6,695 research outputs found
Monoaromatic compounds in ambient air of various cities: A focus on correlations between the xylenes and ethylbenzene
Speciation of o-xylene, m-xylene, p-xylene and ethylbenzene was performed by gas chromatography from ambient air and liquid fuel samples collected at various locations in 19 cities in Europe, Asia and South America. The xylene's mixing ratios were compared to each other from the various locations, which included urban air, traffic air and liquid fuel. For all samples, the xylenes exhibited robust correlations, and the slopes remained constant. The m-xylene/p-xylene ratio was found to be 2.33±0.30, and the m-xylene/o-xylene ratio was found to be 1.84±0.25. These ratios remain persistent even in biomass combustion experiments (in South America and South Africa). Comparing the xylenes to toluene and benzene indicate that combustion, but not fuel evaporation, is the major common source of the xylenes in areas dominated by automotive emissions. Although a wide range of combustion types and combustion efficiencies were encountered throughout all the locations investigated, xylenes and ethylbenzene ratios remained persistent. We discuss the implications of the constancies in the xylenes and ethylbenzene ratios on atmospheric chemistry
Decay of the Z Boson into Scalar Particles
In extensions of the standard model, light scalar particles are often
possible because of symmetry considerations. We study the decay of the Z boson
into such particles. In particular, we consider for illustration the scalar
sector of a recently proposed model of the 17-keV neutrino which satisfies all
laboratory, astrophysical, and cosmological constraints.Comment: 11 pages (2 figures, not included) (Revised, Oct 1992). Some
equations have been corrected and 1 figure has been eliminate
HI Narrow Line Absorption in Dark Clouds
We have used the Arecibo telescope to carry out an survey of 31 dark clouds
in the Taurus/Perseus region for narrow absorption features in HI (
21cm) and OH (1667 and 1665 MHz) emission. We detected HI narrow
self--absorption (HINSA) in 77% of the clouds that we observed. HINSA and OH
emission, observed simultaneously are remarkably well correlated. Spectrally,
they have the same nonthermal line width and the same line centroid velocity.
Spatially, they both peak at the optically--selected central position of each
cloud, and both fall off toward the cloud edges. Sources with clear HINSA
feature have also been observed in transitions of CO, \13co, \c18o, and CI.
HINSA exhibits better correlation with molecular tracers than with CI.
The line width of the absorption feature, together with analyses of the
relevant radiative transfer provide upper limits to the kinetic temperature of
the gas producing the HINSA. Some sources must have a temperature close to or
lower than 10 K. The correlation of column densities and line widths of HINSA
with those characteristics of molecular tracers suggest that a significant
fraction of the atomic hydrogen is located in the cold, well--shielded portions
of molecular clouds, and is mixed with the molecular gas.
The average number density ratio [HI]/[\h2] is . The
inferred HI density appears consistent with but is slightly higher than the
value expected in steady state equilibrium between formation of HI via cosmic
ray destruction of H and destruction via formation of H on grain
surfaces. The distribution and abundance of atomic hydrogen in molecular clouds
is a critical test of dark cloud chemistry and structure, including the issues
of grain surface reaction rates, PDRs, circulation, and turbulent diffusion.Comment: 40 pages, 10 figures, accepted by Ap
Functional Allocation with Airborne Self-Separation Evaluated in a Piloted Simulation
A human-in-the-loop simulation experiment was designed and conducted to evaluate an airborne self-separation concept. The activity supports the National Aeronautics and Space Administration s (NASA) research focus on function allocation for separation assurance. The objectives of the experiment were twofold: (1) use experiment design features in common with a companion study of ground-based automated separation assurance to promote comparability, and (2) assess agility of self-separation operations in managing trajectory-changing events in high traffic density, en-route operations with arrival time constraints. This paper describes the experiment and presents initial results associated with subjective workload ratings and group discussion feedback obtained from the experiment s commercial transport pilot participants
Manifestation of classical wave delays in a fully quantized model of the scattering of a single photon
We consider a fully quantized model of spontaneous emission, scattering, and
absorption, and study propagation of a single photon from an emitting atom to a
detector atom both with and without an intervening scatterer. We find an exact
quantum analog to the classical complex analytic signal of an electromagnetic
wave scattered by a medium of charged oscillators. This quantum signal exhibits
classical phase delays. We define a time of detection which, in the appropriate
limits, exactly matches the predictions of a classically defined delay for
light propagating through a medium of charged oscillators. The fully quantized
model provides a simple, unambiguous, and causal interpretation of delays that
seemingly imply speeds greater than c in the region of anomalous dispersion.Comment: 18 pages, 4 figures, revised for clarity, typos corrrecte
A homeostatic function of CXCR2 signalling in articular cartilage
Funding This work was funded by Arthritis Research UK (grants 17859, 17971, 19654), INNOCHEM EU FP6 (grant LSHB-CT-2005-51867), MRC (MR/K013076/1) and the William Harvey Research FoundationPeer reviewedPublisher PD
How closely do changes in surface and column water vapor follow Clausius-Clapeyron scaling in climate change simulations?
The factors governing the rate of change in the amount of atmospheric water vapor are analyzed in simulations of climate change. The global-mean amount of water vapor is estimated to increase at a differential rate of 7.3% K[superscript â 1] with respect to global-mean surface air temperature in the multi-model mean. Larger rates of change result if the fractional change is evaluated over a finite change in temperature (e.g., 8.2% K [superscript â 1] for a 3 K warming), and rates of change of zonal-mean column water vapor range from 6 to 12% K [superscript â 1] depending on latitude.
ClausiusâClapeyron scaling is directly evaluated using an invariant distribution of monthly-mean relative humidity, giving a rate of 7.4% K â 1 for global-mean water vapor. There are deviations from ClausiusâClapeyron scaling of zonal-mean column water vapor in the tropics and mid-latitudes, but they largely cancel in the global mean. A purely thermodynamic scaling based on a saturated troposphere gives a higher global rate of 7.9% K [superscript â 1].
Surface specific humidity increases at a rate of 5.7% K [superscript â 1], considerably lower than the rate for global-mean water vapor. Surface specific humidity closely follows ClausiusâClapeyron scaling over ocean. But there are widespread decreases in surface relative humidity over land (by more than 1% K â 1 in many regions), and it is argued that decreases of this magnitude could result from the land/ocean contrast in surface warming
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