Gas/particle partitioning of seven volatile polycyclic aromatic hydrocarbons in a heavy traffic urban area

Air samples (vapor- and particle-phase) were taken for 19 sampling events during the period from December 1997 to July 1998 in an urban site in the center of Athens. The urban site is densely populated and characterized by heavy traffic circulation and elevated concentrations of VOCs, NO(x), CO and smoke. Seven volatile polycyclic aromatic hydrocarbons (PAHs) were determined in samples. The temperature dependence of gas-phase atmospheric concentration of PAHs, C(g), was investigated using diagrams of natural logarithm of partial pressures (lnP) vs. reciprocal mid-point temperatures. For the six of seven volatile PAHs, the temperature dependence of lnP was statistically significant (at least at the 90% confidence level) and the temperature accounted for 21-67% of the variability in gas-phase concentrations. The gas-phase concentration C(g) of the very volatile PAHs was affected more significantly by changes in temperature, but the variation of the less-volatile PAHs fluoranthene and pyrene C(g), was better explained by changes in temperature. The temperature dependence of gas/particle partitioning constant K(P) was also examined. Regressions of log(K(P))(-1) vs. T(-1) for fluorene, fluoranthene and pyrene were classified into two different temperature ranges. The gas/particle partitioning of PAHs was studied by correlating the partition constant to the sub-cooled liquid saturation vapor pressure (P(L)(o)). The Junge adsorption model underestimated the particle fraction of volatile PAHs probably due to the presence of non-exchangeable fraction. Slopes (m(r)) of the regressions logK(P) vs. logP(L)(o) were different from the value -1 as Pankow's theory predicts. The short distance between the sampling point and the emission sources is also estimated to be a factor that causes deviations from the theoretical value. Evidence that atmospheric conditions favorable for secondary aerosol formation coincide with higher value of m(r), was provided by limited sampling events. An interrelation was found to exist between the m(r) values, allowing the prediction of the gas/particle partitioning of a series of seven PAHs by the measurement of a single PAH partitioning.     

Bubble interactions for the Mullins–Sekerka problem: Some case examples

The Mullins–Sekerka free boundary problem originates from the study of solidification and liquidation of materials where material is transported by diffusion. In the present paper we explore dynamics of bubbles for the Mullins–Sekerka problem. Using a set of ordinary differential equations for the radii and the centers, we numerically simulate the relevant interactions in both “two-dimensional” and “three-dimensional” settings. Our results illustrate how larger bubbles grow at the expense of smaller ones and highlight the role of additional factors such as the initial inter-bubble distance or weak asymmetries in the bubble position in the ensuing dynamics. One novel feature in comparison with earlier works is the possibility to continue for the three-dimensional case the simulation past the points where one of the bubbles disappears.     

Channel Allocation to Multiservice Wireless Networks: A Collocation—Based Approach to Enhance Spectrum Utilization

A channel allocation (CA) scheme is proposed for wireless networks serving multiple services. The CA scheme is based on collocating the base stations of the wireless networks and restricting their coverage over common cells. The increased coverage granularity makes feasible a more efficient frequency reuse. A genetic algorithm approach is adopted to minimize the number of channels required by the multiservice networks, taking into account the intranetwork and intraservice constraints over a specific coverage area. To validate the proposed scheme and investigate its performance, the Philadelphia benchmark network is employed. Simulation results corresponding to a mix of digital TV and mobile services of various profiles demonstrate the capability of the proposed scheme to optimally allocate channels to multiservice wireless networks.     

Hybrid InAs nanowire-vanadium proximity SQUID

We report the fabrication and characterization of superconducting quantum interference devices (SQUIDs) based on InAs nanowires and vanadium superconducting electrodes. These mesoscopic devices are found to be extremely robust against thermal cycling and to operate up to temperatures of ～ 2.5 K with reduced power dissipation. We show that our geometry allows one to obtain nearly-symmetric devices with very large magnetic field modulation of the critical current. All these properties make these devices attractive for sensitive magnetometry applications and quantum circuit implementation.     

Costimulatory molecules in ocular cicatricial pemphigoid

PURPOSE: To examine normal and inflamed conjunctiva from patients with ocular cicatricial pemphigoid (OCP) for the presence of costimulatory molecule CD28 and its ligands B7-1 (CD80) and B7-2 (CD86). METHODS: Conjunctival biopsy specimens from 12 patients with OCP and from five healthy persons undergoing cataract surgery were analyzed by light microscopy and immunohistochemical examination with monoclonal antibody probes for CD28, B7-1, and B7-2 molecules and for mononuclear cell subtypes. RESULTS: Epithelium of OCP conjunctiva showed more Langerhans' cells, B7-1-positive (+) cells, and B7-2 expression (ratio of B7-2-positive cells to antigen-presenting cells). In the substantia propria, OCP specimens showed significantly increased numbers of T cells (CD3 +), macrophages (CD68+), CD28+ cells, B7-2+ cells (CD86+), Langerhans' cells (CD1a), and B7-1+ cells (CD80). Most of the B7-2+ cells, macrophages, and Langerhans' cells were located subepithelially. B7-2 expression was significantly higher in OCP conjunctival substantia propria compared with normal conjunctiva. CONCLUSIONS: The results of this study indicate that the expression of the costimulatory molecule B7-2 is upregulated in conjunctiva of patients with active OCP. This increased subepithelial B7-2 expression may contribute to the sustained immune activation in OCP conjunctiva.     

Satellite communications at KU, KA, and V bands: Propagation impairments and mitigation techniques

This article surveys the alternative fade mitigation techniques for satellite communication systems operating at Ku, Ka and V frequency bands. The specific phenomena influencing the propagation of radiowaves on Earth-space links are also overviewed. Emphasis is placed on modeling, experimental work carried out in the past, and practical implementations related to each mitigation technique.