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NASA NTRS2000

Methanogenesis, Mesospheric Clouds, and Global Habitability

Hyperthermophilic methanogens can exist in a deep hot biosphere up to 110 C, or 10 km deep. Methane (CH4) itself is thermodynamically stable to depths of 300 km. Geologic (microbial plus abiogenic thermal) methane is transported upward, attested to by its association with helium, to form petroleum pools. Near or at the surface, geologic CH4 mixes with other natural and with anthropogenic CH4 yielding annual emissions into the atmosphere of 500 Tg, of which 200 Tg are natural and 300 Tg are man-made. The atmospheric lifetime of CH4, a greenhouse gas 20 times more effective than CO2 in raising global temperatures, is approximately 10 years. It is removed from the atmosphere mainly by reactions with hydroxyl radical (OH) to form CO2, but also by dry soil and by conversion to H2O in the stratosphere and middle atmosphere. A sudden rise in atmospheric temperatures by 9-12 C some 55 million years ago has been explained by the release in a few thousand years of three trillion tons of CH4 out of 15 trillion tons that had formed beneath the sea floor. What prevented this CH4-induced greenhouse effect from running away? An analog to the CH4-burp of 55 million years ago is the CH4-doubling over the past century which resulted in a increase in upper level H2O from 4.3 ppmv to 6 ppmv. This 30% increase in H2O vapor yielded a tenfold increase in brightness of polar mesospheric clouds because of a strong dependence of the ice particle nucleation rate on the water saturation ratios. Models show that at a given temperature the optical depth of mesospheric clouds scales as [H2O]beta with beta varying between 4 and 8. Radiative transfer tools applied to mesospheric particles suggest that an optical depth of approximately one, or 1000 times the current mesospheric cloud optical depth, would result in tropospheric cooling of about 10 K. Assuming beta=6, a thousandfold increase in optical thickness would require a three-fold increase of H2O, or a 20-fold increase of CH4. At the current rate of anthropogenic emissions this is expected to occur within the next 1000 years. This timescale is also commensurate with what has been assumed for the CH4- burp 55 million years ago.

Pueschel, Rudolf F.↗
NASA NTRS2001

Technology Thrusts for Future Earth Science Applications

This paper presents NASA's recent direction to invest in the critical science instrument and platform technologies in order to realize more reliable, frequent and versatile missions for future Earth Science measurements. Historically, NASA's Earth Science Enterprise has developed and flown science missions that have been large in size, mass and volume. These missions have taken much longer to implement due to technology development time, and have carried a large suite of instruments on a large spacecraft. NASA is now facing an era where the budget for the future years is more or less flat and the possibility for any major new start does not vividly appear on the horizon. Unfortunately, the scientific measurement needs for remote sensing have not shrunk to commensurate with the budget constraints. In fact, the challenges and scientific appetite in search of answers to a score of outstanding questions have been gradually expanding. With these factors in mind, for the last three years NASA has been changing its focus to concentrate on how to take advantage of smaller missions by relying on industry, and minimizing the overall mission life cycle by developing technologies that are independent of the mission implementation cycle. The major redirection of early investment in the critical technologies should eventually have its rewards and significantly reduce the mission development period. Needless to say, in the long run this approach should save money, minimize risk, promote or encourage partnering, allow for a rapid response to measurement needs, and enable frequent missions making a wider variety of earth science measurements. This paper gives an overview of some of the identified crucial technologies and their intended applications for meeting the future Earth Science challenges.

Habib, Shahid↗
NASA NTRS2001

Evaluation of the National Throughput Benefits of the Civil Tilt Rotor

The air transportation system is a key part of the U.S. and global economic infrastructure. In recent years, this system, by any measure of usage - operations, enplanements, or revenue passenger miles (RPMs) - has grown rapidly. The rapid growth in demand has not been matched; however, by commensurate increases in the ability of airports and the airspace system to handle the additional traffic. As a result, the air transportation system is approaching capacity and airlines will face excessive delays or significant constraints on service unless capacity is expanded. To expand capacity, the air traffic management system must be improved. To improve the air traffic management system, the National Aeronautics and Space Administration (NASA) Aerospace Technology Enterprise developed the strategic goal of tripling air traffic throughput over the next 10 years, in all weather conditions, while at least maintaining current safety standards. As the first step in meeting that goal, the NASA Intercenter Systems Analysis Team (ISAT) is evaluating the contribution of existing programs to meet that goal. A major part of the study is an examination of the ability of the National Airspace System (NAS) to meet the predicted growth in travel demand and the potential benefits of technology infusion to expand NAS capacity. We previously analyzed the effects of the addition of two technology elements - Terminal Area Productivity (TAP) and Advanced Air Transportation Technologies (AATT). The next program we must analyze is not specific to airspace or aircraft technology. The program incorporates a fundamentally different vehicle to improve throughput: the civil tilt rotor (CTR). The CTR has the unique operating characteristic of being able to take off and land like a rotorcraft (vertical take off and landing, or VTOL, capability) but cruises like a traditional fixed-wing aircraft. The CTR also can operate in a short take off and landing (STOL) mode; generally, with a greater payload capacity (i.e., more passengers) than when operating in the VTOL mode. CTR could expand access to major airports without interfering with fixed-wing aircraft operating on congested runways and it could add service to new markets without the infrastructure support needed for fixed-wing aircraft. During FY 1999, we preliminarily assessed the feasibility of operating CTRs at two major U.S. airports as part of the annual review of NASA aerospace goals by the ISAT. This current study expands the analysis and concepts of that study to the complete NAS to quantify the national throughput effects of the CTR.

Johnson, Jesse↗
NASA NTRS2001

Quantum Mechanical Study of Nanoscale MOSFET

The steady state characteristics of MOSFETS that are of practical Interest are the drive current, off-current, dope of drain current versus drain voltage, and threshold voltage. In this section, we show that quantum mechanical simulations yield significantly different results from drift-diffusion based methods. These differences arise because of the following quantum mechanical features: (I) polysilicon gate depletion in a manner opposite to the classical case (II) dependence of the resonant levels in the channel on the gate voltage, (III) tunneling of charge across the gate oxide and from source to drain, (IV) quasi-ballistic flow of electrons. Conclusions dI/dV versus V does not increase in a manner commensurate with the increase in number of subbands. - The increase in dI/dV with bias is much smaller then the increase in the number of subbands - a consequence of bragg reflection. Our calculations show an increase in transmission with length of contact, as seen in experiments. It is desirable for molecular electronics applications to have a small contact area, yet large coupling. In this case, the circumferential dependence of the nanotube wave function dictates: - Transmission in armchair tubes saturates around unity - Transmission in zigzag tubes saturates at two.

Svizhenko, Alexei↗
NASA NTRS1995

Chaotic Rotation of Nereid

The shape and spin of Neptune's outermost satellite Nereid are still unknown. Ground-based photometry indicates large brightness variations, but different observers report very different lightcurve amplitudes and periods. On the contrary, Voyager 2 images spanning 12 days show no evidence of variations greater than 0.1 mag. The latter suggest either that Nereid is nearly spherical, or that it is rotating slowly. We propose that tides have already despun Nereid's rotation to a period of a few weeks, during the time before the capture of Triton when Nereid was closer to Neptune. Since Nereid reached its present orbit, tides have further despun Nereid to a period on the order of a month. For Nereid's orbital eccentricity of 0.75, tidal evolution ceases when the spin period is still approximately 1/8 of the orbital period. Furthermore, the synchronous resonance becomes quite weak for such high eccentricities, along with other low-order spin orbit commensurabilities. In contrast, high-order resonances become very strong particularly the 6:1, 6.5:1, 7:1, 7.5:1, and 8:1 spin states. If Nereid departs by more than approximately 1% from a sphere, however, these resonances overlap, generating chaos. Our simulations show that Nereid is likely to be in chaotic rotation for any spin period longer than about 2 weeks.

Dobrovolskis, Anthony R.↗
NASA NTRS2002

Noise, Turbulence, and Thrust of Subsonic Free Jets from Lobed Nozzles

A study of noise benefit, vis-a-vis thrust penalty, and its correlation to turbulence intensities was conducted for free jets issuing from lobed nozzles. Four convergent nozzles with constant exit area were used in the experiments. Three of these were of rectangular lobed configuration having six, ten and fourteen lobes; the fourth was a circular nozzle. Increasing the number of lobes resulted in a progressive reduction in the turbulence intensities as well as in the overall radiated noise. The noise reduction was pronounced at the low frequency end of the spectrum. However, there was an increase in the high frequency noise that rendered the overall benefit less attractive when compared on a scaled-up A-weighted basis. A reduction in noise was accompanied by a commensurate reduction in the turbulent kinetic energy in the flow field. As expected, increasing the number of lobes involved progressive reduction in the thrust coefficient. Among the cases studied, the six-lobed nozzle had the optimum reduction in turbulence and noise with the least thrust penalty.

Zaman, K. B. M. Q.↗
NASA NTRS1995

A FFT Method for the Quasiclassical Selection of Initial Ro-Vibrational States of Triatomic Molecules

This paper describes the use of an exact fast Fourier transform (FFT) method to prepare specified vibrational-rotational states of triatomic molecules. The method determines the Fourier coefficients needed to describe the coordinates and momenta of a vibrating-rotating triatomic molecule. Once the Fourier coefficients of a particular state are determined, it is possible to easily generate as many random sets of initial cartesian coordinates and momenta as desired. All the members of each set will correspond to the particular vibrational-rotational state selected. For example, in the case of the ground vibrational state of a non-rotating water molecule, the calculated actions of 100 sets of initial conditions produced actions within 0.001 h(bar) of the specified quantization values and energies within 5 cm(sup -1) of the semiclassical eigenvalue. The numerical procedure is straightforward for states in which all the fundamental frequencies are independent. However for states for which the fundamental frequencies become commensurate (resonance states), there are additional complications. In these cases it is necessary to determine a new set of "fundamental" frequencies and to modify the quantization conditions. Once these adjustments are made, good results are obtained for resonance states. The major problems are in labelling the large number of Fourier coefficients and the presence of regions of chaotic motion. Results are presented for the vibrational states of H2O and HCN and the ro-vibrational states of H2O.

Eaker, Charles W.↗
NASA NTRS2002

Is Asteroid 951 Gaspra in a Resonant State with Its Spin Increasing Due to YORP?

Asteroid 951 Gaspra appears to be in an obliquity resonance with its spin increasing due to the YORP effect. Gaspra, an asteroid 5.8 km in radius, is a prograde rotator with a rotation period of 7.03 hours. A three million year integration indicates its orbit is stable over at least this time span. From its known shape and spin axis orientation and assuming a uniform density, Gaspra's axial precession period turns out to be nearly commensurate with its orbital precession period, which leads to a resonance condition with consequent huge variations in its obliquity. At the same time its shape is such that the Yarkovsky-O'Keefe-Radzievskii-Paddack effect (YORP effect for short) is increasing its spin rate. The YORP cycle normally leads from spin-up to spin-down and then repeating the cycle; however, it appears possible that resonance trapping can at least temporarily interrupt the YORP cycle, causing spin-up until the resonance is exited. This behavior may partially explain why there is an excess of fast rotators among small asteroids. YORP may also be a reason for small asteroids entering resonances in the first place.

Rubincam, David Parry↗
NASA NTRS1995

Pole-to-Pole Distribution of Stratospheric Black Carbon (Soot) Aerosol from Aircraft

The distribution of black carbon (soot) aerosol (BCA) in the atmosphere is of interest for several reasons: (1) Because BCA has the highest absorption cross section of any compound known, it can absorb solar radiation to cause atmospheric warming. (2) Because it is a strong adsorber of gases, it can catalyze heterogeneous reactions to change the chemical composition of the atmosphere.(3) If aircraft are a major source of BCA, it is an important tracer of aircraft emissions. Analysis for BCA of impactor samples from Arctic and Antarctic deployments, utilizing particle morphology of scanning electron microscopy images, permits the following conclusions: (1) The BCA concentration in the northern stratosphere varies between 0 and 2.6 ng m-3 averaging 0.6 ng/cu m. (2) This BCA loading is commensurate with estimated fuel consumptions in the stratosphere by the current commercial fleet and an emission index E=0.03 g BCA per kg fuel burnt which was measured in jet exhaust at al titude.Thus, most stratospheric BCA in the northern stratosphere results from aircraft emissions. The background BCA concentration in the southern stratosphere varies between 0 and 0.6 ng cu m averaging 0.1 ng/cu m. This strong meridional gradient implies that stratospheric BCA residence time- is shorter than are mixing times between hemispheres. Projected annual fuel consumption of a future supersonic commercial fleet is 7E13 g. This fleet would increase stratospheric BCA loadings by a factor of 2-3, because almost all fuel would be burnt above the tropopause. An improved EI(BCA) by a factor of ten would result in an increase of stratospheric BCA loadings by approximately 50 %.

Pueschel, R. F.↗
NASA NTRS1997

Onward to Petaflops Computing

With the recent demonstration of a computing rate of one Tflop/s at Sandia National Lab, one might ask what lies ahead for high-end computing. The next major milestone is a sustained rate of one Pflop/s (also written one petaflops, or 10(exp 15) floating-point operations per second). It should be emphasized that we could just as well use the term "peta-ops", since it appears that large scientific systems will be required to perform intensive integer and logical computation in addition to floating-point operations, and completely non- floating-point applications are likely to be important as well. In addition to prodigiously high computational performance, such systems must of necessity feature very large main memories, between ten Tbyte (10(exp 13) byte) and one Pbyte (10 (exp 15) byte) depending on application, as well as commensurate I/O bandwidth and huge mass storage facilities. The current consensus of scientists who have performed initial studies in this field is that "affordable" petaflops systems may be feasible by the year 2010, assuming that certain key technologies continue to progress at current rates. A sustained petaflops computing capability however is a daunting challenge; it appears significantly more challenging from today's state-of-the-art than achieving one Tflop/s has been from the level of one Gflop/s about 12 years ago. Challenges are faced in the arena of device technology, system architecture, system software, algorithms and applications. This talk will give an overview of some of these challenges, and describe some of the recent initiatives to address them.

Bailey, David H.↗
NASA NTRS1997

Soot Aerosol from Aircraft: Is there Enough to Cause Middle-Latitude Lower Stratospheric Ozone

Soot aerosol from aircraft has been implicated to cause long-term ozone depletion at mid-latitudes in the lower stratosphere at a rate of approx. equals 5%,per decade. During the 1996 SUCCESS field campaign, we sampling aerosols in the exhaust wake of a Boeing 757 aircraft and determined emission indices for sulfuric acid (EIH2SO4=4.8E-2 and 5.7E- 1. g/kgFUEL for 75 and 675 ppm fuel-sulfur, respectively) and soot (EIsoot=7.5E-4 g/kgFUEL). The corresponding fuel-sulfur to H2SO4 conversion efficiency was 25 % and 30%,respctively. Applying the H2SO4 emission index to the 1990 fuel by the world's commercial fleets of 1.3E11 kg, a conversion efficiency of 30% would have led to an annual contribution to the atmospheric sulfur budget by aircraft of 2.E7 kg H2SO4, if the fuels averaged 500 ppmm.The soot emission index given above yielded a 1990 injection of soot aerosol by aircraft of 1.E5 kg. Thus, soot amounts to only one half of one percent of the aerosol generated by aircraft. The fractal nature of soot may increase its actual surface area by about a factor of 10. The findings, however, of (1) stratospheric soot loadings commensurate with aircraft fuel consumption, based on the emission index given above and the assumption of stratospheric residence times of the order of one year; and (2) a trend in stratospheric soot loading of approx.6% per year since 1981, similar to the annual increase of aircraft operations since that time, implicate aircraft as stratospheric polluters. A trend similar to soot of H2SO4 aerosol loading could not be deciphered, neither from in situ measurements nor SAGE II extinction, against the "noise" due to volcanic eruptions, The current single scatter albedo of the stratospheric aerosol is omega = 0.993+/-0.004.

Pueschel, R. F.↗
NASA NTRS2002

No Compton Reflection In a Chandra/RXTE Observation of Mkn 509: Implications for the Fe-K Line Emission From Accreting X-Ray Sources

We report the results of simultaneous Chandra and RXTE observations of the Seyfert 1 galaxy Mkn 509. We deconvolve the broad and narrow Fe-K emission-line components for which we measure rest-frame equivalent widths of 119+/-18 eV and 57+/-13 eV respectively. The broad line has a FWHM of 57,600((sup 14,400)(sub -21,000)) km/s and the narrow line is unresolved, with an upper limit on the FWHM of 4,940 km/s. Both components must originate in cool matter since we measure rest-frame center energies of 6.36((sup +0.13)(sub -0.12)) keV and 6.42+/-0.01 keV for the broad and narrow line respectively. This rules out He-like and H-like Fe for the origin of both the broad and narrow lines. If, as is widely accepted, the broad Fe-K line originates in Thomson-thick matter (such as an accretion disk), then one expects to observe spectral curvature above approximately 10 keV, (commensurate with the observed broad line), characteristic of the Compton-reflection continuum. However our data sets very stringent limits on deviations of the observed continuum from a power law. Light travel-time delays cannot be invoked to explain anomalies in the relative strengths of the broad Ferry line and Compton-reflection continuum since they are supposed to originate in the same physical location. We are forced to conclude that both the broad and narrow Fe-K lines had to originate in Thomson-thin matter during our observation. This result, for a single observation of just one source, means that our understanding of Fe K line emission and Compton reflection from accreting X-ray sources in general needs to be re-examined. For example, if an irradiated accretion disk existed in Mkn 509 at the time of the observations, the lack of spectral curvature above approximately 10 keV suggests two possibilities. Either the disk was Thomson-thick and highly ionized, having negligible Fe-K line emission and photoelectric absorption or the disk was Thomson-thin producing some or all of the broad Fe-K line emission. In the former case, the broad Fe-K line had to have produced in a Thomson-thin region elsewhere. In both cases the predicted spectral curvature above approximately 10 keV is negligible. An additional implication of our results is that any putative obscuring torus in the system, required by unification models of active galaxies, must also be Thomson-thin. The same applies to the optical broad line region (BLR) if it has a substantial covering factor.

Yaqoob, Tahir↗
NASA NTRS2002

Computation of Turbulent Heat Transfer on the Walls of a 180 Degree Turn Channel With a Low Reynolds Number Reynolds Stress Model

The Low Reynolds number version of the Stress-omega model and the two equation k-omega model of Wilcox were used for the calculation of turbulent heat transfer in a 180 degree turn simulating an internal coolant passage. The Stress-omega model was chosen for its robustness. The turbulent thermal fluxes were calculated by modifying and using the Generalized Gradient Diffusion Hypothesis. The results showed that using this Reynolds Stress model allowed better prediction of heat transfer compared to the k-omega two equation model. This improvement however required a finer grid and commensurately more CPU time.

Ameri, A. A.↗
NASA NTRS2003

Development of Highly-Conductive Polyelectrolytes for Lithium Batteries

Future NASA and Air Force missions require reliable and safe sources of energy with high specific energy and energy density that can provide thousands of charge-discharge cycles at more than 40% depth- of-discharge and that can operate at low temperatures. All solid-state batteries have substantial advantages with respect to stability, energy density, storage fife and cyclability. Among all solid-state batteries, those with flexible polymer electrolytes offer substantial advantages in cell dimensionality and commensurability, low temperature operation and thin film design. The above considerations suggest that lithium-polymer electrolyte systems are promising for high energy density batteries and should be the systems of choice for NASA and US Air Force applications. Polyelectrolytes (single ion conductors) are among most promising avenues for achieving a major breakthrough 'in the applicability of polymer- based electrolyte systems. Their major advantages include unit transference number for the cation, reduced cell polarization, minimal salt precipitation, and favorable electrolyte stability at interfaces. Our research is focused on synthesis, modeling and cell testing of single ion carriers, polyelectrolytes. During the first year of this project we attempted the synthesis of two polyelectrolytes. The synthesis of the first one, the poly(ethyleneoxide methoxy acrylateco-lithium 1,1,2-trifluorobutanesulfonate acrylate, was attempted few times and it was unsuccessful. We followed the synthetic route described by Cowie and Spence. The yield was extremely low and the final product could not be separated from the impurities. The synthesis of this polyelectrolyte is not described in this report. The second polyelectrolyte, comb polysiloxane polyelectrolyte containing oligoether and perfluoroether sidechains, was synthesized in sufficient quantity to study the range of properties such as thermal stability, Li- ion- conductivity and stability toward lithium metal. Also, the batteries containing this polyelectrolyte were assembled and tested. The results are detailed below. The synthesis of another polyelectrolyte similar to polysiloxane polyelectrolyte has been started, however, the synthesis was not completed due to termination of the project.

Shriver, D. F.↗
NASA NTRS2002

Sensor Web Architectural Concepts and Implementation Challenges - An Heuristic Approach

There is a significant interest in the Earth Science remote sensing community to increase the number of observations. The obvious reasons for such a push is to improve the temporal and surface coverage of measurements. However, there is little analysis available in terms of benefits, costs and optimized set of sensors needed to make these necessary observations. In reality, this is a complex problem that should be carefully studied and balanced over many boundaries. For example, the question of technology maturity versus users desire to obtain additional measurements is non congruent. This is further complicated by the limitations of the laws of physics and the economic conditions. With the advent of advance technology, it is anticipated that the cost of the spacecraft technology will become more affordable. However, the specialized detector subsystems, and the precision flying techniques may still require substantial innovation, development time and cost. Additionally, the space deployment scheme should also be given a careful attention because of a high expense. Nonetheless, it is important to carefully examine the science priorities and steer the development efforts that can commensurate with the tangible requirements. This paper outlines a possible set of architectural concepts, operational scenarios and potential benefits of one scheme versus another. It further makes some suggestions where one can draw some boundary conditions to incrementally solve this predicament.

Habib, Shahid↗
NASA NTRS2002

Sensor Web and Intelligent Sensors for Earth Science Applications

There is a significant interest in the Earth Science remote sensing community in substantially increasing the number of observations relative to the current frequency of collection. The obvious reason for such a push is to improve the temporal and surface coverage of measurements. However, there is little analysis available in terms of benefits, costs and optimized set of sensors needed to make these necessary observations. This is a complex problem that should be carefully studied and balanced over many boundaries. For example, the question of technology maturity versus users' desire for obtaining additional measurements is noncongruent. This is further complicated by the limitations of the laws of physics and the economic conditions. With the advent of advanced technology, it is anticipated that developments in spacecraft technology will enable advanced capabilities to become more affordable. However, specialized detector subsystems, and precision flying techniques may still require substantial innovation, development time and cost. Additionally, the space deployment scheme should also be given careful attention because of the high associated expense. Nonetheless, it is important to carefully examine the science priorities and steer the development efforts that can commensurate with the tangible requirements. This presentation will focus on a possible set of architectural concepts beneficial for future Earth science studies and research its and potential benefits.

Habib, Shahid↗
NASA NTRS2003

National Aeronautics and Space Administration (NASA)/American Society of Engineering Education (ASEE) Summer Faculty Fellowship Program - 2000

The 2000 Johnson Space Center (JSC) National Aeronautics and Space Administration (NASA)/American Society for Engineering Education (ASEE) Summer Faculty Fellowship Program was conducted by the University of Houston and JSC. The 10-week program was operated under the auspices of the ASEE. The program at JSC, as well as the programs at other NASA Centers, was funded by the Office of University Affairs, NASA Headquarters, Washington, D.C. The objectives of the program, which began in 1965 at JSC and 1964 nationally, are to (1) further the professional knowledge of qualified engineering and science faculty, (2) stimulate an exchange of ideas between participants and NASA, (3) enrich and refresh the research and teaching activities of participants' institutions, and (4) contribute to the research objectives of the NASA Centers. Each faculty fellow spent at least 10 weeks at JSC engaged in a research project commensurate with her/his interests and background, and worked in collabroation with a NASA/JSC colleague. This document is a compilation of the final reports on the research projects done by the faculty fellows during the summer of 2000.

Bannerot, Richard B.↗
NASA NTRS2003

Regional Impacts of Woodland Expansion on Nitrogen Oxide Emissions from Texas Savannahs: Combining Field, Modeling and Remote Sensing Approaches

Woody encroachment has contributed to documented changes world-wide and locally in the southwestern U.S. Specifically, in North Texas rangelands encroaching mesquite (Prosopis glandulosa var. glandulosa) a known N-fixing species has caused changes in aboveground biomass. While measurements of aboveground plant production are relatively common, measures of soil N availability are scarce and vary widely. N trace gas emissions (nitric and nitrous oxide) flom soils reflect patterns in current N cycling rates and availability as they are stimulated by inputs of organic and inorganic N. Quantification of N oxide emissions from savanna soils may depend upon the spatial distribution of woody plant canopies, and specifically upon the changes in N availability and cycling and subsequent N trace gas production as influenced by the shift from herbaceous to woody vegetation type. The main goal of this research was to determine whether remotely sensible parameters of vegetation structure and soil type could be used to quantify biogeochemical changes in N at local, landscape and regional scales. To accomplish this goal, field-based measurements of N trace gases were carried out between 2000-2001, encompassing the acquisition of imaging spectrometer data from the NASA Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) on September 29, 2001. Both biotic (vegetation type and soil organic N) and abiotic (soil type, soil pH, temperature, soil moisture, and soil inorganic N) controls were analyzed for their contributions to observed spatial and temporal variation in soil N gas fluxes. These plot level studies were used to develop relationships between spatially extensive, field-based measurements of N oxide fluxes and remotely sensible aboveground vegetation and soil properties, and to evaluate the short-term controls over N oxide emissions through intensive field wetting experiments. The relationship between N oxide emissions, remotely-sensed parameters (vegetation cover, and soil type), and physical controls (soil moisture, and temperature) permitted the regional scale quantification of soil N oxides emissions. Landscape scale analysis linking N oxide emissions with cover change revealed an alleviation from N limitation following mesquite invasion. This study demonstrated the advantage of using N trace gases as a measure of ecosystem N availability combined with remote sensing to characterize the spatial heterogeneity in ecosystem parameters at a scale commensurate with field-based measurements of these properties. Woody vegetation encroachment provided an opportunity to capitalize on detection of the remotely-sensible parameter of woody cover as it relates to belowground biogeochemical processes that determine N trace gas production. The first spatially-explicit estimates of NO flux were calculated based on Prosopis fractional cover derived from high resolution remote sensing estimates of fractional woody cover (< 4 m) for a 120 sq km region of North Texas. An assessment of both N stocks and fluxes from the study revealed an alleviation of N limitation at this site experiencing recent woody encroachment. Many arid and semi-arid regions of the world are experiencing woody invasions, often of N-fixing species. The issue of woody encroachment is in the center of an ecological and political debate. Improving the links between biogeochemical processes and remote sensing of ecosystem properties will improve our understanding of biogeochemical processes at the regional scale, thus providing a means to address issues of land-use and land-cover change.

Asner, Gregory P.↗