1,607 research outputs found
The relationships between strength, power, and physical function in older adults with knee osteoarthritis
The Net Global Effects of Alternative U.S. Biofuel Mandates
One of the declared objectives of U.S. biofuel policy is the reduction of greenhouse gas (GHG) emissions from fossil fuel combustion, but many studies have questioned whether such a reduction would actually occur and, if so, how large it would be. This report describes the global market, land use, GHG emissions, and nitrogen use impacts of the U.S. Renewable Fuel Standard (RFS2) and several alternative biofuel policy designs, which differ in terms of mandate magnitude and feedstock composition, over the 2010-2030 period
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Improved estimate of the policy-relevant background ozone in the United States using the GEOS-Chem global model with 1/2° × 2/3° horizontal resolution over North America
The policy-relevant background (PRB) ozone is defined by the US Environmental Protection Agency (EPA) as the surface ozone concentration that would be present over the US in the absence of North American anthropogenic emissions. It is intended to provide a baseline for risk and exposure assessments used in setting the National Ambient Air Quality Standard (NAAQS). We present here three-year statistics (2006–2008) of PRB ozone over the US calculated using the GEOS-Chem global 3-D model of atmospheric composition with 1/2° × 2/3° horizontal resolution over North America and adjacent oceans (2° × 2.5° for the rest of the world). We also provide estimates of the US background (no anthropogenic US emissions) and natural background (no anthropogenic emissions worldwide and pre-industrial methane). Our work improves on previous GEOS-Chem PRB estimates through the use of higher model resolution, 3-year statistics, better representation of stratospheric influence, and updated emissions. PRB is particularly high in the intermountain West due to high elevation, arid terrain, and large-scale subsidence. We present for this region a detailed model evaluation showing that the model is successful in reproducing ozone exceedances up to 70 ppbv. However, the model cannot reproduce PRB-relevant exceptional events associated with wildfires or stratospheric intrusions. The mean PRB estimates for spring–summer are 27 ± 8 ppbv at low-altitude sites and 40 ± 7 ppbv at high-altitude sites. Differences between the PRB simulation and the natural simulation indicate a mean enhancement from intercontinental pollution and anthropogenic methane of 9 ppbv at low-altitude sites and 13 ppbv at high-altitude sites. The PRB is higher than average when ozone exceeds 60 ppbv, particularly in the intermountain West. Our PRB estimates are on average 4 ppbv higher than previous GEOS-Chem studies and we attribute this to higher lighting, increasing Asian emissions, and improved model resolution. Whereas previous studies found no occurrences of PRB exceeding 60 ppbv, we find here some occurrences in the intermountain West. The annual 4th-highest PRB values in the intermountain West are typically 50–60 ppbv, as compared to 35–45 ppbv in the East or on the West Coast. Such high PRB values in the intermountain West suggest that special consideration of this region may be needed if the ozone NAAQS is decreased to a value in the 60–70 ppbv range.Earth and Planetary SciencesEngineering and Applied Science
Pre-M Phase-promoting Factor Associates with Annulate Lamellae in Xenopus Oocytes and Egg Extracts
We have used complementary biochemical and in vivo approaches to study the compartmentalization of M phase-promoting factor (MPF) in prophase Xenopus eggs and oocytes. We first examined the distribution of MPF (Cdc2/CyclinB2) and membranous organelles in high-speed extracts of Xenopus eggs made during mitotic prophase. These extracts were found to lack mitochondria, Golgi membranes, and most endoplasmic reticulum (ER) but to contain the bulk of the pre-MPF pool. This pre-MPF could be pelleted by further centrifugation along with components necessary to activate it. On activation, Cdc2/CyclinB2 moved into the soluble fraction. Electron microscopy and Western blot analysis showed that the pre-MPF pellet contained a specific ER subdomain comprising "annulate lamellae" (AL): stacked ER membranes highly enriched in nuclear pores. Colocalization of pre-MPF with AL was demonstrated by anti-CyclinB2 immunofluorescence in prophase oocytes, in which AL are positioned close to the vegetal surface. Green fluorescent protein-CyclinB2 expressed in oocytes also localized at AL. These data suggest that inactive MPF associates with nuclear envelope components just before activation. This association may explain why nuclei and centrosomes stimulate MPF activation and provide a mechanism for targeting of MPF to some of its key substrates
Origin and Evolution of Saturn's Ring System
The origin and long-term evolution of Saturn's rings is still an unsolved
problem in modern planetary science. In this chapter we review the current
state of our knowledge on this long-standing question for the main rings (A,
Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During
the Voyager era, models of evolutionary processes affecting the rings on long
time scales (erosion, viscous spreading, accretion, ballistic transport, etc.)
had suggested that Saturn's rings are not older than 100 My. In addition,
Saturn's large system of diffuse rings has been thought to be the result of
material loss from one or more of Saturn's satellites. In the Cassini era, high
spatial and spectral resolution data have allowed progress to be made on some
of these questions. Discoveries such as the ''propellers'' in the A ring, the
shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume
provide new constraints on evolutionary processes in Saturn's rings. At the
same time, advances in numerical simulations over the last 20 years have opened
the way to realistic models of the rings's fine scale structure, and progress
in our understanding of the formation of the Solar System provides a
better-defined historical context in which to understand ring formation. All
these elements have important implications for the origin and long-term
evolution of Saturn's rings. They strengthen the idea that Saturn's rings are
very dynamical and rapidly evolving, while new arguments suggest that the rings
could be older than previously believed, provided that they are regularly
renewed. Key evolutionary processes, timescales and possible scenarios for the
rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from
Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009)
537-57
Communication and mutual resource exchange in north Florida hermit crabs
The patterns of shell exchange in three species of hermit crabs which overlap in distribution and shell use were observed in the laboratory. Crabs showed no tendency to initiate more exchanges with conspecifics as compared with nonconspecific individuals and there were no specific size dominance effects. Lack of common communicatory patterns between Clibararius vittatus and Pagurus pollicaris was correlated with minimal actual exchange, while Pagurus impressus exchanged with both species and executed patterns in common with both. The pattern of shell exchanges and preferences indicated that, in some cases, both individuals may gain in interspecific exchanges.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46904/1/265_2004_Article_BF00569198.pd
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
Simian Foamy Virus Transmission from Apes to Humans, Rural Cameroon
Bites from apes efficiently transmit the foamy virus to humans in natural settings in central Africa
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
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