Air flow and concentration fields at urban road intersections for improved understanding of personal exposure

This paper reviews the state of knowledge on modelling air flow and concentration fields at road intersections. The first part covers the available literature from the past two decades on experimental (both field and wind tunnel) and modelling activities in order to provide insight into the physical basis of flow behaviour at a typical cross-street intersection. This is followed by a review of associated investigations of the impact of traffic-generated localised turbulence on the concentration fields due to emissions from vehicles. There is a discussion on the role of adequate characterisation of vehicle-induced turbulence in making predictions using hybrid models, combining the merits of conventional approaches with information obtained from more detailed modelling. This concludes that, despite advancements in computational techniques, there are crucial knowledge gaps affecting the parameterisations used in current models for individual exposure. This is specifically relevant to the growing impetus on walking and cycling activities on urban roads in the context of current drives for sustainable transport and healthy living. Due to inherently longer travel times involved during such trips, compared to automotive transport, pedestrians and cyclists are subjected to higher levels of exposure to emissions. Current modelling tools seem to under-predict this exposure because of limitations in their design and in the empirical parameters employed.     

Sulfur Control In Coal Fired Power Plants: A Probabilistic Approach to Policy Analysis

The optimum level of sulfur pollution control for a coal fired power plant is the point where the sum of societal costs, due to pollution, and control costs is minimized. This basic microeconomic concept has been of limited practical value due to considerable uncertainty in estimating both costs. A probabilistic approach is used to characterize these uncertainties quantitatively for a hypothetical 1000 Mw_e plant located near Pittsburgh, Pennsylvania. Only mortality effects within a distance of 80 km of the plant have been included. The results allow explicit consideration of attitude toward risk and appropriate level of investment to prevent deaths. Limitations of the findings are discussed. Implications are described for policy based on alternative sets of values and assumptions.     

Formation and Removal Reactions of Hazardous Air Pollutants

Current regulatory policies for hazardous air pollutants (HAPs) target the sources of direct emissions. In addition to direct emissions, some of the aromatic, nitrogenated, and oxygenated HAPs can be formed in the atmosphere. Formaldehyde and acetaldehyde, in particular, are produced by almost every hydrocarbon photooxidation reaction. Estimates have been made that, in some urban areas, in situ formation contributes as much as 85 percent of the ambient levels of formaldehyde and 95 percent for acetaldehyde. Over 40 percent of the HAPs being regulated under Title III of the 1990 Clean Air Act Amendments have atmospheric lifetimes of less than one day. The transformation products of these HAPs with low atmospheric persistence are important for assessing risks to human health, especially for cases where the transformation products are more toxic than the HAP itself.     

Temperature and air velocity effects on ethanol emission from corn silage with the characteristics of an exposed silo face

Volatile organic compounds (VOCs) from agricultural sources are believed to be an important contributor to tropospheric ozone in some locations. Recent research suggests that silage is a major source of VOCs emitted from agriculture, but only limited data exist on silage emissions. Ethanol is the most abundant VOC emitted from corn silage; therefore, ethanol was used as a representative compound to characterize the pattern of emission over time and to quantify the effect of air velocity and temperature on emission rate. Ethanol emission was measured from corn silage samples removed intact from a bunker silo. Emission rate was monitored over 12 h for a range in air velocity (0.05, 0.5, and 5 m s^?1) and temperature (5, 20, and 35 °C) using a wind tunnel system. Ethanol flux ranged from 0.47 to 210 g m^?2 h^?1 and 12 h cumulative emission ranged from 8.5 to 260 g m^?2. Ethanol flux was highly dependent on exposure time, declining rapidly over the first hour and then continuing to decline more slowly over the duration of the 12 h trials. The 12 h cumulative emission increased by a factor of three with a 30 °C increase in temperature and by a factor of nine with a 100-fold increase in air velocity. Effects of air velocity, temperature, and air-filled porosity were generally consistent with a conceptual model of VOC emission from silage. Exposure duration, temperature, and air velocity should be taken into consideration when measuring emission rates of VOCs from silage, so emission rate data obtained from studies that utilize low air flow methods are not likely representative of field conditions.     

Ethanol emission from loose corn silage and exposed silage particles

Silage on dairy farms has been identified as a major source of volatile organic compound (VOC) emissions. However, rates of VOC emission from silage are not accurately known. In this work, we measured ethanol (a dominant silage VOC) emission from loose corn silage and exposed corn silage particles using wind tunnel systems. Flux of ethanol was highest immediately after exposing loose silage samples to moving air (as high as 220 g m^?2 h^?1) and declined by as much as 76-fold over 12 h as ethanol was depleted from samples. Emission rate and cumulative 12 h emission increased with temperature, silage permeability, exposed surface area, and air velocity over silage samples. These responses suggest that VOC emission from silage on farms is sensitive to climate and management practices. Ethanol emission rates from loose silage were generally higher than previous estimates of total VOC emission rates from silage and mixed feed. For 15 cm deep loose samples, mean cumulative emission was as high as 170 g m^?2 (80% of initial ethanol mass) after 12 h of exposure to an air velocity of 5 m s^?1. Emission rates measured with an emission isolation flux chamber were lower than rates measured in a wind tunnel and in an open setting. Results show that the US EPA emission isolation flux chamber method is not appropriate for estimating VOC emission rates from silage in the field.     

Participant-based monitoring of indoor and outdoor nitrogen dioxide,volatile organic compounds,and polycyclic aromatic hydrocarbons among MICA-Air households

The Mechanistic Indicators of Childhood Asthma (MICA) study in Detroit, Michigan introduced a participant-based approach to reduce the resource burden associated with collection of indoor and outdoor residential air sampling data. A subset of participants designated as MICA-Air conducted indoor and outdoor residential sampling of nitrogen dioxide (NO₂), volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs). This participant-based methodology was subsequently adapted for use in the Vanguard phase of the U.S. National Children’s Study. The current paper examines residential indoor and outdoor concentrations of these pollutant species among health study participants in Detroit, Michigan.Pollutants measured under MICA-Air agreed well with other studies and continuous monitoring data collected in Detroit. For example, NO₂ and BTEX concentrations reported for other Detroit area monitoring were generally within 10–15% of indoor and outdoor concentrations measured in MICA-Air households. Outdoor NO₂ concentrations were typically higher than indoor NO₂ concentration among MICA-Air homes, with a median indoor/outdoor (I/O) ratio of 0.6 in homes that were not impacted by environmental tobacco smoke (ETS) during air sampling. Indoor concentrations generally exceeded outdoor concentrations for VOC and PAH species measured among non-ETS homes in the study. I/O ratios for BTEX species (benzene, toluene, ethylbenzene, and m/p- and o-xylene) ranged from 1.2 for benzene to 3.1 for toluene. Outdoor NO₂ concentrations were approximately 4.5 ppb higher on weekdays versus weekends. As expected, I/O ratios pollutants were generally higher for homes impacted by ETS.These findings suggest that participant-based air sampling can provide a cost-effective alternative to technician-based approaches for assessing indoor and outdoor residential air pollution in community health studies. We also introduced a technique for estimating daily concentrations at each home by weighting 2- and 7-day integrated concentrations using continuous measurements from regulatory monitoring sites. This approach may be applied to estimate short-term daily or hourly pollutant concentrations in future health studies.     

Analytical model for site-specific isotope fractionation in 13C during sorption: determination by isotopic 13C NMR spectrometry with vanillin as model compound

The aim of this study was to conceive a reactive transport model capable of providing quantitative site-specific enrichment factors for fractionation in ¹³C isotopic content during sorption. As test compound the model treats vanillin, for which the ¹³C isotopic content at natural abundance at each of the 8 carbon positions can be measured by quantitative ¹³C nuclear magnetic resonance spectrometry. This technique determines the isotope ratios with a resolution better than ±1‰ (0.1%) at each carbon position. Site-specific isotope fractionations were recorded in chromatography column experiments with silica RP-18 as stationary phase. The one dimensional reactive transport model accounted for the sorption/desorption behavior of 8 individual ¹³C-isotopomers and one ¹²C-isotopomer of vanillin and reproduced satisfactorily the bulk (average over the whole compound) fractionation observed during elution. After model calibration, the enrichment factors were fitted for each carbon site where a significant fractionation was recorded. To show the interest of such a transport model for environmental studies, the model, extended to three dimensions, was exploited to simulate reactive transport in an aquifer. These results show that significant ¹³C isotope fractionation is expected for 4 out of 8 ¹³C-isotopomers in vanillin, and illustrate that bulk isotope ratios measured by conventional compound specific isotope analysis and mass spectrometry would hardly document significant isotope fractionations in vanillin. It is concluded that modeling of site-specific isotope ratios in molecules is a priori feasible and may help to quantify unknown processes in the environment.     

Temporal variability in dynamic and colloidal metal fractions determined by high resolution in situ measurements in a UK estuary

In recent environmental legislation, such as the Water Framework Directive in the European Union (WFD, 2000/60/EC), the importance of metal speciation and biological availability is acknowledged, although analytical challenges remain. In this study, the Voltammetric In situ Profiler (VIP) was used for high temporal resolution in situ metal speciation measurements in estuarine waters. This instrument simultaneously determines Cd, Cu and Pb species within a size range (ca. <4 nm) that is highly relevant for uptake by organisms. The colloidal metal fraction can be quantified through a combination of VIP measurements and analyses of total dissolved metal concentrations.VIP systems were deployed over tidal cycles in a seasonal study of metal speciation in the Fal Estuary, southwest England. Total dissolved concentrations were 4.97-315 nM Cu, 0.13-8.53 nM Cd and 0.35-5.75 nM Pb. High proportions of Pb (77 ± 17%) and Cu (60 ± 25%) were present as colloids, which constituted a less important fraction for Cd (37 ± 30%). The study elucidated variations in the potentially toxic metal fraction related to river flow, complexation by organic ligands and exchanges between dissolved and colloidal phases and the sediment. Based on published toxicity data, the bioavailable Cu concentrations (1.7-190 nM) in this estuary are likely to severely compromise the ecosystem structure and functioning with respect to species diversity and recruitment of juveniles. The study illustrates the importance of in situ speciation studies at high resolution in pursuit of a better understanding of metal (bio)geochemistry in dynamic coastal systems.     

Transcriptional profiles induced by the Aryl Hydrocarbon Receptor agonists 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran and 2,3,4,7,8-pentachlorodibenzofuran in primary rat hepatocytes

Toxicogenomics was used to examine mRNA expression profiles obtained from primary rat hepatocytes treated for 24 h with 0.01 or 1.0 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), 0.02 or 2.0 nM 2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PeCDF) and 0.1 or 10 nM 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-TCDF). The concentrations of 2,3,4,7,8-PeCDF and 2,3,7,8-TCDF were chosen to be equivalent to 2,3,7,8-TCDD’s concentration based on the toxic equivalency factor/toxic equivalent (TEF/TEQ) method for estimating biological potency. 2,3,7,8-TCDD at 1.0 nM altered the expression of 533 genes; 2,3,4,7,8-PeCDF at 2.0 nM altered 182 genes, and 2,3,7,8-TCDF at 10 nM altered 154 genes. Of these, 57 genes were affected by all three congeners. Agglomerative hierarchical clustering revealed distinct congener-dependent gene subclusters. Principal components analyses of the microarray data revealed that these congeners cluster independently of one another. Data presented here demonstrate that equivalent TEQ concentrations of 2,3,7,8-TCDD, 2,3,4,7,8-PeCDF and 2,3,7,8-TCDF, while altering the expression of a small battery of genes in common, also produce substantial congener specific alterations in gene expression.     

Preparation and X-ray structural characterization of further stereoisomers of 1,2,5,6,9,10-hexabromocyclododecane

Technical 1,2,5,6,9,10-hexabromocyclododecane (HBCD) consists largely of three diastereomers (α-, β-, and γ-HBCD) produced by the trans addition of bromine to cis,trans,trans-cyclododeca-1,5,9-triene (CDT). However, another seven diastereomers are theoretically possible and may be produced by trans addition of bromine across the double bonds of the other three isomers of 1,5,9-CDT. There are indications that small amounts of the minor HBCD isomers may be present in commercial HBCD mixtures or in products containing this brominated flame retardant (BFR). Such minor components may indeed derive from traces of other 1,5,9-CDTs in the cis, trans, trans starting material, however their formation may also be possible through isomerizations during the processing of this BFR or by bioisomerization subsequent to its release into the environment. Two of the seven additional diastereomers (δ- and ε-HBCD) were synthesized previously from trans,trans,trans-CDT. We now report the preparation of the remaining five diastereomers, ζ-, η-, and θ-HBCD from cis,cis,trans-CDT and ι- and κ-HBCD from cis,cis,cis-CDT, and their characterization by ¹H NMR spectroscopy and X-ray crystallography. The availability of these further diastereomers of HBCD should aid in determining if the minor isomers are present in commercial samples of this BFR, in products containing HBCDs, or in environmental samples. We have also carried out an X-ray crystal structure determination on ε-HBCD, so that crystal structures are now available for all 10 HBCD diastereomers.