A Variational Approach to the Structure and Thermodynamics of Linear Polyelectrolytes with Coulomb and Screened Coulomb Interactions

A variational approach, based on a discrete representation of the chain, is used to calculate free energy and conformational properties in polyelectrolytes. The true bond and Coulomb potentials are approximated by a trial isotropic harmonic energy containing force constants between {\em all}monomer-pairs as variational parameters. By a judicious choice of representation and the use of incremental matrix inversion, an efficient and fast-convergent iterative algorithm is constructed, that optimizes the free energy. The computational demand scales as $N^3$ rather than $N^4$ as expected in a more naive approach. The method has the additional advantage that in contrast to Monte Carlo calculations the entropy is easily computed. An analysis of the high and low temperature limits is given. Also, the variational formulation is shown to respect the appropriate virial identities.The accuracy of the approximations introduced are tested against Monte Carlo simulations for problem sizes ranging from $N=20$ to 1024. Very good accuracy is obtained for chains with unscreened Coulomb interactions. The addition of salt is described through a screened Coulomb interaction, for which the accuracy in a certain parameter range turns out to be inferior to the unscreened case. The reason is that the harmonic variational Ansatz becomes less efficient with shorter range interactions. As a by-product a very efficient Monte Carlo algorithm was developed for comparisons, providing high statistics data for very large sizes -- 2048 monomers. The Monte Carlo results are also used to examine scaling properties, based on low-$T$ approximations to end-end and monomer-monomer separations. It is argued that the former increases faster than linearly with the number of bonds.Comment: 40 pages LaTeX, 13 postscript figure

Elucidating the Correlation of the Quasar \ion{Fe}{2}/\ion{Mg}{2} Ratio with Redshift

Interpretation of the \ion{Fe}{2}(UV)/\ion{Mg}{2} emission ratios from quasars has a major cosmological motivation. Both Fe and Mg are produced by short-lived massive stars. In addition, Fe is produced by accreting white dwarf supernovae somewhat after star formation begins. Therefore, we expect that the Fe/Mg ratio will gradually decrease with redshift. We have used data from the Sloan Digital Sky Survey to explore the dependence of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio on redshift and on luminosity in the redshift range of $0.75< z< 2.20$, and we have used predictions from our 830-level model for the \ion{Fe}{2} atom in photoionization calculations to interpret our findings. We have split the quasars into several groups based upon the value of their \ion{Fe}{2}(UV)/\ion{Mg}{2} emission ratios, and then checked to see how the fraction of quasars in each group varies with the increase of redshift. We next examined the luminosity dependence of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio, and we found that beyond a threshold of \ion{Fe}{2}(UV)/\ion{Mg}{2} =~ 5, and $M_{2500} < -25\rm mag$, the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio increases with luminosity, as predicted by our model. We interpret our observed variation of the \ion{Fe}{2}(UV)/\ion{Mg}{2} ratio with redshift as a result of the correlation of redshift with luminosity in a magnitude limited quasar sample.Comment: ApJL accepte

EFFECTS OF TARIFFS AND TECHNICAL BARRIERS ON HIGH- AND LOW-VALUE POULTRY TRADE

A perfectly competitive spatial partial equilibrium model is constructed to evaluate some of the policy effects on world poultry trade. The model simulates the trade flows among six key exporting and importing countries and two aggregate rest-of-world regions. Effects of removal of restrictions based on tariffs, tariff-rate quotas (TRQs) and sanitary regulations are evaluated maintaining a distinction between "high-value" (mostly white meat) and "low-value" (mostly dark meat) poultry products. Results suggest that removal of sanitary barriers alone has relatively little effect compared to removal of tariffs and TRQs, but has more effect if sanitary and other barriers are removed simultaneously. Imposition of new sanitary barriers against US products by Russia would also shift trade flows, with production rising in Brazil.International Relations/Trade,

Economic and environmental effects of border tax adjustments

Taxing imports from regions which are not subject to climate policy and subsidizing exports into these regions have recently been proposed to address presumed negative effects of the EU Emissions Trading Scheme (EU ETS) on industry competitiveness and carbon leakage. This paper analyzes the economic and environmental effects of alternative border tax adjustment (BTA) mechanisms using an extended version of the GTAP-E model that explicitly includes domestic trade and transport margins. The BTAs are imposed on regions which have not committed to emission targets under the Kyoto Protocol or which failed to ratify the Kyoto Protocol. The analyses distinguish between effects of the BTAs on the EU15 countries and on the rest of the EU (REU). Likewise, the analyses single out the effects of climate policy with and without BTAs on domestic output changes which are due to changes in import competition and export competitiveness. Implementing a BTA whose power is equal to the percentage change in production costs in the energy-intensive sectors in the EU has different impacts for those sectors in the EU15 countries compared with the REU countries. In the EU15, the BTA effectively neutralizes import competition in the energy-intensive sectors while enhancing the export competitiveness of these sectors. Conversely, in the REU, the BTA is not effective in neutralizing increased import competition or decreased export competitiveness because the majority of trade by the REU is with countries/regions that are not included in the BTA. Overall, implementing a BTA has little effect on the marginal abatement costs of achieving the emission reductions in the Kyoto Protocol and does little in reducing carbon leakage. --

INTERNATIONAL TRADE: HOW IT AFFECTS VIRGINIA AGRICULTURE

International Relations/Trade,

Submillimeter Atmospheric Transparency at Maunakea, at the South Pole, and at Chajnantor

For a systematic assessment of submillimeter observing conditions at different sites, we constructed tipping radiometers to measure the broad band atmospheric transparency in the window around 350 $\mu$m wavelength. The tippers were deployed on Maunakea, Hawaii, at the South Pole, and in the vicinity of Cerro Chajnantor in northern Chile. Identical instruments permit direct comparison of these sites. Observing conditions at the South Pole and in the Chajnantor area are better than on Maunakea. Simultaneous measurements with two tippers demonstrate conditions at the summit of Cerro Chajnantor are significantly better than on the Chajnantor plateau.Comment: Accepted by PAS

Linking Risk and Economic Assessments in the Analysis of Plant Pest Regulations: The Case of U.S. Imports of Mexican Avocados

Risk and Uncertainty,

Effects of Tariffs and Sanitary Barriers on High- and Low-Value Poultry Trade

A competitive partial-equilibrium spatial model with heterogeneous goods is constructed to evaluate effects of the removal of tariffs, tariff-rate quotas, and sanitary regulations on world poultry trade. The model distinguishes between "highvalue" (mostly white meat) and "low-value" (mostly dark meat) poultry products and simulates the trade flows among eight exporting and importing countries and regions. Removing all barriers simultaneously has a larger impact on trade than removing only tariffs and tariff-rate quotas. Imposition of sanitary barriers against U.S. products by Russia shifts trade flows, but does not have large net impacts on U.S. producers.poultry trade, sanitary barriers, tariffs, Agricultural and Food Policy,

Flight investigation of insect contamination and its alleviation

An investigation of leading edge contamination by insects was conducted with a JetStar airplane instrumented to detect transition on the outboard leading edge flap and equipped with a system to spray the leading edge in flight. The results of airline type flights with the JetStar indicated that insects can contaminate the leading edge during takeoff and climbout. The results also showed that the insects collected on the leading edges at 180 knots did not erode at cruise conditions for a laminar flow control airplane and caused premature transition of the laminar boundary layer. None of the superslick and hydrophobic surfaces tested showed any significant advantages in alleviating the insect contamination problem. While there may be other solutions to the insect contamination problem, the results of these tests with a spray system showed that a continouous water spray while encountering the insects is effective in preventing insect contamination of the leading edges