Average Concentration Ratio Research Articles

Optical performance of fixed east–west aligned CPCs used in China

In this article, a mathematical procedure is developed to estimate the annual collectible radiation captured by fixed compound parabolic concentrators (CPCs, in short) oriented in east-west direction based on the monthly horizontal radiation. Results show that for fixed east–west aligned symmetric CPCs used over the atmosphere, the optimal acceptance half-angle for maximizing its annual energy collection was 25.97°, the yearly optimal tilt-angle of apertures relative to the horizon was equal to the site latitude, and the maximum annual average optical concentration ratio was uniquely related to the site latitude. For CPCs used in China, the optimal acceptance half-angle were in between 25.3° and 26°, the yearly optimal tilt-angle approached the site latitude with a deviation less than 1°, and the maximum annual average optical concentration ratios ranged from 1.45 to 1.74. Results also indicated that CPCs were more favorable to be used in the areas with higher latitude and abundant solar resources. Effects of tilt-angle and azimuth angle of CPCs on its annual solar gain were also presented.

The Effect of Sampling Time on the Overall Performance of Portable Microbial Impactors

In this study we analyzed the sampling time effect on the overall performance of seven portable impactors (SMA MicroPortable, BioCulture, Microflow, MAS-100, Millipore Air Tester, SAS Super 180, and RCS High Flow) as well as a stationary impactor (BioStage) when collecting airborne bacteria and fungi from 2 to 30 minutes indoors and outdoors. Their performance was compared against the reference BioStage impactor collecting samples for 2 minutes. When bacteria were collected outdoors, the average concentration ratio of all test samplers relative to the reference sampler was 0.64, but it decreased to 0.04 when bacteria were collected for 2 min first and subsequently exposed to particle-free air for 28 min (30 min total sampling time). For fungi, the average concentration ratio decreased from 0.35 to 0.03 under the same conditions. When the test impactors sampled particle-free air first and then collected bioaerosols outdoors for 2 min, the concentration ratio for bacteria averaged for all test samplers decr...

Uncertainties in Assessing Annual Nitrate Loads and Concentration Indicators: Part 1. Impact of Sampling Frequency and Load Estimation Algorithms

The objectives of this study are to evaluate the uncertainty in annual nitrate loads and concentrations (such as annual average and median concentrations) as induced by infrequent sampling and by the algorithms used to compute fluxes. A total of 50 watershed-years of hourly to daily flow and concentration data gathered from nine watersheds (5 to 252 km) in Brittany, France, were analyzed. Original (high frequency) nitrate concentration and flow data were numerically sampled to simulate common sampling frequencies. Annual fluxes and concentration indicators calculated from the simulated samples were compared to the reference values calculated from the high-frequency data. The uncertainties contributed by several algorithms used to calculate annual fluxes were also quantified. In all cases, uncertainty increased as sampling intervals increased. Results showed that all the tested algorithms that do not use continuous flow data to compute nitrate fluxes introduced considerable uncertainty. The flow-weighted average concentration ratio method was found to perform best across the 50 annual datasets. Analysis of the bias values suggests that the 90th and 95th percentiles and the maximum concentration values tend to be systematically underestimated in the long term, but the load estimates (using the chosen algorithm) and the average and median concentrations were relatively unbiased. Great variability in the precision of the load estimation algorithms was observed, both between watersheds of different sizes and between years for a particular watershed. This has prevented definitive uncertainty predictions for nitrate loads and concentrations in this preliminary work, but suggests that hydrologic factors, such as the watershed hydrological reactivity, could be a key factor in predicting uncertainty levels.

Numerical estimation of effective diffusion coefficients for charged porous materials based on micro-scale analyses

In order to describe diffusive transport of solutes through a porous material, estimation of effective diffusion coefficients is required. It has been shown theoretically that in the case of uncharged porous materials the effective diffusion coefficient of solutes is a function of the pore morphology of the material and can be described by the tortuosity (tensor) (Bear, 1988 [1]). Given detailed information of the pore geometry at the micro-scale the tortuosity of different materials can be accurately estimated using homogenization procedures. However, many engineering materials (e.g., clays and shales) are characterized by electrical surface charges on particles of the porous material which strongly affect the (diffusive) transport properties of ions. For these type of materials, estimation of effective diffusion coefficients have been mostly based on phenomenological equations with no link to underlying micro-scale properties of these charged materials although a few recent studies have used alternative methods to obtain the diffusion parameters (Jougnot et al., 2009; Pivonka et al., 2009; Revil and Linde, 2006 [2–4]). In this paper we employ a recently proposed up-scaled Poisson–Nernst–Planck type of equation (PNP) and its micro-scale counterpart to estimate effective ion diffusion coefficients in thin charged membranes. We investigate a variety of different pore geometries together with different surface charges on particles. Here, we show that independent of the charges on particles, a (generalized) tortuosity factor can be identified as function of the pore morphology only using the new PNP model. On the other hand, all electro-static interactions of ions and charges on particles can consistently be captured by the ratio of average concentration to effective intrinsic concentration in the macroscopic PNP equations. Using this formulation allows to consistently take into account electrochemical interactions of ions and charges on particles and so excludes any ambiguity generally encountered in phenomenological equations.

Carbonyl compounds in the ambient air of hazy days and clear days in Guangzhou, China

Carbonyl compounds were measured in ambient air of Guangzhou in both hazy and clear days. Air samples were collected in two separate terms, 17–19th of November (the clear period) and 30th of November to 2nd of December 2005 (the hazy period). The total concentrations of carbonyls detected varied from 48.4 to 121.4 µg m − 3 (with a mean of 92.0 µg m − 3 ) in the hazy days and from 21.6 to 45.7 µg m − 3 (with a mean of 32.6 µg m − 3 ) in the clear days. Formaldehyde, acetaldehyde, and acetone were the most abundant carbonyl species in both haze and clear days, while the relative sum of these three compounds in the clear days contributed more than that in the hazy days to the total concentrations of carbonyls (66.0–72.8% vs 42.9–60.5%). The average concentration ratios of formaldehyde/acetaldehyde (C 1/C 2) were fairly constant between the haze and clear days (0.87 vs 0.84, (µg m − 3 )/(µg m − 3 ); 1.28 vs 1.23, ppbv/ppbv), whereas those of acetaldehyde/propionaldehyde (C 2/C 3) showed large variations (ranging from 5.93 to 13.0, (µg m − 3 )/(µg m − 3 ); from 7.82 to 18.3, ppbv/ppbv). The results of correlation analysis and principal component analysis indicated that vehicular exhaust might be the major source of carbonyls in the ambient air, with the biogenic emission as the potential source of some high molecular weight carbonyls. Moreover, biomass burning and accumulation of carbonyls might make significant contribution to the enhancement of carbonyls in the hazy days.

Seasonal Change in the Atmospheric Concentration of Particulate Polycyclic Aromatic Hydrocarbons in Ho Chi Minh City, Vietnam

We analyzed atmospheric particulate polycyclic aromatic hydrocarbons (PAHs) in Ho Chi Minh City, Vietnam, for 19 months. The average concentrations of total PAHs at dry and rainy seasons were 4.28 +/- 2.83 and 15.71 +/- 8.21 ng m(-3), respectively. The use of motorcycles without catalytic converters, estimated to be main emission sources of PAHs, would be higher during the dry season. PAH concentrations show a negative correlation with sunshine duration (r = -0.51). Furthermore, the ratio of average PAH concentration in the dry season to that in the rainy season shows a positive correlation with photolytic half-life (r = 0.94). Thus, seasonal changes in PAH concentrations are attributable to their photolytic degradation.

Factors contributing to seasonal variations in wet deposition fluxes of trace elements at sites along Japan Sea coast

In this study, we measured the wet deposition fluxes of ten trace elements (As, Cd, Cr, Cu, Mn, Ni, Pb, Sb, V and Zn) from December 2002 to March 2006 at three sites along the Japan Sea coast, which have been strongly affected by the long-range transport of air pollutants from the Asian continent. Also, factors, contributing to their seasonal variations were investigated. At the northern and central sites, the monthly wet deposition fluxes of all or most trace elements greatly increased during the cold season (typically, November–April), along with their monthly average (volume-weighted) concentrations in the precipitation. The cold/warm season ratios for the average concentrations of trace elements in precipitation were within the range of 2.7–5.1 at the northern site and 1.8–5.9 at the central site, which were similar to the average scavenging ratios (= concentration in precipitation/concentration in air) at each site. However, there were small differences (0.47–1.2 at the northern site and 0.73–1.7 at the central site) in the ratios of average concentrations in air between the two seasons. These suggest that the increase in the wet deposition fluxes of trace elements during the cold season is due to increases in their scavenging ratios. On the other hand, the result for the southern site was different from those at the other sites. The number of days when the daily maximum wind speed exceeded 10 m s −1 at the meteorological observatories near the study sites increased markedly during the cold season at the northern and central sites, showing that strong winds usually blow during the cold season at those sites, but not at the southern site. Higher wind speed transports larger amounts of constituents into the cloud system, which can result in their increased concentrations in precipitation. Thus, high scavenging ratios of trace elements during the cold season may be caused by the increase in their amounts of discharge into the cloud system owing to high wind speed, suggesting that wind speed is an important factor in the seasonal variations in the wet deposition fluxes.

Characteristics of Carbonaceous Aerosol at a Near–Highway-Traffic Sampling Site During Spring 2006

The concentrations for mass, organic carbon (OC), and elemental carbon (EC) of fine (PM2.5) and inhalable (PM10) particulates were studied at a near–highway-traffic sampling site during March 6 to April 1, 2006, around central Taiwan. The primary OC/EC ratio approach is applied to assess the contribution of secondary organic aerosol (SOA) to the PM2.5 and PM10 mass at the near–highway-traffic sampling site. The mass average concentrations of PM2.5 and inhalable () PM10 particulates were found to be 67.5 μ g m−3 and 107.9 μ g m−3, respectively, during sampling period. Furthermore, analytical results indicated that the average OC and EC concentrations were 10.22 and 3.88 mg m−3, respectively, in PM2.5 particulates. The average OC and EC concentrations were 11.57 and 4.56 mg m−3, respectively, in PM10 particulates. The average mass concentration ratio of PM2.5to PM10 particulates was 1.6 during sampling period. The results also reflected that PM2.5 particulate concentrations were the primary species (average approximately 61%). Additionally, the OC/EC ratio of both PM10 and PM2.5 particulates was at all times greater than 1.7, indicating that OC constituted 66%–79% of total carbon (TC). Experimental results demonstrated that direct emissions of primary organic aerosol (POA) are greater than emissions of carbonaceous material (EC aerosol) for either PM10 or PM2.5 particles. In addition, the results also reflected that OC concentration was the major species at this sampling site.

<b>Heavy metals contamination of topsoil and dispersion in the vicinities of reclaimed

Growing concern about reclamation of auto-repair workshop areas for residential and agricultural purposes makes risk assessment of heavy metal contamination of the study area imperative. In addition, the study is aimed at ascertaining the dispersion of contaminated Zn, Ni, Cr, Hg, and Pb within the soil profile. A total of 75 soil samples from auto-repair workshops and 18 soil samples from control sites were collected. Significant levels of contamination were found in view of elevated levels of these metals above background concentrations in control sites. Average topsoil metal concentrations (0-10 cm) in auto-repair workshop areas were: Zn 0.90 ± 0.5, Ni 11.5 ± 3.3, Cr 5.3 ± 2.3, Hg 9.4 ± 4.6 and Pb 133 ± 66 mg/kg. Lead was the most significant contaminants, and the degree of contamination was highest for Pb followed by Hg. The detailed levels of total metal contamination for Pb and Hg exceeded international thresholds for agricultural use. Enrichment factor within the soil profile metals were Zn 3.37, Ni 3.38, Cr 6.22, Hg 14.5 and Pb 21.1. The enrichment factor reflects the ratio of average concentration at a particular sampling location to average concentration at control sites. The general trend of dispersion of metal contaminants within the soil profile studied is Pb >> Ni > Hg > Cr >>Zn. KEY WORDS: Auto-repair, Lead, Mercury, Soil, Heavy metals, Dispersion Bull. Chem. Soc. Ethiop. 2008 , 22(3), 339-348.

Generic antiepileptic drugs

Generic antiepileptic drugs (AEDs) generally provide safe, effective, lower-cost alternatives to brand-name drugs. To be approved by the US Food and Drug Administration (FDA), manufacturers must show that generic drugs are comparable to brand-name formulations, meeting bioequivalence, dissolution, and manufacturing quality standards. Bioequivalence for most generic formulations is evaluated by measuring blood pharmacokinetic values in a small, crossover study of adult volunteers taking single doses of brand-name and generic AEDs. Bioequivalence standards require that ratios of average peak drug concentrations (C(max)) and total extent of absorption (area under the curve, AUC) for a test drug be within 80% to 125% of the reference brand-name drug, with a confidence interval of 90%. Bioequivalence of most generic AEDs, however, has not been evaluated in patients with epilepsy or in other special populations such as elderly patients or patients taking multiple AEDs and prodrugs. Moreover, evidence is limited regarding the adequacy of FDA generic standards for AEDs, particularly for "narrow therapeutic ratio" medications such as oxcarbazepine, although two carbamazepine studies are supportive. Most patients can successfully initiate therapy with generic AEDs and can safely switch from brand-name to generic AEDs (and sometimes back again). The FDA, however, has not shown safety in generic-to-generic switches, which could potentially cause drug concentration changes of up to 40%. Less expensive generic formulations will soon be available for most of the "second generation" AEDs--onisamide, for example, recently had 17 generic formulations approved in the United States--providing substantial savings in health care costs. In summary, although generic AEDs are generally safe and effective for most patients, the current bioequivalence standards may not be sufficient for certain patient populations and for certain drugs, requiring vulnerable patients to be monitored very carefully when converting to generic AEDs. The adequacy or inadequacy of FDA bioequivalence standards for AEDs has not yet been well evaluated with large, well-controlled studies.

Electrical performance evaluation of low‐concentrating non‐imaging photovoltaic concentrator

AbstractSecond generation prototype photovoltaic facades of reduced costs incorporating devices with optically concentrating elements (PRIDE) incorporate 6 mm wide ‘Saturn’ solar cells at the absorber of the dielectric concentrator. The concentrators were made using injection moulding technique with potential to manufacture in large‐scale applications. Four different concentrator panels have been experimentally verified at outdoors to identify the non‐identical current–voltage (I–V) curves. The I–V curve, fill factor and solar to electrical conversion efficiency of four PRIDE concentrator modules have been evaluated from the 24 manufactured in the ‘IDEOCONTE’ project. The maximum solar to electrical conversion efficiency and the fill factor of the PRIDE concentrator were 9·1 and 70%, respectively. The mismatch loss of the ‘unit concentrators’ has been identified that occurred due to the lack of bonding between the concentrator unit and the solar cell and the rear glass. The average power concentration ratio of PRIDE concentrators manufactured by the improved method was 2·10 compared to a similar non‐concentrating panel and the optical efficiency of the PRIDE system was 83%. Copyright © 2008 John Wiley & Sons, Ltd.

Development of a protocol to measure volatile organic compounds in human breath: a comparison of rebreathing and on-line single exhalations using proton transfer reaction mass spectrometry

Analysis of volatile organic compounds (VOCs) on human breath has great potential as a non-invasive diagnostic technique. It is, therefore, surprising that no single, standard procedure has evolved for breath sampling. Here we present a novel repeated-cycle isothermal rebreathing method, where one cycle comprises five rebreaths, which could be adopted for breath analysis of VOCs. For demonstration purposes, we present measurements of three common breath VOCs: isoprene, acetone and methanol. Their concentrations measured in breath are shown to increase with number of rebreaths until a plateau value is reached by at least 20 rebreaths. The average ratio of plateau concentration to single mixed expired breath concentration was found to be 1.92 ± 0.57 for isoprene, 1.25 ± 0.13 for acetone and 1.12 ± 0.12 for methanol (mean ± standard deviation). Measurements from on-line single exhalations are presented which demonstrate a positive slope in the time-dependent expirograms of isoprene and acetone. The slope of the isoprene expirogram is persistently linear and the end-expired concentration of isoprene is highly variable in the same subject depending on the duration of exhalation. End-expired values of acetone are not as sensitive to the length of exhalation, and are the same to within measurement uncertainty for any duration of exhalation for any subject. It is concluded that uncontrolled single on-line exhalations are not suitable for the reliable measurement of isoprene in the breath and that rebreathing can be the basis of an easily tolerated protocol for the reliable collection of breath samples.

DISSOLVED FRACTION OF STANDARD LABORATORY CLADOCERAN FOOD ALTERS TOXICITY OF WATERBORNE SILVER TO CERIODAPHNIA DUBIA

The biotic ligand model (BLM) for the acute toxicity of cationic metals to aquatic organisms incorporates the toxicity-modifying effects of dissolved organic matter (DOM), but the default parameterization (i.e., assuming 10% of DOM is humic acid) does not differentiate DOM from different sources. We exposed a cladoceran (Ceriodaphnia dubia) to Ag in the presence of DOM from filtered YCT (standard yeast–Cerophyll®–trout chow food recommended by the U.S. Environmental Protection Agency [EPA] for cladocerans), from the Suwannee River (GA, USA; relatively little anthropogenic input), and from the Desjardins Canal in Hamilton (ON, Canada; receives treated municipal wastewater effluent). In all three treatments, the dissolved organic carbon (DOC) concentration was 2 mg/L (the concentration following addition of YCT slurry at the U.S. EPA–recommended volume ratio). The average 48-h median effects concentration (EC50) ratios for dissolved Ag in the presence and absence of DOM [i.e., (EC50 with DOM)/(EC50 without DOM)] were as follows: Suwannee River, 1.6; Desjardins Canal, 2.2; and YCT filtrate, 26.8. Therefore, YCT filtrate provided much more protection against Ag toxicity than that provided by DOM from the surface waters. The major spectral characteristic that differentiated YCT filtrate from the other two types of DOM was a strong tryptophan peak in the excitation–emission matrix for YCT. These results have important implications for interpreting Ag toxicity tests in which organisms are fed YCT, and they suggest BLM-calculated toxicity predictions might be improved by incorporating specific chemical constituents or surrogate indices of DOM. Another component of the protective effect against Ag toxicity, however, might be that the dissolved fraction of YCT served as an energy and/or nutrient source for C. dubia.

The development and application of a mass balance model for mercury (total, elemental and methyl) using data from a remote lake (Big Dam West, Nova Scotia, Canada) and the multi-species multiplier method

Abstract Big Dam West, located in Kejimkujik Park, Nova Scotia, Canada, is a remote lake with elevated Hg concentrations in fish due in part to low pH and high dissolved organic carbon (DOC) concentrations. These features reflect the poor buffering capacity of peat lands in the flat drainage basin. To address the multiple species of Hg, a model was developed by coupling the “multiplier method” for multi-species chemicals with average concentration ratios to the Quantitative Water Air Sediment Interaction (QWASI) model. Elemental Hg was the “key species” modeled. Total Hg fluxes and concentrations were computed using concentration ratios. Many of the concentrations and Hg flux processes within the Big Dam West catchment had been previously measured with concentration ratios that were either computed or taken from the literature when measured data were not available. Measured values for total Hg concentration in each compartment (air, water, sediment) and Hg fluxes (e.g., precipitation, sediment deposition) enable verification of model concentration and flux estimates for total Hg in each environmental compartment. The model was also run with and without the inclusion of upward Hg fluxes from the underlying sediment to determine the significance of this controversial flux. The Hg QWASI model presented in this paper could be a valuable screening tool, especially for remote lakes, due to its ability to provide reasonable Hg estimates with a limited amount of data (water inflow rate, suspended particulate matter, sediment deposition velocity, and concentration of Hg in atmosphere and inflow water).

Semivolatile Fluorinated Organic Compounds in Asian and Western U.S. Air Masses

Semivolatile fluorinated organic compounds (FOCs) were measured in archived air sample extracts collected from Hedo Station Observatory (HSO) on Okinawa, Japan and Mount Bachelor Observatory (MBO), Oregon U.S. during the springs of 2004 (MBO and HSO) and 2006 (MBO). Fluorotelomer alcohols (FTOHs) were measured in both Asian and western U.S. air masses, though western U.S. air masses had significantly higher concentrations. Concentrations of fluorotelomer olefins in Asian air masses and 8:2 fluorotelomer acrylate in U.S. air masses were reported for the first time. N-ethyl perfluorooctane sulfonamide, N-methyl perfluorooctane sulfonamido ethanol, and N-ethyl perfluorooctane sulfonamido ethanol were also measured in Asian and western U.S. air masses but less frequently than FTOHs. The atmospheric sources and fate of FTOHs were investigated by estimating their atmospheric residence times, calculating FTOH concentration ratios, investigating FTOH correlations with nonfluorinated semivolatile organic compound concentrations, and determining air mass back trajectories. Estimated atmospheric residence times for 6:2 FTOH, 8:2 FTOH, and 10:2 FTOH were 50, 80, and 70 d, respectively, and the average concentration ratio of 6:2 FTOH to 8:2 FTOH to 10:2 FTOH at MBO in 2006 was 1.0 to 5.0 to 2.5. The relative order of these atmospheric residence times may explain the observed enhancement of 8:2 FTOH and 10:2 FTOH (relative to 6:2 FTOH) at MBO compared to North American indoor air (FTOH average ratio of 1.0 to 2.0 to 1.0). FTOH concentrations in western U.S. air masses were positively correlated (p < 0.05) with gas-phase polycyclic aromatic hydrocarbon and polychlorinated biphenyl concentrations and negatively correlated (p < 0.05) with agricultural pesticide concentrations. In comparison to western U.S. air masses, trans-Pacific air masses did not contain elevated concentrations of these compounds, whereas lower boundary layer air masses that passed over urban areas of the western U.S. did. This suggests that semivolatile FOCs are emitted from urban areas in the western U.S.

Analysis and design consideration of mean temperature differential Stirling engine for solar application

This article presents a technical innovation, study of solar power system based on the Stirling dish (SD) technology and design considerations to be taken in designing of a mean temperature differential Stirling engine for solar application. The target power source will be solar dish/Stirling with average concentration ratio, which will supply a constant source temperature of 320 °C. Hence, the system design is based on a temperature difference of 300 °C, assuming that the sink is kept at 20 °C. During the preliminary design stage, the critical parameters of the engine design are determined according to the dynamic model with losses energy and pressure drop in heat exchangers was used during the design optimisation stage in order to establish a complete analytical model for the engine. The heat exchangers are designed to be of high effectiveness and low pressure-drop. Upon optimisation, for given value of difference temperature, operating frequency and dead volume there is a definite optimal value of swept volume at which the power is a maximum. The optimal swept volume of 75 cm 3 for operating frequency 75 Hz with the power is 250 W and the dead volume is of 370 cm 3.

Dissolved Fraction of Standard Laboratory Cladoceran Food Alters Toxicity of Waterborne Silver to Ceriodaphnia dubia

The biotic ligand model (BLM) for the acute toxicity of cationic metals to aquatic organisms incorporates the toxicity-modifying effects of dissolved organic matter (DOM), but the default parameterization (i.e., assuming 10% of DOM is humic acid) does not differentiate DOM from different sources. We exposed a cladoceran (Ceriodaphnia dubia) to Ag in the presence of DOM from filtered YCT (standard yeast-Cerophyll(R)-trout chow food recommended by the U.S. Environmental Protection Agency [EPA] for cladocerans), from the Suwannee River (GA, USA; relatively little anthropogenic input), and from the Desjardins Canal in Hamilton (ON, Canada; receives treated municipal wastewater effluent). In all three treatments, the dissolved organic carbon (DOC) concentration was 2 mg/L (the concentration following addition of YCT slurry at the U.S. EPA-recommended volume ratio). The average 48-h median effects concentration (EC50) ratios for dissolved Ag in the presence and absence of DOM [i.e., (EC50 with DOM)/(EC50 without DOM)] were as follows: Suwannee River, 1.6; Desjardins Canal, 2.2; and YCT filtrate, 26.8. Therefore, YCT filtrate provided much more protection against Ag toxicity than that provided by DOM from the surface waters. The major spectral characteristic that differentiated YCT filtrate from the other two types of DOM was a strong tryptophan peak in the excitation- emission matrix for YCT. These results have important implications for interpreting Ag toxicity tests in which organisms are fed YCT, and they suggest BLM-calculated toxicity predictions might be improved by incorporating specific chemical constituents or surrogate indices of DOM. Another component of the protective effect against Ag toxicity, however, might be that the dissolved fraction of YCT served as an energy and/or nutrient source for C. dubia.

Deposition of Polybrominated Diphenyl Ethers, Polychlorinated Biphenyls, and Polycyclic Aromatic Hydrocarbons to a Boreal Deciduous Forest

The atmospheric deposition of several groups of semi-volatile organic compounds to a deciduous forest in Canada was determined using an indirect technique based on ratios of measured canopy interception and air concentrations. Air (gas and particle phase) and bulk deposition were sampled for 14 months from October 2001 to December 2002 at both a forest and a nearby clearing, and extracts were quantified for polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). Long-term average dry deposition velocities for vapors and particle-bound species were then derived for the canopy growing period. The mean dry gaseous deposition velocity for PBDEs and PCBs to the Canadian deciduous forest was 2.7 +/- 0.52 cm x s(-1), which is similar to the only other measured value for a deciduous canopy. Particle-bound deposition velocities to the canopy due to diffusion and impaction were 0.8 cm x s(-1) for the PBDEs and 0.11 cm x s(-1) for the PAHs. Differences in the particle-bound deposition velocities between PBDEs and PAHs and between deciduous canopies in Canada and Germany are explainable by differences in particle size distribution. The interception/concentration ratios for several PAHs were too low to be interpretable as dry gaseous deposition velocities. This is likely because the measured deposition flux under the canopy was less than the deposition flux to the canopy, possibly as a result of photodegradation in the canopy. From the ratio of canopy interception and average gas-phase concentration of less chlorinated PCBs, a predictive relationship between the canopy/air partition coefficient KPA and the octanol/air partition coefficient KOA was derived (KPA = 110 KOA0.67). Despite differences in local climate and canopy composition and structure, the deposition velocities and the canopy uptake capacity measured in Canada were remarkably similar to those reported in Germany, lending credibility to the suggestion that high gaseous deposition velocities are common throughout boreal and temperate deciduous forests. These extraordinarily high deposition velocities of semi-volatile organic compounds to deciduous forest canopies are at the core of the hypothesis of a significant filter effect of forests on a regional and global scale.

Cesium-137 partitioning to wetland sediments and uptake by plants

Studies were undertaken to develop a conceptual model for conducting ecological risk assessment at the R-Canal, located on the Savannah River Site, Aiken, South Carolina. Soil profiles collected from within the canals indicated that the maximum 137 Cs concentrations generally occurred at a depth between 2.5 and 7.5 cm. The original 137 Cs deposits, the result of accidental releases in the early 1960’s, were covered by newly formed organic and/or fluvial deposits. About 60% of the total 137 Cs inventory in each core existed at the organic matter/mineral interface. Based on sequential extraction tests, most of the Cs in this layer was held by either amorphous Fe-oxyhydroxides or organic matter. Sequential extraction tests also indicated that almost half of the Cs was weakly bound to the soil (either exchangeable or associated with amorphous Fe-oxides), a percentage that is five times greater than reported at the Hanford Site. 137 Cs distribution within the profile was controlled more when the 137 Cs was released than by the composition (organic matter concentrations, mineralogy, particle size distribution) of the soil. Netted chain fern had greater plant : soil concentration ratios in drier terrestrial environments than in wetland environments. The cause may be related to the Cs existing in a more bioavailable form or to the better overall health of the plants under drier soil conditions. The average concentration ratio was greater for the netted chain fern 2.78±2.96 than for rice cutgrass 0.18±0.13. The overall average plant : soil concentration ratio, 1.05±2.01, was large and points out, yet again, that Cs bioavailability is greater on the SRS than at most other sites.

Change in Corneal Permeability to Timolol After Laser In Situ Keratomileusis and Photorefractive Keratectomy in Rabbit

To evaluate the change in corneal permeability to timolol 1 month after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) in rabbits. LASIK or PRK was performed on one eye of each of 20 rabbits. One month later, 20 microl of 0.5% timolol was instilled into the 10 LASIK eyes, 10 PRK eyes, and the 20 control contralateral eyes. One hour later, 150 microl of aqueous humor was collected from all eyes, and the concentration of timolol in the aqueous humor was measured by high-performance liquid chromatography. The effect of LASIK and PRK on the corneal permeability to timolol was analyzed. The average timolol concentrations in the aqueous humor of the ten eyes 1 month after LASIK and in the ten contralateral eyes were 1.16 +/- 0.58 microg/ml and 1.07 +/- 0.39 microg/ml (average +/- SD, P = 0.678), respectively. The average timolol concentrations in the aqueous humor of the ten eyes 1 month after PRK and in the ten contralateral eyes were 1.24 +/- 0.43 microg/ml and 1.12 +/- 0.30 microg/ml (P = 0.483), respectively. The average ratio of timolol concentration in the aqueous humor of the LASIK or PRK eyes 1 month after surgery to that of the contralateral normal eyes was 1.18 +/- 0.58 or 1.19 +/- 0.67 (P = 0.972), respectively. LASIK and PRK do not affect corneal permeability to timolol 1 month after surgery.