1,906 research outputs found
Modeling nitrous acid and its impact on ozone and hydroxyl radical during the Texas Air Quality Study 2006
Nitrous acid (HONO) mixing ratios for the Houston metropolitan area were simulated with the Community Multiscale Air Quality (CMAQ) Model for an episode during the Texas Air Quality Study (TexAQS) II in August/September 2006 and compared to in-situ MC/IC (mist-chamber/ion chromatograph) and long path DOAS (Differential Optical Absorption Spectroscopy) measurements at three different altitude ranges. Several HONO sources were accounted for in simulations, such as gas phase formation, direct emissions, nitrogen dioxide (NO<sub>2</sub>) hydrolysis, photo-induced formation from excited NO<sub>2</sub> and photo-induced conversion of NO<sub>2</sub> into HONO on surfaces covered with organic materials. Compared to the gas-phase HONO formation there was about a tenfold increase in HONO mixing ratios when additional HONO sources were taken into account, which improved the correlation between modeled and measured values. Concentrations of HONO simulated with only gas phase chemistry did not change with altitude, while measured HONO concentrations decrease with height. A trend of decreasing HONO concentration with altitude was well captured with CMAQ predicted concentrations when heterogeneous chemistry and photolytic sources of HONO were taken into account. Heterogeneous HONO production mainly accelerated morning ozone formation, albeit slightly. Also HONO formation from excited NO<sub>2</sub> only slightly affected HONO and ozone (O<sub>3</sub>) concentrations. Photo-induced conversion of NO<sub>2</sub> into HONO on surfaces covered with organic materials turned out to be a strong source of daytime HONO. Since HONO immediately photo-dissociates during daytime its ambient mixing ratios were only marginally altered (up to 0.5 ppbv), but significant increase in the hydroxyl radical (OH) and ozone concentration was obtained. In contrast to heterogeneous HONO formation that mainly accelerated morning ozone formation, inclusion of photo-induced surface chemistry influenced ozone throughout the day
The TAOS Project Stellar Variability I. Detection of Low-Amplitude delta Scuti Stars
We analyzed data accumulated during 2005 and 2006 by the Taiwan-American
Occultation Survey (TAOS) in order to detect short-period variable stars
(periods of <~ 1 hour) such as delta Scuti. TAOS is designed for the detection
of stellar occultation by small-size Kuiper Belt Objects (KBOs) and is
operating four 50cm telescopes at an effective cadence of 5Hz. The four
telescopes simultaneously monitor the same patch of the sky in order to reduce
false positives. To detect short-period variables, we used the Fast Fourier
Transform algorithm (FFT) inasmuch as the data points in TAOS light-curves are
evenly spaced. Using FFT, we found 41 short-period variables with amplitudes
smaller than a few hundredths of a magnitude and periods of about an hour,
which suggest that they are low-amplitude delta Scuti stars (LADS). The
light-curves of TAOS delta Scuti stars are accessible online at the Time Series
Center website (http://timemachine.iic.harvard.edu)Comment: Accepted for publication in A
On the new concept of the available water climatology and its application
We propose a new concept of climatology called the Available Water Climate (AWC). Available water is 'the remained water usable in every moment' that is calculated regardless of any time intervals or the amounts of precipitation. With this concept, the Available Water Resources Index (AWRI) has been digitized following the earlier work of Byun and Lee (2002). The applicability of AWRI not only to the assessment and prediction of water related disasters but also to the academic researches has been tested. Resulted merits are as follows. Firstly, the threshold value of AWRI for the occurrence of all water related disasters like flood, drought, inundation landslide, and drought each region became clear, therefore the assessment and the prediction of them became much more precise than before. It became clear that the more extreme the AWRI value is, the severer the related disasters become. As example, all disasters caused by heavy rains, even though a small inundation, became predictable at the time step of heavy rain warning with the help of the Long-term remained water index(LWI). As another example, the drought intensity and its dates on start and end are defined with more reasonably and precisely than any other drought indexes with help of the Effective drought index (EDI) using sliding time scale. Secondly, the spatiotemporal distribution of water environment were digitized clearly and objectively using AWRI and new concepts of the Water Abundant Season (WAS) and the Little Water Season (LIWAS), their dates on start and end, and their strength were defined, which is very beneficial for agriculture, forestry, and all other water controls. Also, the differences of water environments among regions were clearly digitized and the improvement of the climate classification by Köppen etc. became possible. Thirdly, other merits will be found continuously afterwards
Mechanisms of grain refinement by intensive shearing of AZ91 alloy melt
The official published version of the article can be accessed at the link below.It has been demonstrated recently that intensive melt shearing can be an effective approach to the grain refinement of both shape casting and continuous casting of Mg alloys. In the present study, the mechanisms of grain refinement by intensive melt shearing were investigated through a combination of both modelling and experimental approaches. The measurement of the cooling curves during solidification, quantification of grain size of the solidified samples, and image analysis of the MgO particle size and size distribution in the pressurized filtration samples were performed for the AZ91 alloy with and without intensive melt shearing. The experimental results were then used as input parameters for the free growth model to investigate the mechanisms of grain refinement by intensive melt shearing. The experimental results showed that, although intensive melt shearing does not change the nucleation starting temperature, it increases the nucleation finishing temperature, giving rise to a reduced nucleation undercooling. The theoretical modelling using the free growth model revealed quantitatively that intensive melt shearing can effectively disperse MgO particles densely populated in the oxide films into more individual particles in the alloy melt, resulting in an increase in the MgO particle density by three orders of magnitude and the density of active nucleating MgO particles by a factor of 20 compared with those of the non-sheared melt. Therefore, the grain refining effect of intensive melt shearing can be confidently attributed to the significantly increased refining efficiency of the naturally occurring MgO particles in the alloy melt as potent nucleation sites.Financial support under Grant EP/H026177/1 from the EPSRC
The Taiwanese-American Occultation Survey: The Multi-Telescope Robotic Observatory
The Taiwanese-American Occultation Survey (TAOS) operates four fully
automatic telescopes to search for occultations of stars by Kuiper Belt
Objects. It is a versatile facility that is also useful for the study of
initial optical GRB afterglows. This paper provides a detailed description of
the TAOS multi-telescope system, control software, and high-speed imaging.Comment: 11 pages, 11 figure
The TAOS Project: Statistical Analysis of Multi-Telescope Time Series Data
The Taiwanese-American Occultation Survey (TAOS) monitors fields of up to
~1000 stars at 5 Hz simultaneously with four small telescopes to detect
occultation events from small (~1 km) Kuiper Belt Objects (KBOs). The survey
presents a number of challenges, in particular the fact that the occultation
events we are searching for are extremely rare and are typically manifested as
slight flux drops for only one or two consecutive time series measurements. We
have developed a statistical analysis technique to search the multi-telescope
data set for simultaneous flux drops which provides a robust false positive
rejection and calculation of event significance. In this paper, we describe in
detail this statistical technique and its application to the TAOS data set.Comment: 15 pages, 14 figures. Submitted to PAS
The Mechanical Behavior of a 25Cr Super Duplex Stainless Steel at Elevated Temperature
Peer reviewedPostprin
Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8
The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained
simultaneous near and far ultraviolet light curves of the ROTSE-I Catalog RR
Lyrae ab-type variable star J143753.84+345924.8. A series of 38 GALEX Deep
Imaging Survey observations well distributed in phase within the star's
0.56432d period shows an AB=4.9mag variation in the far UV (1350-1750A) band
and an AB=1.8mag variation in the near UV (1750-2750A) band, compared with only
a 0.8mag variation in the broad, unfiltered ROTSE-I (4500-10000A) band. These
GALEX UV observations are the first to reveal a large RR Lyrae amplitude
variation at wavelengths below 1800A. We compare the GALEX and ROTSE-I
observations to predictions made by recent Kurucz stellar atmosphere models. We
use published physical parameters for the comparable period (0.57433d),
well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and
ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree
with the Kurucz predictions for [Fe/H]=-1.25 to within AB=0.2mag in the GALEX
NUV and ROTSE-I bands, and within 0.5mag in the FUV. At all metallicities
between solar and one hundredth solar, the Kurucz models predict 6-8mag of
variation at wavelengths between 1000-1700A. Other variable stars with similar
temperature variations, such as Cepheids, should also have large-amplitude FUV
light curves, observable during the ongoing GALEX imaging surveys.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http:/www.galex.caltech.edu/PUBLICATIONS after
November 22, 200
The Taiwanese-American Occultation Survey Project Stellar Variability. II. Detection of 15 Variable Stars
The Taiwanese-American Occultation Survey (TAOS) project has collected more than a billion photometric measurements since 2005 January. These sky survey data—covering timescales from a fraction of a second to a few hundred days—are a useful source to study stellar variability. A total of 167 star fields, mostly along the ecliptic plane, have been selected for photometric monitoring with the TAOS telescopes. This paper presents our initial analysis of a search for periodic variable stars from the time-series TAOS data on one particular TAOS field, No. 151 (R.A. = 17^(h)30^(m)6^(s).7, decl. = 27°17'30", J2000), which had been observed over 47 epochs in 2005. A total of 81 candidate variables are identified in the 3 deg^2 field, with magnitudes in the range 8 < R < 16. On the basis of the periodicity and shape of the light curves, 29 variables, 15 of which were previously unknown, are classified as RR Lyrae, Cepheid, δ Scuti, SX Phonencis, semi-regular, and eclipsing binaries
A single-column ocean biogeochemistry model (GOTM–TOPAZ) version 1.0
Recently, Earth system models (ESMs) have begun to
consider the marine ecosystem to reduce errors in climate simulations.
However, many models are unable to fully represent the ocean-biology-induced
climate feedback, which is due in part to significant bias in the simulated
biogeochemical properties. Therefore, we developed the Generic Ocean
Turbulence Model–Tracers of Phytoplankton with Allometric Zooplankton
(GOTM–TOPAZ), a single-column ocean biogeochemistry model that can be used
to improve ocean biogeochemical processes in ESMs. This model was developed
by combining GOTM, a single-column model that can simulate the physical
environment of the ocean, and TOPAZ, a biogeochemical module. Here, the
original form of TOPAZ has been modified and modularized to allow easy
coupling with other physical ocean models. To demonstrate interactions
between ocean physics and biogeochemical processes, the model was designed
to allow ocean temperature to change due to absorption of visible light by
chlorophyll in phytoplankton. We also added a module to reproduce upwelling
and the air–sea gas transfer process for oxygen and carbon dioxide,
which are of particular importance for marine ecosystems. The simulated
variables (e.g., chlorophyll, oxygen, nitrogen, phosphorus, silicon) of
GOTM–TOPAZ were evaluated by comparison against observations. The temporal
variability in the observed upper-ocean (0–20 m) chlorophyll is well
captured by the GOTM–TOPAZ with a correlation coefficient of 0.53 at point 107 in the Sea of Japan. The
surface correlation coefficients among GOTM–TOPAZ oxygen, nitrogen,
phosphorus, and silicon are 0.47, 0.31, 0.16, and 0.19, respectively. We
compared the GOTM–TOPAZ simulations with those from MOM–TOPAZ and found that
GOTM–TOPAZ showed relatively lower correlations, which is most likely due to
the limitations of the single-column model.
Results also indicate that source–sink terms may contribute to the biases in
the surface layer (<60 m), while initial values are important for
realistic simulations in the deep sea (>250 m). Despite this
limitation, we argue that our GOTM–TOPAZ model is a good starting point for
further investigation of key biogeochemical processes and is also useful to
couple complex biogeochemical processes with various oceanic global
circulation models.</p
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