364 research outputs found
Mobile air monitoring data-processing strategies and effects on spatial air pollution trends
The collection of real-time air quality measurements while in motion (i.e.,
mobile monitoring) is currently conducted worldwide to evaluate in situ
emissions, local air quality trends, and air pollutant exposure. This
measurement strategy pushes the limits of traditional data analysis with
complex second-by-second multipollutant data varying as a function of time
and location. Data reduction and filtering techniques are often applied to
deduce trends, such as pollutant spatial gradients downwind of a highway.
However, rarely do mobile monitoring studies report the sensitivity of their
results to the chosen data-processing approaches. The study being reported
here utilized 40 h (> 140 000 observations) of mobile monitoring data
collected on a roadway network in central North Carolina to explore common
data-processing strategies including local emission plume detection,
background estimation, and averaging techniques for spatial trend analyses.
One-second time resolution measurements of ultrafine particles (UFPs), black
carbon (BC), particulate matter (PM), carbon monoxide (CO), and nitrogen
dioxide (NO<sub>2</sub>) were collected on 12 unique driving routes that were
each sampled repeatedly. The route with the highest number of repetitions was
used to compare local exhaust plume detection and averaging methods. Analyses
demonstrate that the multiple local exhaust plume detection strategies
reported produce generally similar results and that utilizing a median of
measurements taken within a specified route segment (as opposed to a mean)
may be sufficient to avoid bias in near-source spatial trends. A time-series-based method of estimating background concentrations was shown to produce
similar but slightly lower estimates than a location-based method. For the
complete data set the estimated contributions of the background to the mean
pollutant concentrations were as follows: BC (15%), UFPs (26%), CO (41%),
PM<sub>2.5-10</sub> (45%), NO<sub>2</sub> (57%), PM<sub>10</sub> (60%), PM<sub>2.5</sub>
(68%). Lastly, while temporal smoothing (e.g., 5 s averages) results
in weak pair-wise correlation and the blurring of spatial trends, spatial
averaging (e.g., 10 m) is demonstrated to increase correlation and refine
spatial trends
Evaluating local anthropogenic impact on remote Arctic monitoring stations: a case study at Summit, Greenland
International audienceSummit, Greenland is a remote Arctic research station allowing for field measurements at the highest point of the Greenland Ice Sheet. Due to the current reliance on diesel generators for electricity at Summit, unavoidable local emissions are a potential contamination threat to the measurement of combustion-related species in the air and snow. The effect of fossil-fuel combustion on particulate elemental carbon (EC) is assessed by a combination of ambient measurements (~1 km from the main camp), a series of snow pits (up to 20 km from Summit Camp), and Gaussian plume modeling. Ambient measurements indicate that the air directly downwind of the research station generators experiences particulate absorption coefficient (closely related to EC) values that are up to a factor of 200 higher than the summer 2006 non-camp-impacted ambient average. Local anthropogenic influence on snow EC content is also evident. The average EC concentration in 1-m snow pits in the "clean air" sector of Summit Camp are a factor of 1.8?2.4 higher than in snow pits located 10 km and 20 km to the north ("downwind") and south ("upwind") of the research site. Gaussian plume modeling performed using meteorological data from years 2003?2006 suggests a strong angular dependence of anthropogenic impact, with highest risk to the northwest of Summit Camp and lowest to the southeast. Along a transect to the southeast (5 degree angle bin), the modeled frequency of significant camp contribution to atmospheric EC (i.e. camp-produced EC>2006 summer average EC) at a distance of 0.5 km, 10 km, and 20 km is 1%, 0.2%, and 0.05%, respectively. According to both the snow pit and model results, a distance exceeding 10 km towards the southeast is expected to minimize risk of contamination. These results also suggest that other remote Arctic monitoring stations powered by local fuel combustion may need to account for local air and snow contamination in field sampling design and data interpretation
Local anthropogenic impact on particulate elemental carbon concentrations at Summit, Greenland
Summit, Greenland is a remote Arctic research station allowing for field measurements at the highest point of the Greenland Ice Sheet. Due to the current reliance on diesel generators for electricity at Summit, unavoidable local emissions are a potential contamination threat to the measurement of combustion-related species in the air and snow. The effect of fossil-fuel combustion on particulate elemental carbon (EC) is assessed by a combination of ambient measurements (~1 km from the main camp), a series of snow pits, and Gaussian plume modeling. Ambient measurements indicate that the air directly downwind of the research station generators experiences particulate absorption coefficient (closely related to EC) values that are up to a factor of 200 higher than the summer 2006 non-camp-impacted ambient average. Local anthropogenic influence on snow EC content is also evident. The average EC concentration in 1-m snow pits in the "clean air" sector of Summit Camp are a factor of 1.8&ndash;2.4 higher than in snow pits located 10 km and 20 km to the north ("downwind") and south ("upwind") of the research site. Gaussian plume modeling performed using meteorological data from years 2003&ndash;2006 suggests a strong angular dependence of anthropogenic impact, with highest risk to the northwest of Summit Camp and lowest to the southeast. Along a transect to the southeast (5 degree angle bin), the modeled frequency of significant camp contribution to atmospheric EC (i.e. camp-produced EC&gt;summer 2006 average EC) at a distance of 0.5 km, 10 km, and 20 km is 1%, 0.2%, and 0.05%, respectively. According to both the snow pit and model results, a distance exceeding 10 km towards the southeast is expected to minimize risk of contamination. These results also suggest that other remote Arctic monitoring stations powered by local fuel combustion may need to account for local air and snow contamination in field sampling design and data interpretation
Nucleon structure with two flavors of dynamical domain-wall fermions
We present a numerical lattice quantum chromodynamics calculation of
isovector form factors and the first few moments of the isovector structure
functions of the nucleon. The calculation employs two degenerate dynamical
flavors of domain-wall fermions, resulting in good control of chiral symmetry
breaking. Non-perturbative renormalization of the relevant quark currents is
performed where necessary. The inverse lattice spacing, , is about 1.7
GeV. We use degenerate up and down dynamical quark masses around 1, 3/4 and 1/2
the strange quark mass. The physical volume of the lattice is about
. The ratio of the isovector vector to axial charges, ,
trends a bit lower than the experimental value as the quark mass is reduced
toward the physical point. We calculate the momentum-transfer dependences of
the isovector vector, axial, induced tensor and induced pseudoscalar form
factors. The Goldberger-Treiman relation holds at low momentum transfer and
yields a pion-nucleon coupling, , where the quoted
error is only statistical. We find that the flavor non-singlet quark momentum
fraction and quark helicity fraction
overshoot their experimental values after linear chiral extrapolation. We
obtain the transversity, in
at 2 GeV and a twist-3 polarized moment, , appears small, suggesting that
the Wandzura-Wilczek relation holds approximately. We discuss the systematic
errors in the calculation, with particular attention paid to finite-volume
effects, excited-state contamination, and chiral extrapolations.Comment: 28 pages in two columns; 37 figures, 12 table
Long-lived photoexcited states in polydiacetylenes with different molecular and supramolecular organization
With the aim of determining the importance of the molecular and supramolecular organization on the excited states of polydiacetylenes, we have studied the photoinduced absorption spectra of the red form of poly[1,6-bis(3,6-didodecyl-N-carbazolyl)-2,4-hexadiyne] (polyDCHD-S) and the results compared with those of the blue form of the same polymer. An interpretation of the data is given in terms of both the conjugation length and the interbackbone separation also in relation to the photoinduced absorption spectra of both blue and red forms of poly[1,6-bis(N-carbazolyl)-2,4-hexadiyne] (polyDCHD), which does not carry the alkyl substituents on the carbazolyl side groups. Information on the above properties is derived from the analysis of the absorption and Raman spectra of this class of polydiacetylenes
Ab-initio calculation of the electronic and optical excitations in polythiophene: effects of intra- and interchain screening
We present an calculation of the electronic and optical excitations of an
isolated polythiophene chain as well as of bulk polythiophene. We use the GW
approximation for the electronic self-energy and include excitonic effects by
solving the electron-hole Bethe-Salpeter equation. The inclusion of interchain
screening in the case of bulk polythiophene drastically reduces both the
quasi-particle band gap and the exciton binding energies, but the optical gap
is hardly affected. This finding is relevant for conjugated polymers in
general.Comment: 4 pages, 1 figur
Ab-initio prediction of the electronic and optical excitations in polythiophene: isolated chains versus bulk polymer
We calculate the electronic and optical excitations of polythiophene using
the GW approximation for the electronic self-energy, and include excitonic
effects by solving the electron-hole Bethe-Salpeter equation. Two different
situations are studied: excitations on isolated chains and excitations on
chains in crystalline polythiophene. The dielectric tensor for the crystalline
situation is obtained by modeling the polymer chains as polarizable line
objects, with a long-wavelength polarizability tensor obtained from the
ab-initio polarizability function of the isolated chain. With this model
dielectric tensor we construct a screened interaction for the crystalline case,
including both intra- and interchain screening. In the crystalline situation
both the quasi-particle band gap and the exciton binding energies are
drastically reduced in comparison with the isolated chain. However, the optical
gap is hardly affected. We expect this result to be relevant for conjugated
polymers in general.Comment: 15 pages including 4 figures; to appear in Phys. Rev. B, 6/15/200
Long-term evaluation of air sensor technology under ambient conditions in Denver, Colorado
Air pollution sensors are quickly proliferating for use in a wide variety of
applications, with a low price point that supports use in high-density
networks, citizen science, and individual consumer use. This emerging
technology motivates the assessment under real-world conditions, including
varying pollution levels and environmental conditions. A seven-month,
systematic field evaluation of low-cost air pollution sensors was performed
in Denver, Colorado, over 2015–2016; the location was chosen to evaluate the
sensors in a high-altitude, cool, and dry climate. A suite of particulate
matter (PM), ozone (O3), and nitrogen dioxide (NO2) sensors were
deployed in triplicate and were collocated with federal equivalent method
(FEM) monitors at an urban regulatory site. Sensors were evaluated for their
data completeness, correlation with reference monitors, and ability to
reproduce trends in pollution data, such as daily concentration values and
wind-direction patterns. Most sensors showed high data completeness when data
loggers were functioning properly. The sensors displayed a range of
correlations with reference instruments, from poor to very high (e.g.,
hourly-average PM Pearson correlations with reference measurements varied
from 0.01 to 0.86). Some sensors showed a change in response to laboratory
audits/testing from before the sampling campaign to afterwards, such as
Aeroqual, where the O3 response slope changed from about 1.2 to 0.6.
Some PM sensors measured wind-direction and time-of-day trends similar to
those measured by reference monitors, while others did not. This study showed
different results for sensor performance than previous studies performed by
the U.S. EPA and others, which could be due to different geographic location,
meteorology, and aerosol properties. These results imply that continued field
testing is necessary to understand emerging air sensing technology.</p
Quark Soup al dente: Applied Superstring Theory
We discuss the application of the AdS/CFT correspondence to possibly gain new
physical insights for the strongly coupled quark-gluon plasma. This article
provides an informal summary of a talk given by RCM at the 18th International
Conference on General Relativity and Gravitation in July 2007.Comment: This article provides an informal summary of a talk given by RCM at
the 18th International Conference on General Relativity and Gravitation in
July 200
The size of electron-hole pairs in pi conjugated systems
We have performed momentum dependent electron energy-loss studies of the
electronic excitations in sexithiophene and compared the results to those from
parent oligomers. Our experiment probes the dynamic structure factor
S(q,omega)and we show that the momentum dependent intensity variation of the
excitations observed can be used to extract the size of the electron-hole pair
created in the excitation process. The extension of the electron-hole pairs
along the molecules is comparable to the length of the molecules and thus maybe
only limited by structural constraints. Consequently, the primary
intramolecular electron-hole pairs are relatively weakly bound. We find no
evidence for the formation of excitations localized on single thiophene units.Comment: RevTex, 3 figures, to appear in Physical Review Letter
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