end of its life. When environments are complex and include many different factors known to impact solar absorptivity degradation, such as UV, charged particles, contamination, or lunar dust coverage, combining data from several different studies can be done to estimate total degradation. While the most conservative way is to stack delta absorptivity from each factor to come up with a total delta, this may be overly conservative for long or complex missions with multiple mission phases. As absorptivity increases the rate of degradation decreases, and synergistic effects need to be taken into account. In this report an alternate method is proposed using Gateway materials as an example. This method determines which environmental factor will be dominant at what time (such as contamination or charged particles) and uses that degradation rate for that mission phase. For the next mission phase, the new absorptivity can be used instead of the original BOL absorptivity to determine degradation rate. This approach prevents overestimating optical degradation when considering degradation from interconnected sources.

thermo-optical properties↗
NASA NTRS1972

Enhanced indirect optical absorption in AlAs and GaP.

Measurement and analysis of indirect interband optical absorption in GaP. Deviations from a simple indirect absorption law are shown to be consistent with those given by a model which takes into account both the variation of the energy denominators with photon energy and indirect absorption to higher-energy conduction-band valleys. The analysis of the GaP data indicates that such an analysis on data of limited absorption-constant range can provide reasonably accurate values of the direct band gap. The value of the direct band gap best fitting this model for AlAs at 6 K is 3.13 eV. In GaP and AlAs there is evidence of a higher set of extrema giving rise to an additional indirect absorption component 0.34 and 0.20 eV, respectively, above that due to the lowest-energy conduction-band minima. The strength of the absorption due to these higher-energy valleys indicates that they contain states of X sub 3 symmetry.

Dumke, W. P.↗
NASA NTRS1975

Measurements of NH3 absorption coefficients with a C-13/O-16/2 laser

Measurements of NH3 absorption coefficients are presented for several transitions of a C-13(O-16)2 laser for small concentrations of NH3(p less than 1 torr) for absorption lines broadened to 1 atm with N2. NH3 absorption coefficients were determined for laser transitions R(8)(920.2194 wavelengths/cm) to R(28)(933.8808 wavelengths/cm) of the 00 1 - (10 0,02 0)I band. The strongest absorption coefficient K = 36.09 + or - 1.43 per (atm-cm) was measured for the R(18) transition for the NH3 line, aQ(6,6), and is larger than has been found in any previous measurements with a CO2 laser. The dependence of K on total pressure was also obtained for select transitions, and the frequency separation between the R(18) laser transition and the neighboring NH3 line aQ(6,6) was determined to be 550 + or - 50 MHz. These results are significant for long path absorption monitoring of NH3 with CO2 lasers since the path length can be reduced by approximately 40% and for heterodyne detection of NH3 since the relative position of the laser transition to the NH3 absorption line is well within the bandpass of Hg-Cd-Te photomixers.

Allario, F.↗
NASA NTRS1977

On the origin of the absorption spectra of quasistellar and BL Lacertae objects

Following the earlier work of Wagoner (1967) we have reexamined the possibility that the absorption spectra of QSOs are due to gas in the disks, coronae, or halos of intervening galaxies. Comparison of the expected number of absorption systems with the number observed in all of the QSOs and BL Lacertae objects so far cataloged shows that there is a gross discrepancy in the sense that the multiplicity of absorption systems already found is far higher than the numbers expected if the absorption is due to gas in known disks or hypothetical small coronae. A discussion of the selection effects suggests that the number of absorption systems so far reported is an underestimate, so that an even greater discrepancy is likely to be present. High multiplicities can be explained only if very large halos or clouds containing heavy elements are commonly present, and there is no independent evidence for this. We conclude that the bulk of the absorption is likely to be intrinsic to the QSOs.

Burbidge, G.↗
NASA NTRS1978

A study of the absorption-line spectra of six high-redshift quasi-stellar objects

High-resolution spectrograms of six QSOs with apparent visual magnitudes of 17.0 to 18.5 and redshifts greater than 2 are studied for absorption lines. Accurate wavelengths are measured for all the absorption lines that could be detected, attempts are made to identify redshift systems, and the redshifts in each object are examined. Analysis of the absorption-line spectra shows that: (1) a large number of redshifts covering a wide range of values are present in five of the QSOs; (2) about two-thirds of the absorption lines remain unidentified, cannot be of Galactic origin, and are not associated with existing redshift systems; (3) many comparatively small absorption redshifts exist which, if they are due to intervening galaxies, must be produced by very large halos; and (4) in one QSO there is a possible redshift system containing five lines with an absorption redshift that is significantly greater than the emission redshift.

Roberts, D. H.↗
NASA NTRS1984

Synopsis of Mid-latitude Radio Wave Absorption in Europe

Radio wave absorption data covering almost two years from Europe to Central Asia are presented. They are normalized by relating them to a reference absorption. Every day these normalized data are fitted to a mathematical function of geographical location in order to obtain a daily synopsis of radio wave absorption. A film of these absorption charts was made which is intended to reveal movements of absorption or absorption anomaly. In addition, radiance (temperature) data from the lower D-region are also plotted onto these charts.

Torkar, K. M.↗
NASA NTRS1986

Monochromatic calculations of atmospheric radiative transfer due to molecular line absorption

Sensitivity studies related to the effects of line cutoff, spectral resolution, and temperature and pressure interpolations in radiative transfer have been performed so that a data set of absorption coefficients for water vapor, CO2, and O3 may be created efficiently. Results show that computations of absorption coefficients are affected only slightly by cutting a line off at a wave number 190 times the Lorentz half width from the center, or equivalently, cutting off 0.33 percent of the line intensity from the wings. To achieve a relative cooling rate error smaller than 2 percent, it is sufficient to precompute the absorption coefficient at three temperatures (210, 250, and 290 K) and 19 pressures with Delta (log 10 p) = 0.2. The absorption coefficient at other conditions can be interpolated linearly with pressure and exponentially with a quadratic in temperature. For the spectral resolution the absorption coefficients can be adequately computed at 0.01, 0.002, 0.005, and 0.025/cm intervals in the thermal water vapor, the CO2 and O3 bands, and the solar water vapor bands, respectively, which limits the error to only a few percent in the cooling and heating rates. Using the precomputed absorption coefficients, repeated monochromatic calculations of atmospheric heating/cooling rates for radiation model developments and for comparison with less detailed calculations are no longer difficult.

Chou, M.-D.↗
NASA NTRS1986

Atmospheric-water absorption features near 2.2 micrometers and their importance in high spectral resolution remote sensing

Selective absorption of electromagnetic radiation by atmospheric gases and water vapor is an accepted fact in terrestrial remote sensing. Until recently, only a general knowledge of atmospheric effects was required for analysis of remote sensing data; however, with the advent of high spectral resolution imaging devices, detailed knowledge of atmospheric absorption bands has become increasingly important for accurate analysis. Detailed study of high spectral resolution aircraft data at the U.S. Geological Survey has disclosed narrow absorption features centered at approximately 2.17 and 2.20 micrometers not caused by surface mineralogy. Published atmospheric transmission spectra and atmospheric spectra derived using the LOWTRAN-5 computer model indicate that these absorption features are probably water vapor. Spectral modeling indicates that the effects of atmospheric absorption in this region are most pronounced in spectrally flat materials with only weak absorption bands. Without correction and detailed knowledge of the atmospheric effects, accurate mapping of surface mineralogy (particularly at low mineral concentrations) is not possible.

Kruse, F. A.↗
NASA NTRS1985

High temperature measurement of water vapor absorption

An investigation was undertaken to measure the absorption coefficient, at a wavelength of 10.6 microns, for mixtures of water vapor and a diluent gas at high temperature and pressure. The experimental concept was to create the desired conditions of temperature and pressure in a laser absorption wave, similar to that which would be created in a laser propulsion system. A simplified numerical model was developed to predict the characteristics of the absorption wave and to estimate the laser intensity threshold for initiation. A non-intrusive method for temperature measurement utilizing optical laser-beam deflection (OLD) and optical spark breakdown produced by an excimer laser, was thoroughly investigated and found suitable for the non-equilibrium conditions expected in the wave. Experiments were performed to verify the temperature measurement technique, to screen possible materials for surface initiation of the laser absorption wave and to attempt to initiate an absorption wave using the 1.5 kW carbon dioxide laser. The OLD technique was proven for air and for argon, but spark breakdown could not be produced in helium. It was not possible to initiate a laser absorption wave in mixtures of water and helium or water and argon using the 1.5 kW laser, a result which was consistent with the model prediction.

Keefer, Dennis↗
NASA NTRS1989

Stratospheric infrared continuum absorptions observed by the ATMOS instrument

A quantitative analysis of infrared continuum absorption features observed in ATMOS/Spacelab 3 (1985) spectra of the lower stratosphere is reported. Continuous absorption produced primarily by the collision-induced fundamental vibration-rotation band of O2 and to a lesser extent by the superposition of H2O far line wings has been observed in the 1400 to 1800/cm interval below tangent heights of about 25 km. Continuum optical depths measured in microwindows nearly free of atmospheric line absorption are 0.78 + or - 0.06 times those calculated with the O2 absorption coefficients of Timofeyev and Tonkov (1978). Transmittance measurements in microwindows between 2395 and 2535/cm have been used to study continuous absorption from the collision induced fundamental vibration-rotation band of N2 and the far wings of strong CO2 lines. The measured transmittances have been analyzed to derive best fit absorption coefficients for the N2 pressure-induced band at lower stratospheric temperatures (about 210 K).

Rinsland, C. P.↗
NASA NTRS1990

Intersubband absorption in Si(1-x)Ge(x/Si superlattices for long wavelength infrared detectors

Researchers calculated the absorption strengths for intersubband transitions in n-type Si(1-x)Ge(x)/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si(1-x)Ge(x)/Si supperlattice detectors is the ability to detect normally incident light; in Ga(1-x)Al(x)As/GaAs superlattices, intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. Researchers present detailed calculation of absorption coefficients, and peak absorption wavelengths for (100), (111) and (110) Si(1-x)Ge(x)/Si superlattices. Peak absorption strengths of about 2000 to 6000 cm(exp -1) were obtained for typical sheet doping concentrations (approx. equals 10(exp 12)cm(exp -2)). Absorption comparable to that in Ga(1-x)Al(x)As/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications.

Rajakarunanayake, Yasantha↗
NASA NTRS1991

The analysis of apparent optical depth profiles for interstellar absorption lines

Attention is given to the apparent optical depth method, a procedure for analyzing interstellar absorption lines. Observed absorption-line profiles are converted into profiles of apparent optical depth, and apparent column density per unit velocity. By comparing the latter for a given interstellar species having two or more absorption lines which differ in the product, the presence or absence of unresolved saturated structure in the profiles can be directly inferred. The method is illustrated using absorption-line data from the IUE satellite for the highly ionized gas toward HD 64760. Additional illustrations and a study of the method's accuracy are provided through a series of numerical simulations of multicomponent interstellar absorption situations. The method is compared to the standard growth curve method for deriving interstellar column densities. The principal value of the apparent optical depth method is that the absorption-line data are directly converted into a form that provides for direct scientific interpretations of the physical conditions in the interstellar absorbing medium as a function of velocity.

Savage, Blair D.↗
NASA NTRS1991

Direct evidence for stellar wind X-ray absorption in O stars

The observed X-ray spectral distributions from O stars are highly dependent upon the absorbing media traversed. These media include the cold ISM and the 'local' highly ionized dense surrounding medium commonly referred to as the stellar wind or envelope. Since this local absorption may exceed the ISM absorption, the usual practice of neglecting the local component may lead to underestimates of the intrinsic X-ray fluxes by several orders of magnitude. Previously, the neglect of the local component was justified, since no direct observational evidence supporting local absorption existed. Here evidence is presented for local absorption by showing a correlation between the observed X-ray hardness ratios (an indicator of absorption) and the 6 cm radio data (a measure of the stellar wind column density). It is proposed that this relation may be the first 'positive' confirmation of the existence of the local absorption component.

Waldron, Wayne L.↗
NASA NTRS1993

The absorption of p-modes by sunspots - Variations with degree and order

A spherical harmonic decomposition of the p-modes into inward and outward propagating waves is employed to investigate the absorption of solar p-modes by an isolated sunspot. The absorption coefficient (averaged over frequency and azimuthal order) is found to increase with increasing horizontal wavenumber k over the range 0-0.8/Mm. For larger horizontal wavenumbers, in the range 0.8-1.5/Mm, the absorption coefficient decreases with increasing k. The absorption along each individual p-mode ridge tends to peak at an intermediate value of the spherical harmonic degree in the range 200-400. The highest absorption is found along the p(1) ridge, and the absorption decreases with increasing radial order.

Bogdan, Thomas J.↗