463 research outputs found
Ion size effects at ionic exclusion from dielectric interfaces and slit nanopores
A previously developed field-theoretic model [R.D. Coalson et al., J. Chem.
Phys. 102, 4584 (1995)] that treats core collisions and Coulomb interactions on
the same footing is investigated in order to understand ion size effects on the
partition of neutral and charged particles at planar interfaces and the ionic
selectivity of slit nanopores. We introduce a variational scheme that can go
beyond the mean-field (MF) regime and couple in a consistent way pore modified
core interactions, steric effects, electrostatic solvation and image-charge
forces, and surface charge induced electrostatic potential. We show that in the
dilute limit, the MF and the variational theories agree well with MC simulation
results, in contrast to a recent RPA method. The partition of charged Yukawa
particles at a neutral dielectric interface (e.g air-water or protein-water
interface) is investigated. It is shown that as a result of the competition
between core collisions that push the ions towards the surface, and repulsive
solvation and image forces that exclude them from the interface, a
concentration peak of finite size ions sets in close to the dielectric
interface. We also characterize the role played by the ion size on the ionic
selectivity of neutral slit nanopores. We show that the complex interplay
between electrostatic forces, excluded volume effects induced by core
collisions and steric effects leads to an unexpected reversal in the ionic
selectivity of the pore with varying pore size: while large pores exhibits a
higher conductivity for large ions, narrow pores exclude large ions more
efficiently than small ones
The MERRA-2 Aerosol Reanalysis, 1980 Onward. Part I: System Description and Data Assimilation Evaluation
The Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) updates NASA's previous satellite era (1980 - onward) reanalysis system to include additional observations and improvements to the Goddard Earth Observing System, Version 5 (GEOS-5) Earth system model. As a major step towards a full Integrated Earth Systems Analysis (IESA), in addition to meteorological observations, MERRA-2 now includes assimilation of aerosol optical depth (AOD) from various ground- and space-based remote sensing platforms. Here, in the first of a pair of studies, we document the MERRA-2 aerosol assimilation, including a description of the prognostic model (GEOS-5 coupled to the GOCART aerosol module), aerosol emissions, and the quality control of ingested observations. We provide initial validation and evaluation of the analyzed AOD fields using independent observations from ground, aircraft, and shipborne instruments. We demonstrate the positive impact of the AOD assimilation on simulated aerosols by comparing MERRA-2 aerosol fields to an identical control simulation that does not include AOD assimilation. Having shown the AOD evaluation, we take a first look at aerosol-climate interactions by examining the shortwave, clear-sky aerosol direct radiative effect. In our companion paper, we evaluate and validate available MERRA-2 aerosol properties not directly impacted by the AOD assimilation (e.g. aerosol vertical distribution and absorption). Importantly, while highlighting the skill of the MERRA-2 aerosol assimilation products, both studies point out caveats that must be considered when using this new reanalysis product for future studies of aerosols and their interactions with weather and climate
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
Host model uncertainties in aerosol radiative forcing estimates: results from the AeroCom Prescribed intercomparison study
Simulated multi-model âdiversityâ in aerosol direct
radiative forcing estimates is often perceived as a measure
of aerosol uncertainty. However, current models used
for aerosol radiative forcing calculations vary considerably
in model components relevant for forcing calculations and
the associated âhost-model uncertaintiesâ are generally convoluted
with the actual aerosol uncertainty. In this AeroCom
Prescribed intercomparison study we systematically isolate
and quantify host model uncertainties on aerosol forcing experiments
through prescription of identical aerosol radiative
properties in twelve participating models.
Even with prescribed aerosol radiative properties, simulated
clear-sky and all-sky aerosol radiative forcings show
significant diversity. For a purely scattering case with globally
constant optical depth of 0.2, the global-mean all-sky
top-of-atmosphere radiative forcing is â4.47Wmâ2 and the
inter-model standard deviation is 0.55Wmâ2, corresponding
to a relative standard deviation of 12 %. For a case
with partially absorbing aerosol with an aerosol optical
depth of 0.2 and single scattering albedo of 0.8, the forcing
changes to 1.04Wmâ2, and the standard deviation increases
to 1.01Wâ2, corresponding to a significant relative standard
deviation of 97 %. However, the top-of-atmosphere forcing
variability owing to absorption (subtracting the scattering
case from the case with scattering and absorption) is low,
with absolute (relative) standard deviations of 0.45Wmâ2
(8 %) clear-sky and 0.62Wmâ2 (11 %) all-sky.
Scaling the forcing standard deviation for a purely scattering
case to match the sulfate radiative forcing in the Aero-
Com Direct Effect experiment demonstrates that host model
uncertainties could explain about 36% of the overall sulfate
forcing diversity of 0.11Wmâ2 in the AeroCom Direct Radiative
Effect experiment
Observations of the Interaction and Transport of Fine Mode Aerosols With Cloud and/or Fog in Northeast Asia From Aerosol Robotic Network and Satellite Remote Sensing
Analysis of Sun photometer measured and satellite retrieved aerosol optical depth (AOD) datahas shown that major aerosol pollution events with very highfine mode AOD (>1.0 in midvisible) in theChina/Korea/Japan region are often observed to be associated with significant cloud cover. This makesremote sensing of these events difficult even for high temporal resolution Sun photometer measurements.Possible physical mechanisms for these events that have high AOD include a combination of aerosolhumidification, cloud processing, and meteorological covariation with atmospheric stability andconvergence. The new development of Aerosol Robotic Network Version 3 Level 2 AOD with improved cloudscreening algorithms now allow for unprecedented ability to monitor these extremefine mode pollutionevents. Further, the spectral deconvolution algorithm (SDA) applied to Level 1 data (L1; no cloud screening)provides an even more comprehensive assessment offine mode AOD than L2 in current and previous dataversions. Studying the 2012 winter-summer period, comparisons of Aerosol Robotic Network L1 SDA dailyaveragefine mode AOD data showed that Moderate Resolution Imaging Spectroradiometer satellite remotesensing of AOD often did not retrieve and/or identify some of the highestfine mode AOD events in thisregion. Also, compared to models that include data assimilation of satellite retrieved AOD, the L1 SDAfinemode AOD was significantly higher in magnitude, particularly for the highest AOD events that were oftenassociated with significant cloudiness
Integrated Methane Inversion (IMI 1.0): a user-friendly, cloud-based facility for inferring high-resolution methane emissions from TROPOMI satellite observations
We present a user-friendly, cloud-based facility for quantifying methane emissions with 0.25ââĂâ0.3125â
(ââ25âkmâĂâ25âkm) resolution by inverse analysis of satellite observations from the TROPOspheric Monitoring
Instrument (TROPOMI). The facility is built on an Integrated Methane Inversion optimal estimation workflow (IMI 1.0) and supported for use on the
Amazon Web Services (AWS) cloud. It exploits the GEOS-Chem chemical transport model and TROPOMI data already resident on AWS, thus avoiding
cumbersome big-data download. Users select a region and period of interest, and the IMI returns an analytical solution for the Bayesian optimal
estimate of period-average emissions on the 0.25ââĂâ0.3125â grid including error statistics, information content, and
visualization code for inspection of results. The inversion uses an advanced research-grade algorithm fully documented in the literature. An
out-of-the-box inversion with rectilinear grid and default prior emission estimates can be conducted with no significant learning curve. Users can
also configure their inversions to infer emissions for irregular regions of interest, swap in their own prior emission inventories, and modify
inversion parameters. Inversion ensembles can be generated at minimal additional cost once the Jacobian matrix for the analytical inversion has been
constructed. A preview feature allows users to determine the TROPOMI information content for their region and time period of interest before
actually performing the inversion. The IMI is heavily documented and is intended to be accessible by researchers and stakeholders with no expertise
in inverse modelling or high-performance computing. We demonstrate the IMI's capabilities by applying it to estimate methane emissions from the US
oil-producing Permian Basin in May 2018.</p
Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment
In this study we examine the performance of 31 global model radiative transfer schemes in cloud-free conditions with prescribed gaseous absorbers and no aerosols (Rayleigh atmosphere), with prescribed scattering-only aerosols, and with more absorbing aerosols. Results are compared to benchmark results from high-resolution, multi-angular line-by-line radiation models. For purely scattering aerosols, model bias relative to the line-by-line models in the top-of-the atmosphere aerosol radiative forcing ranges from roughly â10 to 20%, with over- and underestimates of radiative cooling at lower and higher solar zenith angle, respectively. Inter-model diversity (relative standard deviation) increases from ~10 to 15% as solar zenith angle decreases. Inter-model diversity in atmospheric and surface forcing decreases with increased aerosol absorption, indicating that the treatment of multiple-scattering is more variable than aerosol absorption in the models considered. Aerosol radiative forcing results from multi-stream models are generally in better agreement with the line-by-line results than the simpler two-stream schemes. Considering radiative fluxes, model performance is generally the same or slightly better than results from previous radiation scheme intercomparisons. However, the inter-model diversity in aerosol radiative forcing remains large, primarily as a result of the treatment of multiple-scattering. Results indicate that global models that estimate aerosol radiative forcing with two-stream radiation schemes may be subject to persistent biases introduced by these schemes, particularly for regional aerosol forcing
Solvation free energy profile of the SCN- ion across the water-1,2-dichloroethane liquid/liquid interface. A computer simulation study
The solvation free energy profile of a single SCN- ion is calculated across the water-1,2-dichloroethane liquid/liquid interface at 298 K by the constraint force method. The obtained results show that the free energy cost of transferring the ion from the aqueous to the organic phase is about 70 kJ/mol, The free energy profile shows a small but clear well at the aqueous side of the interface, in the subsurface region of the water phase, indicating the ability of the SCN- ion to be adsorbed in the close vicinity of the interface. Upon entrance of the SCN- ion to the organic phase a coextraction of the water molecules of its first hydration shell occurs. Accordingly, when it is located at the boundary of the two phases the SCN- ion prefers orientations in which its bulky S atom is located at the aqueous side, and the small N atom, together with its first hydration shell, at the organic side of the interface
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The Mercury Project: A High Average Power, Gas-Cooled Laser For Inertial Fusion Energy Development
Hundred-joule, kilowatt-class lasers based on diode-pumped solid-state technologies, are being developed worldwide for laser-plasma interactions and as prototypes for fusion energy drivers. The goal of the Mercury Laser Project is to develop key technologies within an architectural framework that demonstrates basic building blocks for scaling to larger multi-kilojoule systems for inertial fusion energy (IFE) applications. Mercury has requirements that include: scalability to IFE beamlines, 10 Hz repetition rate, high efficiency, and 10{sup 9} shot reliability. The Mercury laser has operated continuously for several hours at 55 J and 10 Hz with fourteen 4 x 6 cm{sup 2} ytterbium doped strontium fluoroapatite (Yb:S-FAP) amplifier slabs pumped by eight 100 kW diode arrays. The 1047 nm fundamental wavelength was converted to 523 nm at 160 W average power with 73% conversion efficiency using yttrium calcium oxy-borate (YCOB)
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