245 research outputs found
Atmospheric correction for MASTER image data using localized modelled and observed meteorology and trace gases
Atmospheric correction for remote sensing-based studies typically does not use information from spatio-temporally resolved meteorological models. We assessed the effect of using observations and mesoscale weather and chemical transport models on multispectral retrievals of land and ocean properties. We performed two atmospheric corrections on image data acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS)/Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) airborne simulator over Monterey Bay, California. One correction used local atmospheric profiles of meteorology and trace gases at overpass and the other used the 1976 US Standard default atmospheric profile in the MODTRAN4 radiative transfer model.We found only minor impacts from atmospheric correction in the Fluorescence Line Height index of ocean chlorophyll, but substantive differences in retrievals of surface temperature and the Normalized Difference Vegetation Index. Improvements in sea surface temperature retrieval were validated by in situ measurements. Results indicate that spatio-temporally specific atmospheric correction factors from mesoscale models can improve retrievals of surface properties from remotely sensed image data
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Secondary organic aerosol formation from in-use motor vehicle emissions using a potential aerosol mass reactor.
Secondary organic aerosol (SOA) formation from in-use vehicle emissions was investigated using a potential aerosol mass (PAM) flow reactor deployed in a highway tunnel in Pittsburgh, Pennsylvania. Experiments consisted of passing exhaust-dominated tunnel air through a PAM reactor over integrated hydroxyl radical (OH) exposures ranging from ∼ 0.3 to 9.3 days of equivalent atmospheric oxidation. Experiments were performed during heavy traffic periods when the fleet was at least 80% light-duty gasoline vehicles on a fuel-consumption basis. The peak SOA production occurred after 2-3 days of equivalent atmospheric oxidation. Additional OH exposure decreased the SOA production presumably due to a shift from functionalization to fragmentation dominated reaction mechanisms. Photo-oxidation also produced substantial ammonium nitrate, often exceeding the mass of SOA. Analysis with an SOA model highlight that unspeciated organics (i.e., unresolved complex mixture) are a very important class of precursors and that multigenerational processing of both gases and particles is important at longer time scales. The chemical evolution of the organic aerosol inside the PAM reactor appears to be similar to that observed in the atmosphere. The mass spectrum of the unoxidized primary organic aerosol closely resembles ambient hydrocarbon-like organic aerosol (HOA). After aging the exhaust equivalent to a few hours of atmospheric oxidation, the organic aerosol most closely resembles semivolatile oxygenated organic aerosol (SV-OOA) and then low-volatility organic aerosol (LV-OOA) at higher OH exposures. Scaling the data suggests that mobile sources contribute ∼ 2.9 ± 1.6 Tg SOA yr(-1) in the United States, which is a factor of 6 greater than all mobile source particulate matter emissions reported by the National Emissions Inventory. This highlights the important contribution of SOA formation from vehicle exhaust to ambient particulate matter concentrations in urban areas
A dual-chamber method for quantifying the effects of atmospheric perturbations on secondary organic aerosol formation from biomass burning emissions
Biomass burning (BB) is a major source of atmospheric pollutants. Field and laboratory studies indicate that secondary organic aerosol (SOA) formation from BB emissions is highly variable. We investigated sources of this variability using a novel dual-smog-chamber method that directly compares the SOA formation from the same BB emissions under two different atmospheric conditions. During each experiment, we filled two identical Teflon smog chambers simultaneously with BB emissions from the same fire. We then perturbed the smoke with UV lights, UV lights plus nitrous acid (HONO), or dark ozone in one or both chambers. These perturbations caused SOA formation in nearly every experiment with an average organic aerosol (OA) mass enhancement ratio of 1.78 ± 0.91 (mean ± 1σ). However, the effects of the perturbations were highly variable ranging with OA mass enhancement ratios ranging from 0.7 (30% loss of OA mass) to 4.4 across the set of perturbation experiments. There was no apparent relationship between OA enhancement and perturbation type, fuel type, and modified combustion efficiency. To better isolate the effects of different perturbations, we report dual-chamber enhancement (DUCE), which is the quantity of the effects of a perturbation relative to a reference condition. DUCE values were also highly variable, even for the same perturbation and fuel type. Gas measurements indicate substantial burn-to-burn variability in the magnitude and composition of SOA precursor emissions, even in repeated burns of the same fuel under nominally identical conditions. Therefore, the effects of different atmospheric perturbations on SOA formation from BB emissions appear to be less important than burn-to-burn variability
Effect of promoter architecture on the cell-to-cell variability in gene expression
According to recent experimental evidence, the architecture of a promoter,
defined as the number, strength and regulatory role of the operators that
control the promoter, plays a major role in determining the level of
cell-to-cell variability in gene expression. These quantitative experiments
call for a corresponding modeling effort that addresses the question of how
changes in promoter architecture affect noise in gene expression in a
systematic rather than case-by-case fashion. In this article, we make such a
systematic investigation, based on a simple microscopic model of gene
regulation that incorporates stochastic effects. In particular, we show how
operator strength and operator multiplicity affect this variability. We examine
different modes of transcription factor binding to complex promoters
(cooperative, independent, simultaneous) and how each of these affects the
level of variability in transcription product from cell-to-cell. We propose
that direct comparison between in vivo single-cell experiments and theoretical
predictions for the moments of the probability distribution of mRNA number per
cell can discriminate between different kinetic models of gene regulation.Comment: 35 pages, 6 figures, Submitte
Stable, Precise, and Reproducible Patterning of Bicoid and Hunchback Molecules in the Early Drosophila Embryo
Precise patterning of morphogen molecules and their accurate reading out are of key importance in embryonic development. Recent experiments have visualized distributions of proteins in developing embryos and shown that the gradient of concentration of Bicoid morphogen in Drosophila embryos is established rapidly after fertilization and remains stable through syncytial mitoses. This stable Bicoid gradient is read out in a precise way to distribute Hunchback with small fluctuations in each embryo and in a reproducible way, with small embryo-to-embryo fluctuation. The mechanisms of such stable, precise, and reproducible patterning through noisy cellular processes, however, still remain mysterious. To address these issues, here we develop the one- and three-dimensional stochastic models of the early Drosophila embryo. The simulated results show that the fluctuation in expression of the hunchback gene is dominated by the random arrival of Bicoid at the hunchback enhancer. Slow diffusion of Hunchback protein, however, averages out this intense fluctuation, leading to the precise patterning of distribution of Hunchback without loss of sharpness of the boundary of its distribution. The coordinated rates of diffusion and transport of input Bicoid and output Hunchback play decisive roles in suppressing fluctuations arising from the dynamical structure change in embryos and those arising from the random diffusion of molecules, and give rise to the stable, precise, and reproducible patterning of Bicoid and Hunchback distributions
Secondary Organic Aerosol Formation from Intermediate-Volatility Organic Compounds: Cyclic, Linear, and Branched Alkanes
Intermediate volatility organic compounds (IVOCs) are
an important
class of secondary organic aerosol (SOA) precursors that have not
been traditionally included in chemical transport models. A challenge
is that the vast majority of IVOCs cannot be speciated using traditional
gas chromatography-based techniques; instead they are classified as
an unresolved complex mixture (UCM) that is presumably made up of
a complex mixture of branched and cyclic alkanes. To better understand
SOA formation from IVOCs, a series of smog chamber experiments was
conducted with different alkanes, including cyclic, branched, and
linear compounds. The experiments focused on freshly formed SOA from
hydroxyl (OH) radical-initiated reactions under high-NO<sub><i>x</i></sub> conditions at typical atmospheric organic aerosol
concentrations (<i>C</i><sub>OA</sub>). SOA yields from
cyclic alkanes were comparable to yields from linear alkanes three
to four carbons larger in size. For alkanes with equivalent carbon
numbers, branched alkanes had the lowest SOA mass yields, ranging
between 0.05 and 0.08 at a <i>C</i><sub>OA</sub> of 15 μg
m<sup>–3</sup>. The SOA yield of branched alkanes also depends
on the methyl branch position on the carbon backbone. High-resolution
aerosol mass spectrometer data indicate that the SOA oxygen-to-carbon
ratios were largely controlled by the carbon number of the precursor
compound. Depending on the precursor size, the mass spectrum of SOA
produced from IVOCs is similar to the semivolatile-oxygenated and
hydrocarbon-like organic aerosol factors derived from ambient data.
Using the new yield data, we estimated SOA formation potential from
diesel exhaust and predict the contribution from UCM vapors to be
nearly four times larger than the contribution from single-ring aromatics
and comparable to that of polycyclic aromatic hydrocarbons after several
hours of oxidation at typical atmospheric conditions. Therefore, SOA
from IVOCs may be an important contributor to urban OA and should
be included in SOA models; the yield data presented in this study
are suitable for such use
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A dual-chamber method for quantifying the effects of atmospheric perturbations on secondary organic aerosol formation from biomass burning emissions
Gas-Particle Partitioning of Vehicle Emitted Primary Organic Aerosol Measured in a Traffic Tunnel
We
measured the gas-particle partitioning of vehicle emitted primary
organic aerosol (POA) in a traffic tunnel with three independent methods:
artifact corrected bare-quartz filters, thermodenuder (TD) measurements,
and thermal-desorption gas-chromatography mass-spectrometry (TD-GC-MS).
Results from all methods consistently show that vehicle emitted POA
measured in the traffic tunnel is semivolatile under a wide range
of fleet compositions and ambient conditions. We compared the gas-particle
partitioning of POA measured in both tunnel and dynamometer studies
and found that volatility distributions measured in the traffic tunnel
are similar to volatility distributions measured in the dynamometer
studies, and predict similar gas-particle partitioning in the TD.
These results suggest that the POA volatility distribution measured
in the dynamometer studies can be applied to describe gas-particle
partitioning of ambient POA emissions. The POA volatility distribution
measured in the tunnel does not have significant diurnal or seasonal
variations, which indicate that a single volatility distribution is
adequate to describe the gas-particle partitioning of vehicle emitted
POA in the urban environment
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