Impacts of the U.S.-Central America-Dominican Republic Free Trade Agreement (CAFTA-DR) on U.S. Fruit Exports - the Apple Case

The U.S.-Central America-Dominican Republic Free Trade Agreement (CAFTA-DR) levels the playing field of trade between the United States and the six CAFTA-DR partner countries. Half of U.S. farm products gain immediate tariff-free access to the markets of the CAFTA-DR region. All Tariffs will be eliminated in 20 years. Under CAFTA-DR, tariffs on an important U.S. fresh fruit export to the region, fresh apples, declined from an initial base of 15%-25% in CAFTA-DR countries to zero immediately upon enforcement. The specific objective of this research is to analyze the impact of tariff elimination under CAFTA-DR on the trade of U.S. fresh apples. Generalized Method of Moments (GMM) is used for the analysis involving an excess-supply-excess-demand model with monthly trade data from January 2000 to December 2010. The more telling empirical results indicate that for each of the six CAFTA-DR countries, tariff elimination positively promotes U.S. apple exports to this region.CAFTA-DR, trade liberalization, tariff elimination, Generalized Method of Moments(GMM), Agricultural and Food Policy, International Relations/Trade, Q17, F13, F15,

Enhancing energy absorption in quantum dot solar cells via periodic light-trapping microstructures

Colloidal quantum dot (CQD) solar cells prove to be promising devices for optoelectronic applications due to their tunable absorption range, deep infrared absorption capabilities, and straightforward processability. However, there remains a need to further enhance their device performance - particularly when one has to adhere to strict physical limitations on their physical structure. Here we present a three-dimensional numerical model of CQD solar cells in COMSOL Multiphysics based on the finite element method. With this model we have simulated the optical characteristics of several CQD solar cells across varying photonic structures and physical parameters to investigate how distinct photonic structures may enhance the light absorption and current output of CQD solar cells using identical physical parameters. Of the many cells simulated, one notable model increased the predicted current in the active layer PbS by 69.33% as compared to a flat solar cell with identical physical parameters, and produced a current of 24.18 mA cm-2 by implementing a cross-shaped photonic structure built on top of a flat substrate of glass and ITO. This cross-shaped model serves as a key example of how unique photonic structures can be implemented to further enhance light absorption

Ground state of a polydisperse electrorheological solid: Beyond the dipole approximation

The ground state of an electrorheological (ER) fluid has been studied based on our recently proposed dipole-induced dipole (DID) model. We obtained an analytic expression of the interaction between chains of particles which are of the same or different dielectric constants. The effects of dielectric constants on the structure formation in monodisperse and polydisperse electrorheological fluids are studied in a wide range of dielectric contrasts between the particles and the base fluid. Our results showed that the established body-centered tetragonal ground state in monodisperse ER fluids may become unstable due to a polydispersity in the particle dielectric constants. While our results agree with that of the fully multipole theory, the DID model is much simpler, which offers a basis for computer simulations in polydisperse ER fluids.Comment: Accepted for publications by Phys. Rev.

Modeling photovoltaic performance in periodic patterned colloidal quantum dot solar cells

Colloidal quantum dot (CQD) solar cells have attracted tremendous attention mostly due to their wide absorption spectrum window and potentially low processability cost. The ultimate efficiency of CQD solar cells is highly limited by their high trap state density. Here we show that the overall device power conversion efficiency could be improved by employing photonic structures that enhance both charge generation and collection efficiencies. By employing a two-dimensional numerical model, we have calculated the characteristics of patterned CQD solar cells based of a simple grating structure. Our calculation predicts a power conversion efficiency as high as 11.2%, with a short circuit current density of 35.2 mA/cm2, a value nearly 1.5 times larger than the conventional flat design, showing the great potential value of patterned quantum dot solar cells

B→η(η′)K(π)B \to \eta(\eta') K(\pi) in the Standard Model with Flavor Symmetry

The observed branching ratios for $B\to K \eta'$ decays are much larger than factorization predictions in the Standard Model (SM). Many proposals have been made to reconcile the data and theoretical predictions. In this paper we study these decays within the SM using flavor U(3) symmetry. If small annihilation amplitudes are neglected, one needs 11 hadronic parameters to describe $B\to PP$ decays where $P$ can be one of the $\pi$, $K$, $\eta$ and $\eta'$ nonet mesons. We find that existing data are consistent with SM with flavor U(3) symmetry. We also predict several measurable branching ratios and CP asymmetries for $B \to K (\pi) \eta(\eta')$, $\eta(\eta')\eta(\eta')$ decays. Near future experiments can provide important tests for the Standard Model with flavor U(3) symmetry.Comment: 13 pages, 4 table

Electromigration-Induced Flow of Islands and Voids on the Cu(001) Surface

Electromigration-induced flow of islands and voids on the Cu(001) surface is studied at the atomic scale. The basic drift mechanisms are identified using a complete set of energy barriers for adatom hopping on the Cu(001) surface, combined with kinetic Monte Carlo simulations. The energy barriers are calculated by the embedded atom method, and parameterized using a simple model. The dependence of the flow on the temperature, the size of the clusters, and the strength of the applied field is obtained. For both islands and voids it is found that edge diffusion is the dominant mass-transport mechanism. The rate limiting steps are identified. For both islands and voids they involve detachment of atoms from corners into the adjacent edge. The energy barriers for these moves are found to be in good agreement with the activation energy for island/void drift obtained from Arrhenius analysis of the simulation results. The relevance of the results to other FCC(001) metal surfaces and their experimental implications are discussed.Comment: 9 pages, 13 ps figure

Effect of aerosol composition on the performance of low-cost optical particle counter correction factors

There is considerable interest in using low-cost optical particle counters (OPCs) to supplement existing routine air quality networks that monitor particle mass concentrations. In order to do this, low-cost OPC data need to be comparable with particle mass reference instrumentation; however, there is currently no widely agreed upon methodology to accomplish this. Aerosol hygroscopicity is known to be a key parameter to consider when correcting particle mass concentrations derived from low-cost OPCs, particularly at high ambient relative humidity (RH). Correction factors have been developed that apply κ-Köhler theory to correct for the influence of water uptake by hygroscopic aerosols. We have used datasets of co-located reference particle measurements and low-cost OPC (OPC-N2, Alphasense) measurements, collected in four cities on three continents, to explore the performance of this correction factor. We provide evidence that the elevated particle mass concentrations, reported by the low-cost OPC relative to reference instrumentation, are due to bulk aerosol hygroscopicity under different RH conditions, which is determined by aerosol composition and, in particular, the levels of hygroscopic aerosols (sulfate and nitrate). We exploit measurements made in volcanic plumes in Nicaragua, which are predominantly composed of sulfate aerosol, as a natural experiment to demonstrate this behaviour in the ambient atmosphere; the observed humidogram from these measurements closely resembles the calculated pure sulfuric acid humidogram. The results indicate that the particle mass concentrations derived from low-cost OPCs during periods of high RH (>60 %) need to be corrected for aerosol hygroscopic growth. We employed a correction factor based on κ-Köhler theory and observed that the corrected OPC-N2 PM2.5 mass concentrations were within 33 % of reference measurements at all sites. The results indicated that a κ value derived in situ (using suitable reference instrumentation) would lead to the most accurate correction relative to co-located reference instruments. Applying a κ values from the literature in the correction factor also resulted in improved OPC-N2 performance, with the measurements being within 50 % of the reference values. Therefore, for areas where suitable reference instrumentation for developing a local correction factor is lacking, using a literature κ value can result in a reasonable correction. For locations with low levels of hygroscopic aerosols and low RH values, a simple calibration against gravimetric measurements (using suitable reference instrumentation) would likely be sufficient. Whilst this study generated correction factors specific for the Alphasense OPC-N2 sensor, the calibration methodology developed is likely amenable to other low-cost PM sensors

Respiratory plasticity in response to changes in oxygen supply and demand

Aerobic organisms maintain O2 homeostasis by responding to changes in O2 supply and demand in both short and long time domains. In this review, we introduce several specific examples of respiratory plasticity induced by chronic changes in O2 supply (environmental hypoxia or hyperoxia) and demand (exercise-induced and temperature-induced changes in aerobic metabolism). These studies reveal that plasticity occurs throughout the respiratory system, including modifications to the gas exchanger, respiratory pigments, respiratory muscles, and the neural control systems responsible for ventilating the gas exchanger. While some of these responses appear appropriate (e.g., increases in lung surface area, blood O2 capacity, and pulmonary ventilation in hypoxia), other responses are potentially harmful (e.g., increased muscle fatigability). Thus, it may be difficult to predict whole-animal performance based on the plasticity of a single system. Moreover, plastic responses may differ quantitatively and qualitatively at different developmental stages. Much of the current research in this field is focused on identifying the cellular and molecular mechanisms underlying respiratory plasticity. These studies suggest that a few key molecules, such as hypoxia inducible factor (HIF) and erythropoietin, may be involved in the expression of diverse forms of plasticity within and across species. Studying the various ways in which animals respond to respiratory challenges will enable a better understanding of the integrative response to chronic changes in O2 supply and deman