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DOE OSTI2024

Investigating Mountain Watershed Headwater‐To‐Groundwater Connections, Water Sources, and Storage Selection Behavior With Dynamic‐Flux Particle Tracking

Abstract Climate change will impact mountain watershed streamflow both directly—with changing precipitation amounts and variability—and indirectly—through temperature shifts altering snowpack, melt, and evapotranspiration. To understand how these complex processes will affect ecosystem functioning and water resources, we need tools to distinguish connections between water sources (rain/snowmelt), groundwater storage, and exit fluxes (streamflow/evapotranspiration), and to determine how these connections change seasonally and as climate shifts. Here, we develop novel watershed‐scale approaches to understand water source, storage, and exit flux connections using a dynamic‐flux particle tracking model (EcoSLIM) applied in California's Cosumnes Watershed, which connects the Sierra Nevada and Central Valley. This work develops new visualizations and applications to provide mechanistic understanding that underpins the interpretation of isotopic field data at watershed scales to distinguish sources, flow paths, residence times, and storage selection. In our simulations, streamflow comes primarily from snow‐derived water while evapotranspiration generally comes from rain. Most streamflow starts above 1,000 m while evapotranspiration is sourced relatively evenly across the watershed and is generally younger than streamflow. Modeled streamflow consists primarily of water sourced from precipitation in the previous 5 years but before the current water year, while ET consists primarily of water from precipitation in the current water year. ET, and to a lesser extent streamflow, are both younger than water in groundwater storage. However, snowmelt‐derived streamflow preferentially discharges older water from snow‐derived storage. Dynamic‐flux particle tracking and new approaches presented here enable novel model‐tracer comparisons in large‐scale watersheds to better understand watershed behavior in a changing climate.

54 ENVIRONMENTAL SCIENCES↗

NASA NTRS1992

Biostratigraphic and chemostratigraphic correlation of Neoproterozoic sedimentary successions: upper Tindir Group, northwestern Canada, as a test case

Recent advances in Proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of Neoproterozoic (1000-540 Ma) successions is possible. Because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available, the upper Tindir Group of northwestern Canada provides an opportunity to test this hypothesis. The age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc-shaped-scale microfossils that may provide insights into the early evolution of biomineralization. A reinterpretation of upper Tindir microfossil assemblages suggests a late Riphean age. Although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates, least altered samples indicate that delta 13C of contemporaneous seawater was at least +4.7%, typical of Neoproterozoic, but not Cambrian, carbonates. Strontium isotopic compositions of the least altered samples yield values of approximately 0.7065, which can be uniquely correlated with late Riphean seawater. Together, micropaleontology and the isotopic tracers of C and Sr constrain the upper Tindir carbonates and their unique fossils to be late Riphean, likely between 620 and 780 Ma.

NASA Discipline Exobiology↗

DOE OSTI2023

The Rest-frame Submillimeter Spectrum of High-redshift, Dusty, Star-forming Galaxies from the SPT-SZ Survey

We present the average rest-frame spectrum of the final catalog of dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope's SPT-SZ survey and measured with Band 3 of the Atacama Large Millimeter/submillimeter Array. This work builds on the previous average rest-frame spectrum, given in Spilker et al. (2014) for the first 22 sources, and is comprised of a total of 78 sources, normalized by their respective apparent dust masses. The spectrum spans 1.9 < z < 6.9 and covers rest-frame frequencies of 240–800 GHz. Combining this data with low-J CO observations from the Australia Telescope Compact Array, we detect multiple bright line features from 12 CO, [Ci], and H 2 O, as well as fainter molecular transitions from 13 CO, HCN, HCO + , HNC, CN, H 2 O + , and CH. We use these detections, along with limits from other molecules, to characterize the typical properties of the interstellar medium (ISM) for these high-redshift DSFGs. We are able to divide the large sample into subsets in order to explore how the average spectrum changes with various galaxy properties, such as effective dust temperature. We find that systems with hotter dust temperatures exhibit differences in the bright 12 CO emission lines, and contain either warmer and more excited dense gas tracers or larger dense gas reservoirs. These observations will serve as a reference point to studies of the ISM in distant luminous DSFGs (L IR > 10 12 L ⊙ ), and will inform studies of chemical evolution before the peak epoch of star formation at z = 2–3.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2026

High sensitivity measurement of femtogram-level 238 Pu by thermal ionization mass spectrometry with correction of background 238 U

Low-concentration plutonium isotopic measurements provide a valuable fingerprint to identify possible material origins in the context of environmental contamination monitoring, environmental tracing, nuclear safeguards, nuclear forensics, and treaty monitoring. Thermal ionization mass spectrometry (TIMS) has long been recognized as one of the most sensitive techniques for plutonium isotopic measurement, but has not generally been utilized for 238 Pu determination, especially at low concentrations, due to an isobaric interference from 238 U, prevalent as background. Here, this work demonstrates a new analytical technique for correction of background 238 U from the 238 Pu counts collected during a TIMS analysis. The technique takes advantage of the higher ionization temperature of U relative to Pu and uses a 235 U tracer added after plutonium purification chemistry for 238 U semi-quantitative determination. This technique has been successfully applied to the determination of 238 Pu concentration and the 238 Pu/ 239 Pu ratio in sample types including single-isotope standards, isotopic certified reference materials, matrix-containing environmental reference materials, and simulated nuclear detonation debris. The results have been directly compared on a sample-by-sample basis to alpha spectrometry results. The detection limit by this new technique is 0.23 fg 238 Pu, which was previously unachievable by mass spectrometry and is possible in around 1 h of analysis time post-chemistry compared to weeks of required counting time by alpha spectrometry (at the same atom amount). We also present original, high precision data for Pu isotope concentrations in IAEA-384, Fangataufa Sediment, including 242 Pu results for the first time. The new technique allows measurement of a key isotope, 238 Pu, that was previously not measured in small environmental or nuclear forensics collections.

238Pu↗

DOE OSTI2021

Hundreds of weak lensing shear-selected clusters from the Hyper Suprime-Cam Subaru Strategic Program S19A data

Abstract We use the Hyper Suprime-Cam Subaru Strategic Program S19A shape catalog to construct weak lensing shear-selected cluster samples. From aperture mass maps covering ∼510 deg2 created using a truncated Gaussian filter, we construct a catalog of 187 shear-selected clusters that correspond to mass map peaks with signal-to-noise ratio larger than 4.7. Most of the shear-selected clusters have counterparts in optically selected clusters, from which we estimate the purity of the catalog to be higher than 95%. The sample can be expanded to 418 shear-selected clusters with the same signal-to-noise ratio cut by optimizing the shape of the filter function and by combining weak lensing mass maps created with several different background galaxy selections. We argue that dilution and obscuration effects of cluster member galaxies can be mitigated by using background source galaxy samples and adopting a filter function with its inner boundary larger than about 2′. The large samples of shear-selected clusters that are selected without relying on any baryonic tracer are useful for detailed studies of cluster astrophysics and cosmology.

Oguri, Masamune↗

DOE OSTI2020

The EGS Collab Project: Learnings from Experiment 1

The primary objective of the EGS Collab Project sponsored by DOE is to increase the understanding needed to efficiently implement enhanced geothermal systems (EGS). One goal of the EGS Collab project is to create a collaborative research environment in which to study stimulation of crystalline rock at the 10 meter scale. Key to this effort is the collection of high quality data to allow comparison to numerical coupled process models in an effort to build confidence in the codes and modeling techniques used. In response to this, the EGS Collab team has created an underground test bed at the Sanford Underground Research Facility (SURF) in Lead SD at a depth of approximately 1.5 km to examine hydraulic fracturing (Experiment 1). We are currently designing a second test bed aimed at investigating shear stimulation (Experiment 2). At the Experiment 1 location, we have characterized our host rock using laboratory testing and numerous field-based geophysical and geological techniques, and created a well-instrumented test bed to allow us to carefully monitor stimulation events and flow tests. In addition to the installed geophysical sensors, we have used tracer tests, differences in the ambient microbial communities at flow collection locations, and cold water injection to inform us about dynamic flow pathways. In Experiment 1, we have hydraulically stimulated the host rock in a number times at several locations in one well, creating new fractures that connect to existing fractures between the injection and production boreholes. We have performed long-term ambient and chilled water injection tests as an analog to EGS, and have monitored system changes resulting from these water injections through geophysical monitoring, flow and pressure measurements, tracer tests, and microbiology. Here, we summarize the tests performed, issues identified including poroelastic and thermoelastic effects, Joule-Thomson effects, restarting effects, indications of flow channeling, and the primary learnings from Experiment 1.

Enhanced Geothermal Systems, EGS Collab, stimulati↗

DOE OSTI2025

Modeling of the high column density systems in the Lyman-Alpha forest

The Lyman-α forests observed in the spectra of high-redshift quasars can be used as a tracer of the cosmological matter density to study baryon acoustic oscillations (BAO) and the Alcock-Paczynski effect. Extraction of cosmological information from these studies requires modeling of the forest correlations. While the models depend most importantly on the bias parameters of the intergalactic medium (IGM), they also depend on the numbers and characteristics of high-column-density systems (HCDs) ranging from Lyman-limit systems with column densities log N HI /1cm -2 > 17 to damped Lyman-α systems (DLAs) with log N HI /1cm -2 > 20.2. These HCDs introduce broad damped absorption characteristic of a Voigt profile. Consequently they imprint a component on the power spectrum whose modes in the radial direction are suppressed, leading to a scale-dependent bias. Using mock data sets of known HCD content, we test a model that describes this effect in terms of the distribution of column densities of HCDs, the Fourier transforms of their Voigt profiles and the bias of the halos containing the HCDs. Our results show that this physically well-motivated model describes the effects of HCDs with an accuracy comparable to that of the ad-hoc models used in published forest analyses. We also discuss the problems of applying the model to real data, where the HCD content and their bias is uncertain.

Lyman alpha forest↗

DOE OSTI2023

Target Selection and Validation of DESI Emission Line Galaxies

The Dark Energy Spectroscopic Instrument (DESI) will precisely constrain cosmic expansion and the growth of structure by collecting ~40 million extragalactic redshifts across ~80% of cosmic history and one-third of the sky. The Emission Line galaxy (ELG) sample, which will comprise about one-third of all DESI tracers, will be used to probe the universe over the 0.6 < z < 1.6 range, including the 1.1 < z < 1.6 range, which is expected to provide the tightest constraints. We present the target selection for the DESI Survey Validation (SV) and Main Survey ELG samples, which relies on the imaging of the Legacy Surveys. The Main ELG selection consists of a g-band magnitude cut and a (g – r) versus (r – z) color box, while the SV selection explores extensions of the Main selection boundaries. The Main ELG sample is composed of two disjoint subsamples, which have target densities of about 1940 deg –2 and 460 deg –2 , respectively. We first characterize their photometric properties and density variations across the footprint. We then analyze the DESI spectroscopic data that have been obtained from 2020 December to 2021 December in the SV and Main Survey. We establish a preliminary criterion for selecting reliable redshifts, based on the [O ιι] flux measurement, and assess its performance. Using this criterion, we are able to present the spectroscopic efficiency of the Main ELG selection, along with its redshift distribution. We thus demonstrate that the Main selection 1940 deg –2 subsample alone should provide 400 deg –2 and 460 deg –2 reliable redshifts in the 0.6 < z < 1.1 and the 1.1 < z < 1.6 ranges, respectively.

79 ASTRONOMY AND ASTROPHYSICS↗

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 NTRS1987

In situ cosmogenic H-3, C-14, and Be-10 for determining the net accumulation and ablation rates of ice sheets

The usefulness of the in situ cosmogenic H-3, C-14, and Be-10 produced by spallation of oxygen nuclei in ice, as tracers to determine net accumulation/ablation rates of ice sheets is explored. The application of the in situ H-3 and Be-10 is severely constrained because at deposition, ice contains appreciable amounts of these isotopes from the atmosphere. The case is much more favorable for C-14, which is not carried with wet precipitations; atmospheric C-14 gets mechanically trapped in the ice during deposition. It is pointed out that cosmogenic C-14 would probably exist as (C-14)O in ice. This seems to be supported by the published results of Fireman and Norris (1981). Considering their inherent amounts in the ice and the expected in situ production rates, conditions under which these isotopes can be used to study net accumulation and ablation rates are discussed. Available data on C-14 and Be-10 on polar ice from accumulation and ablation zones is also discussed. It is concluded that H-3 and C-14 should find wide applications in studying ice dynamics and Be-10 in very special circumstances.

Lal, D.↗

NASA NTRS1994

Evidence for eddy formation in the eastern Arabian Sea during the northeast monsoon

The seasonal formation of a large (500-800 km diameter) anticyclonic eddy in the upper 300-400 m of the eastern Arabian Sea during the northest monsoon period (December-April) is indicated fom hydrographic and satellite altimetry sea level observations, as well as from numerical model experiments. The center of the eddy circulation is approximately 10 deg N, 70 deg E, just to the west of the north-south Laccadive Island chain. In this paper the eddy is called the Laccadive High (LH). In some ways it is like a mirrorlike counterpart to the Great Whirl that develops during the southwest monsoon of the Somali coast (western Arabian Sea). The LH occurs at the same latitude but on the opposite side of the basin during the reversed monsoon. It is different from the Great Whirl, however, in its formation process, its intensity, and its decay. The hydrographic data obtained from surveys all during a single season give sufficiently close station spacing to allow reasonable contouring of the geopotential surfaces and of the properties within and around the LH region with minimum time aliasing. The Geostat altimeter record extends over 4 years, during which the seasonal variability of the LH indicates a dynamic relief of approximately 15-20 cm, which is in good agreement with the hydrographics observations. The altimetry time series also suggests a westward translation of the LH by January with a subsequent dissipation in midbasin. The model used is a wind-forced three-layer primitive equation model which depicts a LH agreement with the timing, position, and amplitude of both the hydrographic and altimetric measurements. The numerical simulation includes a passive tracer located in the Western Bay of Bengal; the western advection of the tracer around the south coasts of Sri Lanka and India in December and January is consistent with the appearance of low-salinity water observed to extend into the Arabian Sea during this period. The modeling studies suggest that both local and remote forcing are important in formation of the LH.

Bruce, John G.↗

DOE OSTI2025

High-redshift millennium and astrid galaxies in effective field theory at the field level

Effective field theory (EFT) modeling is expected to be a useful tool in the era of future higher-redshift galaxy surveys such as DESI-II and Spec-S5 due to its robust description of various large-scale structure tracers. However, large values of EFT bias parameters of higher-redshift galaxies could jeopardize the convergence of the perturbative expansion. Here, in this paper we measure the bias parameters and other EFT coefficients from samples of two types of star-forming galaxies in the state-of-the-art MilleniumTNG and astrid hydrodynamical simulations. Our measurements are based on the field-level EFT forward model that allows for precision EFT parameter measurements by virtue of cosmic variance cancellation. Specifically, we consider approximately representative samples of Lyman-break galaxies (LBGs) and Lyman-𝛼 emitters (LAEs) that are consistent with the observed (angular) clustering and number density of these galaxies at 𝑧 = 3. Reproducing the linear biases and number densities observed from existing LAE and LBG data, we find quadratic bias parameters that are roughly consistent with those predicted from the halo model coupled with a simple halo occupation distribution model. We also find nonperturbative velocity contributions (fingers of God) of a similar size for LBGs to the familiar case of luminous red galaxies. However, these contributions are quite small for LAEs despite their large satellite fraction values of up to ∼ 30%. Our results indicate that the effective momentum reach 𝑘 max at 𝑧 = 3 for LAEs (LBGs) will be in the range 0.3−0.6⁢ℎ Mpc −1 (0.2−0.8⁢ℎ Mpc −1 ), suggesting that EFT will perform well for high-redshift galaxy clustering. This work provides the first step toward obtaining realistic simulation-based priors on EFT parameters for LAEs and LBGs.

Sullivan, James M. [Massachusetts Inst. of Technol↗

NASA NTRS2002

Measurements of Oxygenated Organic Chemicals In the Pacific Troposphere During TRACE-P: Higher Aldehydes (less than C(sub 1)), Their Sources, and Potential Role In Atmospheric Oxidation

Airborne measurements of a large number of oxygenated organics were carried out in the Pacific troposphere (to 12 km) in the Spring of 2001 (Feb. 24-April 10). Specifically these measurements included acetaldehyde, propanaldehyde, acetone, methylethyl ketone, methanol, ethanol, PAM and organic nitrates. Independent measurements of formaldehyde, peroxides, and tracers were also available. Highly polluted as well as pristine air masses were sampled. Oxygenated organics were abundant in the clean In troposphere and were greatly enhanced in the outflow regions from Asia. Extremely high concentrations of aldehydes could be measured in the troposphere. It is not possible to explain the large abundances of aldehydes in the background troposphere without invoking significant oceanic sources. A strong correlation between the observed mixing ratios of formaldehyde and acetaldehyde is present. We infer that higher aldehydes (such as acetaldehyde and propanaldehyde) may provide a large source of formaldehyde and sequester Cox throughout the troposphere. The atmospheric behavior of acetone, methylethyl ketone, and methanol is generally indicative of their common terrestrial sources with a Image contribution from biomass/biofuel burning. A vast body of data has been collected and it is being analyzed both statistically and with the help of models to better understand the role that oxygenated organics play in the atmosphere and to unravel their sources and sinks. These results will be presented.

Singh, Hanwant B.↗

NASA NTRS2003

Large-scale Distribution of CH4 in the Western North Pacific: Sources and Transport from the Asian Continent

Methane (CH4) mixing ratios in the northern Pacific Basin were sampled from two aircraft during the TRACE-P mission (Transport and Chemical Evolution over the Pacific) from late February through early April 2001 using a tunable diode laser system. Described in more detail by Jacob et al., the mission was designed to characterize Asian outflow to the Pacific, determine its chemical evolution, and assess changes to the atmosphere resulting from the rapid industrialization and increased energy usage on the Asian continent. The high-resolution, high-precision data set of roughly 13,800 CH4 measurements ranged between 1602 ppbv in stratospherically influenced air and 2149 ppbv in highly polluted air. Overall, CH4 mixing ratios were highly correlated with a variety of other trace gases characteristic of a mix of anthropogenic industrial and combustion sources and were strikingly correlated with ethane (C2H6) in particular. Averages with latitude in the near-surface (0-2 km) show that CH4 was elevated well above background levels north of 15 deg N close to the Asian continent. In the central and eastern Pacific, levels of CH4 were lower as continental inputs were mixed horizontally and vertically during transport. Overall, the correlation between CH4 and other hydrocarbons such as ethane (C2H6), ethyne (C2H2), and propane (C3H8) as well as the urban/industrial tracer perchloroethene (C2Cl4), suggests that for CH4 colocated sources such as landfills, wastewater treatment, and fossil fuel use associated with urban areas dominate regional inputs at this time. Comparisons between measurements made during TRACE-P and those of PEM-West B, flown during roughly the same time of year and under a similar meteorological setting 7 years earlier, suggest that although the TRACE-P CH4 observations are higher, the changes are not significantly greater than the increases seen in background air over this time interval.

Bartlett, Karen B.↗

NASA NTRS2013

Radiogenic Ingrowth of 40CA from Decay of 40K Provides a Powerful Tracer for Understanding the Origins of Felsic Magmas

Over time high K/Ca continental crust produces a unique Ca isotopic reservoir, with measurable 40Ca excesses compared to Earth's mantle (ɛCa=0). Thus, values of ɛCai > 1 indicate a significant crustal contribution to a magma. Values of ɛCai (<1) indistinguishable from mantle Ca indicate that the Ca in those magmas is either directly from the mantle, or is from partial melting of newly formed crust. So, whereas 40Ca excesses clearly define crustal contributions, mantle-like 40Ca/44Ca ratios are not as definitive. Here we present Ca isotopic measurements of intermediate to felsic igneous rocks from the western United States, and two crustal xenoliths found within the Fish Canyon Tuff (FCT). The two crustal xenoliths found within the 28.2 Ma FCT of the southern Rocky Mountain volcanic field (SRMVF) yield ɛCa values of ~4 and ~7.5, respectively. The 40Ca excesses of these possible source rocks are due to long-term in situ 40K decay and suggest that they are Precambrian in age. However, the FCT (ɛCai ~0.3) is within uncertainty of the mantle 40Ca/44Ca. Together, these data indicate that little Precambrian crust was involved in the petrogenesis of the FCT. Nd isotopic analyses of the FCT imply that it was generated from 10- 75% of an enriched component, and the Ca isotopic data appear to restrict that component to newly formed lower crust, or enriched mantle. However, the Ca isotopic data do permit assimilation of some crust with low Ca/Nd; decreasing the 143Nd/144Nd without adding much excess 40Ca to the FCT. Several other large tuffs from the SRMVF and from Yellowstone have ɛCai indistinguishable from the mantle. However, a few large tuffs from the SRMVF show significant 40Ca excesses. These tuffs (Wall Mountain, Blue Mesa, and Grizzly Peak) are likely sourced from near, or within the Colorado Mineral Belt. New isotopic measurements of Mesozoic and Tertiary granites from across the northern Great Basin show a range of ɛCai from 0 to ~3. In these samples ɛCai is generally correlated with ɛSri and is broadly negatively correlated with ɛNdi. However, for granites with similar ɛNdi at a given general location ɛCai can vary significantly (1 to 2 epsilon units). In rocks where low ɛNdi could also be due to melting from enriched reservoirs in the mantle lithosphere, the combination of high ɛCai with low ɛNdi clearly identifies crustal melts.

Mills, Ryan D.↗

NASA NTRS1994

Detection of CO Cameron band emission in comet P/Hartley 2 (1991 XV) with the Hubble Space Telescope

Ultraviolet (UV) spectra of comet P/Hartley 2 (1991 XV) taken with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in 1991 September reveal several bands of the Cameron system of CO (a 3 Pi-X 1 Sigma). These band are most likely due to 'prompt' emission from CO2 and, thus, provide a direct tracer of the CO2 abundance in the nucleus. Photodissociative excitation of CO2 is probably the largest contributor to the Cameron band emission, but significant contributions from electron impact excitation of CO, electron impact dissociation of CO2, and dissociative recombination of CO2(+), are also possible. Using our estimate that photodissociative excitation is responsible for approximately 60% of the total excitation of the Cameron system, we derive Q(sub CO2) approximately 2.6 x 10(exp 27) molecules/s, which implies CO2/H20 approximately 4%. If all of the Cameron band emission is due to photodissociative excitation, then CO2/H2O = 7 +/- 2%. For the largest possible contributions from the other excitation mechanisms considered, the CO2 abundance could be as a small as aproximately 2-3%. We did not detect CO Fourth Positive Group emission in our data and derive an upper limit of CO/H2O less than or equal to 1% (3 sigma) for CO coming directly from the nucleus. Comparison of the relative CO2 and CO abundances in P/Hartley 2 to those in P/Halley (CO2/H2O approximately 3%-4%, CO/H20 approximately 4% for the nucleus source) indicates that selective devolatilization of the nucleus may have occurred for P/Hartley 2. A relatively large CO2/CO ratio (i.e., approximately greater than 1) seems to be a common property of cometary nuclei. Since gas phase chemistry, in either the solar nebula or the interstellar medium (ISM), appears incapable of producing large relative CO2 abundances, the CO2 in cometary nuclei is probably produced either by UV and/or cosmic ray irradiation of ISM grains prior to the formation of the Solar System, or by condensation fractionation in the solar nebula.

Weaver, H. A.↗

NASA NTRS2007

Reactive Nitrogen Distribution and Partitioning in the North American Troposphere and Lowermost Stratosphere

A comprehensive group of reactive nitrogen species (NO, NO2, HNO3, HO2NO2, PANs, alkyl nitrates, and aerosol-NO3) were measured in the troposphere and lowermost stratosphere over North America and the Atlantic during July/August 2004 (INTEX-A) from the NASA DC-8 platform (0.1-12 km). Less reactive nitrogen species (HCN and CH3CN), that are also unique tracers of biomass combustion, were also measured along with a host of other gaseous (CO, VOC, OVOC, halocarbon) and aerosol tracers. Clean background air as well as air with influences from biogenic emissions, anthropogenic pollution, biomass combustion, and stratosphere was sampled both over continental U. S., Atlantic and Pacific. The North American upper troposphere was found to be greatly influenced by both lightning NO(x) and surface pollution lofted via convection and contained elevated concentrations of PAN, ozone, hydrocarbons, and NO(x). Under polluted conditions PAN was a dominant carrier of reactive nitrogen in the upper troposphere while nitric acid dominated in the lower troposphere. Peroxynitric acid (HO2NO2) was present in sizable concentrations always peaking at around 8 km. Aerosol nitrate appeared to be mostly contained in large soil based particles in the lower troposphere. Plumes from Alaskan fires contained large amounts of PAN and very little enhancement in ozone. Observational data suggest that lightning was a far greater contributor to NO(x) in the upper troposphere than previously believed. NO(x) and NO(y) reservoir appeared to be in steady state only in the middle troposphere where NO(x)/NO(y) was independent of air mass age. A first comparison of observed data with simulations from four 3-D models shows significant differences between observations and models as well as among models. These uncertainties likely propagate themselves in satellites derived NOx data. Observed data are interpreted to suggest that soil sinks of HCN/CH3CN are at best very small. We investigate the partitioning and interplay of the reactive nitrogen species within characteristic air masses and further examine their role in ozone formation.

Singh, H. B.↗

NASA NTRS1991

Seasonal variations in the subauroral electron temperature enhancement

A statistical study of the seasonal variations of the subauroral electron temperature enhancement was undertaken using data from the Langmuir probe experiment on the DE 2 satellite throughout most of the mission (1981-1982). In the winter hemisphere the nighttime background electron temperature is the highest and the magnitude of the peak Te responds most weakly to the geomagnetic activity. This behavior can be explained by seasonal trends in the nighttime downward heat flux due to conjugate photoelectrons. Moreover, model results indicate that a factor of about three increase in heat inflow during equinox relative to solstice is required to raise the electron temperature to a given level. This is a consequence of the higher electron densities at the Te peak near equinox. The Te peak occurs on field lines which thread the outer plasmasphere in the vicinity if the plasmapause and thus can be used as a tracer of the plasmapause position.

Fok, M.-C.↗