A second look at the Palmes' diffusive sampler

The Palmes' tube, the first diffusive sampler incorporating a fixed path length, has received wide usage for the sampling of a large number of gaseous pollutants. But despite numerous previous studies, questions remain regarding the accuracy of these inexpensive, simple-to-construct, open-ended samplers. Here the mass transfer resistance in a Palmes' diffusive sampler was measured using the loss of cyclohexane from a Palmes' tube containing liquid cyclohexane at its base. The average loss rates, at factorial combinations of five air incidence angles evenly spaced from 270 degrees to 90 degrees, and five air speeds from 0.5 m/sec to 2.5 m/sec ranged from 46% to 121% higher than rates calculated from the physical dimensions of the sampler, proving the need to calibrate these samplers rather than relying on a theoretical calculation. The mass transfer resistance was nearly constant when the airflow was perpendicular to the sampler and sufficiently high to avoid stagnation, a finding that may explain the widespread acceptance of the results obtained using this sampler.     

PCB mass transfer coefficients determined by application of a water surface sampler

A water surface sampler (WSS) was employed in combination with greased surface deposition plates (GSDPs) to measure the particulate dry deposition and gas exchange of polychlorinated biphenyls (PCBs) in Chicago, IL. Vapor phase PCB fluxes were calculated by subtracting the particulate fluxes obtained from GSDPs from total (particulate+gas) fluxes obtained from the WSS. Vapor phase PCB fluxes were divided by ambient air concentrations measured with a high volume sampler to calculate overall gas phase PCB mass transfer coefficients (K(G)). The calculated average PCB MTC was 0.54+/-0.47 cm s(-1). This experimentally determined average gas phase overall mass transfer coefficient, K(G), agreed well with the ones reported from studies using similar techniques and agreed well with modeled values obtained using MTC correlations developed for the WSS.     

Evaluation of performance reference compounds in polyethylene-based passive air samplers

Polyethylene-based passive air samplers (PSDs) were loaded with performance reference compounds (PRCs) and deployed in a wind tunnel to examine the effects of wind speed on sampler performance. PRCs could be loaded reproducibly into PSDs, with coefficients of variation only exceeding 20% for the more volatile compounds. When PSDs were exposed to low (0.5-1.5 m s-1) and high (3.5-4.5 m s-1) wind speeds, PRC loss rate constants generally increased with increasing wind speed and decreased with increasing sampler/air partition coefficients. PSD-based air concentrations calculated using PRC loss rate constants and sampler/air partition coefficients and air concentrations measured using active samplers compared closely. PRCs can be used to account for the effect of differences in wind speeds on sampler performance and measure air concentrations with reasonable accuracy.     

Calibration of polyurethane foam (PUF) disk passive air samplers for quantitative measurement of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs): factors influencing sampling rates

PUF disk passive air samplers are increasingly employed for monitoring of POPs in ambient air. In order to utilize them as quantitative sampling devices, a calibration experiment was conducted. Time integrated indoor air concentrations of PCBs and PBDEs were obtained from a low volume air sampler operated over a 50 d period alongside the PUF disk samplers in the same office microenvironment. Passive sampling rates for the fully-sheltered sampler design employed in our research were determined for the 51 PCB and 7 PBDE congeners detected in all calibration samples. These values varied from 0.57 to 1.55 m3 d(-1) for individual PCBs and from 1.1 to 1.9 m3 d(-1) for PBDEs. These values are appreciably lower than those reported elsewhere for different PUF disk sampler designs (e.g. partially sheltered) employed under different conditions (e.g. in outdoor air), and derived using different calibration experiment configurations. This suggests that sampling rates derived for a specific sampler configuration deployed under specific environmental conditions, should not be extrapolated to different sampler configurations. Furthermore, our observation of variable congener-specific sampling rates (consistent with other studies), implies that more research is required in order to understand fully the factors that influence sampling rates. Analysis of wipe samples taken from the inside of the sampler housing, revealed evidence that the housing surface scavenges particle bound PBDEs.     

The effects of ozone/limonene reactions on indoor secondary organic aerosols

An indoor air quality model was used to predict dynamic particle mass concentrations based on homogeneous chemical mechanisms and partitioning of semi-volatile products to particles. The ozone–limonene reaction mechanism was combined with gas-phase chemistry of common atmospheric organic and inorganic compounds and incorporated into the indoor air quality model. Experiments were conducted in an environmental chamber to investigate secondary particle formation resulting from ozone/limonene reactions. Experimental results indicate that significant fine particle growth occurs due to the interaction of ozone and limonene and subsequent intermediate by-products. Secondary particle mass concentrations were estimated from the measured particle size distribution. Predicted particle mass concentrations were in good agreement with experimental results—generally within ∼25% at steady-state conditions. Both experimental and predicted results suggest that air exchange rate plays a significant role in determining secondary fine particle levels in indoor environments. Secondary particle mass concentrations are predicted to increase substantially with lower air exchange rates, an interesting result given a continuing trend toward more energy efficient buildings. Lower air exchange rates also shifted the particle size distribution toward larger particle diameters. Secondary particle mass concentrations are also predicted to increase with higher outdoor ozone concentrations, higher outdoor particle concentrations, higher indoor limonene emission rates, and lower indoor temperatures.     

Evaluation of the use of diffusive air samplers for determining temporal and spatial variation of volatile organic compounds in the ambient air of urban communities

The Houston-Galveston metropolitan area has a relatively high density of point and mobile sources of air toxics, and determining and understanding the relationship between emissions and ambient air concentrations of air toxics is important for evaluating potential impacts on public health and formulating effective regulatory policies to control this impact, both in this region and elsewhere. However, conventional ambient air monitoring approaches are limited with regard to expense, siting limitations, and representative sampling necessary for adequate exposure assessment. The overall goal of this multiphase study is to evaluate the use of simple passive air samplers to determine temporal and spatial variability of the ambient air concentrations of selected volatile organic compounds (VOCs) in urban areas. Phase 1 of this study, reported here, was a field evaluation of 3M organic vapor monitors (OVMs) involving limited comparisons with commonly used active sampling methods, an assessment of sampler precision, a determination of optimal sampling duration, and an investigation of the utility of a simple modification of the commercial sampler. The results indicated that a sampling duration of 72 hr exhibited generally low bias relative to automated continuous gas chromatography measurements, good overall precision, and an acceptable number of measurements above detection limits. The modified sampler showed good correlation with the commercial sampler, with higher sampling rates, although lower than expected.     

Passive sampling of ambient, gaseous air pollutants: an assessment from an ecological perspective

During some past two decades there has been a growing interest among air pollution-vegetation effects-scientists to use passive sampling systems for quantifying ambient, gaseous air pollutant concentrations, particularly in remote and wilderness areas. On the positive side, excluding the laboratory analysis costs, passive samplers are inexpensive, easy to use and do not require electricity to operate. Therefore, they are very attractive for use in regional-scale air quality assessments. Passive samplers allow the quantification of cumulative air pollutant exposures, as total or average pollutant concentrations over a sampling duration. Such systems function either by chemical absorption or by physical adsorption of the gaseous pollutant of interest onto the sampling medium. Selection of a passive sampler must be based on its known or tested characteristics of specificity and linearity of response to the chemical constituent being collected. In addition, the effects of wind velocity, radiation, temperature and relative humidity must be addressed in the context of absorbent/adsorbent performance and sampling rate. Because of all these considerations, passive samplers may provide under- or overestimations of the cumulative exposures, compared to the corresponding data from co-located continuous monitors or active samplers, although such statistical variance can be minimized by taking necessary precautions. On the negative side, cumulative exposures cannot identify short-term (相似文献   

Evaluation of the Use of Diffusive Air Samplers for Determining Temporal and Spatial Variation of Volatile Organic Compounds in the Ambient Air of Urban Communities

Abstract

The Houston-Galveston metropolitan area has a relatively high density of point and mobile sources of air toxics, and determining and understanding the relationship between emissions and ambient air concentrations of air toxics is important for evaluating potential impacts on public health and formulating effective regulatory policies to control this impact, both in this region and elsewhere. However, conventional ambient air monitoring approaches are limited with regard to expense, siting limitations, and representative sampling necessary for adequate exposure assessment. The overall goal of this multiphase study is to evaluate the use of simple passive air samplers to determine temporal and spatial variability of the ambient air concentrations of selected volatile organic compounds (VOCs) in urban areas. Phase 1 of this study, reported here, was a field evaluation of 3M organic vapor monitors (OVMs) involving limited comparisons with commonly used active sampling methods, an assessment of sampler precision, a determination of optimal sampling duration, and an investigation of the utility of a simple modification of the commercial sampler. The results indicated that a sampling duration of 72 hr exhibited generally low bias relative to automated continuous gas chromatography measurements, good overall precision, and an acceptable number of measurements above detection limits. The modified sampler showed good correlation with the commercial sampler, with higher sampling rates, although lower than expected.     

In-vivo passive sampling to measure elimination kinetics in bioaccumulation tests

The application of in-tissue passive sampling to quantify chemical kinetics in fish bioconcentration experiments was investigated. A passive sampler consisting of an acupuncture needle covered with a PDMS tube was developed together with a method for its deployment in rainbow trout. The time to steady state for chemical uptake into the passive sampler was >1 d, so it was employed as a kinetically limited sampler with a deployment time of 2 h. The passive sampler was employed in parallel with the established whole tissue extraction method to study the elimination kinetics of 10 diverse chemicals in rainbow trout. 4-n-nonylphenol and 2,4,6-tri-tert-butylphenol were close to or below the limit of quantification in the sampler. For chlorpyrifos, musk xylene, hexachlorobenzene, 2,5-dichlorobiphenyl and p,p′-DDT, the elimination rate constants determined with the passive sampler method and the established method agreed within 18%. Poorer agreement (35%) was observed for 2,3,4-trichloroanisole and p-diisopropylbenzene because fewer data were obtained with the passive sampling method due to its lower sensitivity. The work shows that in-tissue passive sampling can be employed to measure contaminant elimination kinetics in fish. This opens up the possibility of studying contaminant kinetics in individual fish, thereby reducing the fish requirements and analytical costs for the determination of bioconcentration factors.     

Irrigation model of bleached Kraft mill wastewater through volcanic soil as a pollutants attenuation process

An irrigation process through volcanic soil columns was evaluated for bleached Kraft mill effluent pollutants retention. The system was designed to remove color and phenolic compounds and a simple kinetic model for determining the global mass transfer coefficient and the adsorption rate constant was used. The results clearly indicate that the global mass transfer coefficient values (K(c)a) and the adsorption rate constants are higher for the irrigation processes onto acidified soil. This means that the pretreatment of washing the volcanic soil with an acid solution has a positive effect on the adsorption rate for both pollutant groups. The enhanced adsorption capacity is partially explained by the activation of the metal oxides present in the soil matrix during the acid washing process. Increasing the flow rate from 1.5 to 2.5 ml/min yielded higher (K(c)a) values and adsorption rate constants for both pollutant groups. For instance, regarding color adsorption onto acidified soil, there is an increment of 43% in the (K(c)a) value for the experiment with a flow rate of 2.5 ml/min. Increasing the porosity of the column from 0.55 to 0.59, yielded a decrease in the (K(c)a) values for color and phenolic compounds adsorption processes. Onto natural soil for example, these decreases reached 21% and 24%, respectively. Therefore, the (K(c)a) value is dependent on both the liquid-phase velocity (external resistance) and the soil fraction in the column (internal resistance); making forced convection and diffusion to be the main transport mechanisms involved in the adsorption process. Analyzing the adsorption rate constants (K(c)a)/m, phenolic compounds and color adsorption rates onto acidified soil of 2.25 x 10(-6) and 2.62 x 10(-6) l/mg min were achieved for experiment 1. These adsorption rates are comparable with other adsorption systems and adsorbent materials.     

The potential effect of differential ambient and deployment chamber temperatures on PRC derived sampling rates with polyurethane foam (PUF) passive air samplers

Performance reference compound (PRC) derived sampling rates were determined for polyurethane foam (PUF) passive air samplers in both sub-tropical and temperate locations across Australia. These estimates were on average a factor of 2.7 times higher in summer than winter. The known effects of wind speed and temperature on mass transfer coefficients could not account for this observation. Sampling rates are often derived using ambient temperatures, not the actual temperatures within deployment chambers. If deployment chamber temperatures are in fact higher than ambient temperatures, estimated sampler-air partition coefficients would be greater than actual partition coefficients resulting in an overestimation of PRC derived sampling rates. Sampling rates determined under measured ambient temperatures and estimated deployment chamber temperatures in summer ranged from 7.1 to 10 m³ day⁻¹ and 2.2-6.8 m³ day⁻¹ respectively. These results suggest that potential differences between ambient and deployment chamber temperatures should be considered when deriving PRC-based sampling rates.     

New passive samplers for chlorinated semivolatile organic pollutants in ambient air

Two new types of passive samplers were designed and tested on semivolatile organic compounds. The first type (a spiral-rod sampler) consists of a low-density polyethylene membrane acting as a permeation film and a silicone elastomer as the receiving material; the second (a stir-bar sampler) has the same membrane material but a polydimethylsiloxane-coated stir bar acting as the collector phase and installed radially symmetrically in the sampler. The advantages of the new samplers are their simple design, low costs, and their easy processing via thermodesorption coupled with capillary gas chromatography and mass selective detection. In both samplers, the uptake of selected analytes was integrative over exposure periods of up to 384 h. The sampling rates calculated from a laboratory calibration study using the chlorinated semivolatiles hexachlorobenzene, hexachlorocyclohexane isomers and polychlorinated biphenyls ranged from 88.1 ml h-1 for delta-hexachlorocyclohexane to 3443 ml h-1 for 2,2',5,5'-tetrachlorobiphenyl. A field trial at a hazardous waste dump near Bitterfeld, Germany, for up to 21 days combined with periodical determinations of air concentrations using low-volume sampling indicated that the new samplers can in principle be used in the field, although the sampling rates derived from the field results differed considerably from the laboratory findings. Nevertheless the preliminary results suggest that the new sampler types are promising for the long-term air monitoring of semivolatiles.     

Polycyclic aromatic hydrocarbon sampling in wastewaters using semipermeable membrane devices: accuracy of time-weighted average concentration estimations of truly dissolved compounds

Semipermeable membrane devices (SPMDs) previously spiked with performance reference compounds were exposed in wastewater. After 6 days of exposure, 13 polycyclic aromatic hydrocarbons (PAHs) were quantified in SPMDs. Exchange rate constants and time-weighted average (TWA) concentrations of SPMD-available PAHs in water were calculated. The bias of using SPMDs to estimate an actual TWA concentration if the concentration in water fluctuates, as can be expected in wastewater, was studied with numerical simulations. The bias increased with the exchange rate constant. However, most exchange rate constants evaluated in SPMDs exposed in wastewater were small enough for SPMDs to estimate a TWA concentration of PAHs even when the water concentration varied. TWA-SPMD-available concentrations were always below total dissolved (operationally defined as 0.7 microm) concentrations, indicating that part of the dissolved PAHs was not available for sampling. In situ partitioning coefficients K(DOC) were computed and found to be slightly higher than data from the literature. This confirms that only truly dissolved PAHs should be sampled by SPMDs in wastewater.     

Field validation of an active sampling cartridge as a passive sampler for long-term carbonyl monitoring

A carbonyl sampler originally designed for the active sampling method (Sep-Pak XPoSure) was used for long-term passive sampling, and its applicability as a passive sampler was examined through field experiments. The uptake rates of passive sampling were determined experimentally from collocated passive and active samplings for various sampling periods. The obtained uptake rates of formaldehyde, acetaldehyde, and acetone were 1.48, 1.23, and 1.08 mL/min, respectively. These uptake rates were consistent for a wide range of the sampling term (12 hr-2 weeks). Uptake rates of each carbonyl were proportional to the diffusion coefficients of each. Therefore, the ratios of diffusion coefficients were used to calculate the uptake rates of carbonyls for which the rates were not determined experimentally. Lower limits of determination were 2.16-17.5 microg/m3 for 2-week sampling. It was confirmed that 2-week monitoring of carbonyl concentrations up to 118-229 microg/m3 was possible. Relative standard deviations of the passive method generated from the repeatability test were 2-12.3% error for five samplings, and the recovery efficiencies were larger than 90%. Thus, the passive sampler was found to be highly suitable for long-term monitoring of carbonyl compounds.     

Factors affecting mass transfer limited biodegradation in saturated porous media

Microbial degradation rates in the subsurface are not only limited by the physiological capacity of the organisms, but also by inefficient supply of nutrients to the microbes. Although mass transfer limitation of biodegradation in the subsurface has been postulated for years, experimental evidence is still scarce. In the column experiments described here, diffusive transport of 4-nitroanisole from the bulk solution to cells of Rhodococcus opacus strain AS2 immobilized on glass beads or sand appeared to be responsible for the slow transformation rates observed. Assuming steady state, we applied a coupled transformation/transport equation to these data (Best equation) and apparent bead-related mass transfer coefficients were found to increase in proportion to the surface area covered with bacteria. This implies that mass transfer coefficients for individual cells remained constant. In an idealized oligotrophic environment where cells are only loosely clustered and do not shield each other, we would therefore expect biodegradation rates to be independent from the longitudinal distribution of the total biomass along a given flow path. Moreover, apparent mass transfer coefficients increased with the grain size of the column fillings, but did not change upon varying the flow rate. With a limiting external transport step, overall transformation fluxes do not become saturated at concentrations as low as predicted for Michaelis-Menten-type kinetics. Mass transfer limitation thus offers a justification for the common assumption that biodegradation rates in the subsurface follow first order kinetics in a wide concentration range.     

An alternative method for predicting organic solute volatilization rates under gas and liquid turbulence

A method for predicting organic compound volatilization rates under turbulent liquid and gas conditions is developed. The reference compounds are classified according to their physico-chemical properties. The mass transfer coefficient (K(OL)) ratios for organic solutes to the reference compounds are constant for a wide range of environmental conditions, including liquid or air turbulence, or both at once. The obtained results indicate that when the environmental conditions are the same the determination of the volatilization rates is strongly dependent on the solute properties and the chemical structure. The presented method can more effectively estimate the volatilization rates of the solutes than the traditional one under various environmental conditions especially for low volatility solutes. The advantages and disadvantages of the traditional method are also discussed.     

A directional passive air sampler for monitoring polycyclic aromatic hydrocarbons (PAHs) in air mass

A passive air sampler was developed for collecting polycyclic aromatic hydrocarbons (PAHs) in air mass from various directions. The airflow velocity within the sampler was assessed for its responses to ambient wind speed and direction. The sampler was examined for trapped particles, evaluated quantitatively for influence of airflow velocity and temperature on PAH uptake, examined for PAH uptake kinetics, calibrated against active sampling, and finally tested in the field. The airflow volume passing the sampler was linearly proportional to ambient wind speed and sensitive to wind direction. The uptake rate for an individual PAH was a function of airflow velocity, temperature and the octanol-air partitioning coefficient of the PAH. For all PAHs with more than two rings, the passive sampler operated in a linear uptake phase for three weeks. Different PAH concentrations were obtained in air masses from different directions in the field test.     

Polyurethane foam (PUF) disks passive air samplers: wind effect on sampling rates

Different passive sampler housings were evaluated for their wind dampening ability and how this might translate to variability in sampler uptake rates. Polyurethane foam (PUF) disk samplers were used as the sampling medium and were exposed to a PCB-contaminated atmosphere in a wind tunnel. The effect of outside wind speed on PUF disk sampling rates was evaluated by exposing polyurethane foam (PUF) disks to a PCB-contaminated air stream in a wind tunnel over air velocities in the range 0 to 1.75 m s-1. PUF disk sampling rates increased gradually over the range 0-0.9 m s-1 at approximately 4.5-14.6 m3 d-1 and then increased sharply to approximately 42 m3 d-1 at approximately 1.75 m s-1 (sum of PCBs). The results indicate that for most field deployments the conventional 'flying saucer' housing adequately dampens the wind effect and will yield approximately time-weighted air concentrations.     

Modeling air/sea flux parameters in a coastal area: A comparative study of results from the TOGA COARE model and the NOAA Buoy model

Because estuaries and coastal regions are particularly susceptible to nutrient over-enrichment due to their close proximity to source-rich regions, a goal of the BRACE study was to improve estimates of nitrogen air/sea transfer rates in the Tampa Bay Estuary. Our objective was to critically evaluate two air/sea gas exchange models to determine their efficacy for use in a coastal region, with the ultimate goal of improving nitrogen exchange estimates in Tampa Bay. We used meteorological data and oceanographic parameters collected hourly at an instrumented tower located in Middle Tampa Bay, Florida. The data was used to determine the friction velocity and the turbulent flux of heat and moisture across the air/sea interface and then compared with modeled parameters at the same offshore site. On average both models underpredicted sensible heat flux and there was considerable scatter in the data during stable conditions, indicating that nitrogen gas exchange rates may also be underestimated. Model improvement, however, was observed with friction velocity comparisons. Model inter-comparisons of sensible heat flux and friction velocity suggest excellent agreement between the TOGA COARE and the NOAA Buoy models, but model estimated heat transfer coefficients and latent heat fluxes did not agree as well. Based on our analysis, we conclude that both models are suitable for use in a coastal environment to estimate nitrogen air/sea gas exchange, although the NOAA Buoy model requires fewer meteorological inputs. However, if the purpose is to conduct more sophisticated microscale modeling of air/sea interactions, we recommend the TOGA COARE model.     

The Development of a Sampling System for the Determination of Odor Emission Rates from Areal Surfaces: Part II. Mathematical Model

Abstract

An improved design for an odor emission hood was examined in the laboratory using ammonia emission from a water surface. The experimental ammonia convective mass transfer coefficients from a diluted ammonia solution were determined at velocities of 0.3 m/s to 0.8 m/s using the odor emission hood. The theoretical ammonia convective mass transfer coefficients were also predicted by boundary layer theory under laminar flow conditions. It was found that experimental data were only 10% below theoretical predictions at an air velocity of 0.3 m/s to 0.6 m/s. The maximum velocity was limited to 0.6 m/s by the geometric size, shape and aerodynamic stability of the hood. At 0.33 m/s, the smallest variation of mass transfer coefficients was measured. The odor emission rate was found to be a function of air velocity to the power 0.5 in accordance with boundary layer theory. An odor sampling system based upon the odor emission hood provides a way to quantify the potential odor emission strength in sewage treatment plants, for odor dispersion modeling and odor control.