Computational fluid dynamic modeling of two passive samplers

To effectively use a passive sampler for monitoring trace contaminants in the gas-phase, its sampling characteristics as a function of ambient wind conditions must be known. In this study two commonly used passive samplers were evaluated using computational fluid dynamics. Contaminant uptake by the polyurethane foam (PUF) was modeled using a species transport model. The external-internal flow interactions in the sampler were characterized, and the uptake rates of contaminant species were quantified. The simulations show that flow fields in the samplers have strong velocity gradients, and single-point velocity measurements do not capture flow interactions accurately. Sampling rates calculated for a PUF in freestream are in good agreement with sampling rates for PUFs in the passive samplers studied for the same average velocity over the PUF. The calculated sampling rates are in general agreement with those obtained experimentally by other researchers.     

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.     

Performance evaluation of a tailor-made passive sampler for monitoring of tropospheric ozone

Introduction

This study presents the performance evaluation of a tailor-made passive sampler developed for the monitoring of tropospheric ozone.

Methods

The performance of the passive sampler was tested in the field conditions in terms of accuracy, precision, blank values, detection limit, effects of some parameters such as sampling site characteristics and sampling period on the field blanks, self-consistency, experimental and theoretical uptake rates, shelf life and comparison with commercial passive samplers.

Results

There was an agreement (R ²?=?0.84) between the responses of passive sampler and the continuous automatic analyser. The accuracy of the sampler, expressed as percent relative error, was obtained lower than 15%. Method precision in terms of coefficient of variance for three simultaneously applied passive samplers was 12%. Sampler detection limit was 2.42???g?m^?3 for an exposure period of 1?week, and the sampler can be stored safely for a period of up to 8?weeks before exposure. Satisfactory self-consistency results showed that extended periods gave the same integrated response as a series of short-term samplers run side by side. The uptake rate of ozone was found to be 10.21?mL?min^?1 in a very good agreement with the theoretical uptake rate (10.32?mL?min^?1). The results of the comparison study conducted against a commercially available diffusion tube (Gradko diffusion tube) showed a good linear relationship (R ²?=?0.93) between two passive samplers.

Conclusions

The sampler seems suitable to be used in large-scale measurements of ozone where no data are available or the number of existing automated monitors is not sufficient.     

Passive air sampling theory for semivolatile organic compounds

The mathematical modelling underlying passive air sampling theory can be based on mass transfer coefficients or rate constants. Generally, these models have not been inter-related. Starting with basic models, the exchange of chemicals between the gaseous phase and the sampler is developed using mass transfer coefficients and rate constants. Importantly, the inter-relationships between the approaches are demonstrated by relating uptake rate constants and loss rate constants to mass transfer coefficients when either sampler-side or air-side resistance is dominating chemical exchange. The influence of sampler area and sampler volume on chemical exchange is discussed in general terms and as they relate to frequently used parameters such as sampling rates and time to equilibrium. Where air-side or sampler-side resistance dominates, an increase in the surface area of the sampler will increase sampling rates. Sampling rates are not related to the sampler/air partition coefficient (K(SV)) when air-side resistance dominates and increase with K(SV) when sampler-side resistance dominates.     

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.     

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.     

Passive sampler for PM10-2.5 aerosol

This study investigates the use of a small passive sampler for aerosol particles to determine particulate matter (PM)10-2.5 concentrations in outdoor air. The passive sampler collects particles by gravity, diffusion, and convective diffusion onto a glass coverslip that is then examined with an optical microscope; digital images are processed with free software and the resultant PM10-2.5 concentrations determined. Both the samplers and the analyses are relatively inexpensive. Passive samplers were collocated with Federal Reference Method (FRM) samplers in Chapel Hill, NC; Phoenix, AZ; and Birmingham, AL; for periods from 5 to 15 days. Particles consisted primarily of inorganic dusts at some sites and a mix of industrial and inorganic materials at other sites. Measured concentrations ranged from < 10 microg/m3 to approximately 40 microg/m3. Overall, PM10-2.5 concentrations measured with the passive samplers were within approximately 1 standard deviation of concentrations measured with the FRM samplers. Concentrations determined with passive samplers depend on assumptions about particle density and shape factors and may also depend somewhat on local wind speed and turbulence; accurate values for these parameters may not be known. The degree of agreement between passive and FRM concentrations measured here suggests that passive measurements may not be overly dependent on accurate knowledge of these parameters.     

Calibration and field performance of membrane-enclosed sorptive coating for integrative passive sampling of persistent organic pollutants in water

Membrane-enclosed sorptive coating (MESCO) is a miniaturised monitoring device that enables integrative passive sampling of persistent, hydrophobic organic pollutants in water. The system combines the passive sampling with solventless preconcentration of organic pollutants from water and subsequent desorption of analytes on-line into a chromatographic system. Exchange kinetics of chemicals between water and MESCO was studied at different flow rates of water, in order to characterize the effect of variable environmental conditions on the sampler performance, and to identify a method for in situ correction of the laboratory-derived calibration data. It was found that the desorption of chemicals from MESCO into water is isotropic to the absorption of the analytes onto the sampler under the same exposure conditions. This allows for the in situ calibration of the uptake of pollutants using elimination kinetics of performance reference compounds and more accurate estimates of target analyte concentrations. A field study was conducted to test the sampler performance alongside spot sampling. A good agreement of contaminant patterns and water concentrations was obtained by the two sampling techniques.     

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.     

Calibration of the Chemcatcher passive sampler for the monitoring of priority organic pollutants in water

An integrative passive sampler consisting of a C18 Empore disk receiving phase saturated with n-octanol and fitted with low-density polyethylene diffusion membrane was calibrated for the measurement of time-weighted average concentrations of hydrophobic micropollutants, including polyaromatic hydrocarbons and organochlorine pesticides, in water. The effect of temperature and water turbulence on kinetic and thermodynamic parameters characterising the exchange of analytes between the sampler and water was studied in a flow-through system under controlled conditions. It was found that the absorption of test analytes from water to the sampler is related to their desorption to water. This allows for the in situ calibration of the uptake of pollutants using offload kinetics of performance reference compounds. The sampling kinetics are dependent on temperature, and for most of the tested analytes also on the flow velocity. Sampler-water partition coefficients did not significantly change with temperature.     

Uptake of PAHs into polyoxymethylene and application to oil-soot (lampblack)-impacted soil samples

Polyoxymethylene (POM) is a polymeric material used increasingly in passive sampling of hydrophobic organic contaminants such as PAHs and PCBs in soils and sediments. In this study, we examined the sorption behavior of 12 PAH compounds to POM and observed linear isotherms spanning two orders of magnitude of aqueous concentrations. Uptake kinetic studies performed in batch systems for up to 54 d with two different volume ratios of POM-to-aqueous phase were evaluated with coupled diffusion and mass transfer models to simulate the movement of PAHs during the uptake process and to assess the physicochemical properties and experimental conditions that control uptake rates. Diffusion coefficients of PAHs in POM were estimated to be well correlated with diffusants' molecular weights as D(POM) proportional, variant(MW)(-3), descending from 2.3 x 10(-10) cm(2) s(-1) for naphthalene to 7.0 x 10(-11) cm(2) s(-1) for pyrene. The uptake rates for PAHs with log K(ow)<5.8 were controlled by the POM phase and the hydrophobicity of PAH compounds. For more hydrophobic PAH compounds, the aqueous boundary layer played an increasingly important role in determining the overall mass transfer rate. The POM partitioning technique was demonstrated to agree well with two other procedures for measuring PAH soil-water distribution coefficients in oil-soot (lampblack) containing soil samples.     

Distribution of ozone and other air pollutants in forests of the Carpathian Mountains in central Europe.

Ozone (O3) concentrations were monitored during the 1997-1999 growing seasons in 32 forest sites of the Carpathian Mountains. At all sites (elevation between 450 and 1320 m) concentrations of O3, nitrogen dioxide (NO2), and sulfur dioxide (SO2) were measured with passive samplers. In addition, in two western Carpathian locations, Vychodna and Gubalówka, ozone was continuously monitored with ultraviolet (UV) absorption monitors. Highest average hourly O3 concentrations in the Vychodna and Guba?ówka sites reached 160 and 200 microg/m3 (82 and 102 ppb), respectively (except for the AOT40 values, ozone concentrations are presented as microg/m3; and at 25 degrees C and 760 mm Hg, 1 microg O3/m3 = 0.51 ppb O3). These sites showed drastically different patterns of diurnal 03 distribution, one with clearly defined peaks in the afternoon and lowest values in the morning, the other with flat patterns during the entire 24-h period. On two elevational transects, no effect of elevation on O3 levels was seen on the first one, while on the other a significant increase of O3 levels with elevation occurred. Concentrations of O3 determined with passive samplers were significantly different between individual monitoring years, monitoring periods, and geographic location of the monitoring sites. Results of passive sampler monitoring showed that high O3 concentrations could be expected in many parts of the Carpathian range, especially in its western part, but also in the eastern and southern ranges. More than four-fold denser network of monitoring sites is required for reliable estimates of O3 distribution in forests over the entire Carpathian range (140 points). Potential phytotoxic effects of O3 on forest trees and understory vegetation are expected on almost the entire territory of the Carpathian Mountains. This assumption is based on estimates of the AOT40 indices for forest trees and natural vegetation. Concentrations of NO2 and SO2 in the entire Carpathian range were typical for this part of Europe and below the expected levels of phytotoxicity.     

Characterization of ambient volatile organic compounds at a landfill site in Guangzhou,South China

Ambient air monitoring was conducted at Datianshan landfill, Guangzhou, South China in 1998 to investigate the seasonal and horizontal variations of trace volatile organic compounds (VOCs). Twelve sampling points over the Datianshan landfill were selected and samples were collected simultaneously using Carbontrap(TM) adsorption tubes. Thirty eight VOCs were detected in the winter, whereas 60 were detected in the summer. The VOC levels measured in summer were alkanes, 0.5-6.5 microg/m(3); aromatics, 2.3-1667 microg/m(3); chlorinated species, 0.2-31 microg/m(3); terpines, 0.1-34 microg/m(3); carbonyl species, 0.3-5.6 microg/m(3) and naphthalene and its derivatives, 0.4-27 microg/m(3). Compared to the summer samples the VOC levels in winter were much lower (mostly 1-2 orders of magnitude lower). The aromatics are dominant VOCs in landfill air both in winter and summer. High levels of alkylbenzene and terpines such as methyl-isopropylbenzene (max 1667 microg/m(3)) and limonene (max 162 microg/m(3)) cause undesirable odor. The similar correlation coefficients of BTEX in summer and winter suggest VOCs emissions were from landfill site sources. The variation of BTEX ratio at landfill site is different from that in the urban area of Guangzhou. It shows that the ambient VOCs at landfill site were different from the urban areas.     

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 and field application of a solvent-based cellulose membrane passive sampling device for the monitoring of polar herbicides

A passive sampler device selective for hydrophilic analytes was constructed from cellulose membrane (40microm thickness) pre-stained with ruthenium red for 96-168h to impede degradation of the cellulose. The sampling device consisted of pre-stained cellulose membrane tubing containing a binary mixture of the solvents 1-dodecanol and 2,2,4-trimethylpentane as the sequestering medium. A laboratory flow-through system was used to investigate the rates of uptake of herbicides into the solvent mixture of the device and their release. The target herbicides were diuron, atrazine, metolachlor and molinate. Uptake of the herbicides into the solvent mixture of the cellulose membrane device was linear for up to 22 days, and daily sampling rates were determined. Release half-lives from the solvent mixture of the sampling device varied from 14 days for diuron, 15 days for atrazine, 84 days for metolachlor and 28 days for molinate. A field study was undertaken to determine if herbicide concentrations in agricultural drainage water derived from the passive sampler devices deployed for periods from 7 to 22 days, using the laboratory-derived sampling rates, would compare closely with time-weighted average herbicide concentrations determined from extractions of daily composite water samples. The concentrations of diuron, atrazine, metolachlor and molinate determined using the cellulose membrane devices were within twofold of the cumulated mean of the daily drainage water extractions.     

Determination of deployment specific chemical uptake rates for SDB-RPD Empore disk using a passive flow monitor (PFM)

The use of the adsorbent styrenedivinylbenzene-reverse phase sulfonated (SDB-RPD) Empore disk in a chemcatcher type passive sampler is routinely applied in Australia when monitoring herbicides in aquatic environments. One key challenge in the use of passive samplers is mitigating the potentially confounding effects of varying flow conditions on chemical uptake by the passive sampler. Performance reference compounds (PRCs) may be applied to correct sampling rates (R_s) for site specific changed in flow and temperature however evidence suggests the use of PRCs is unreliable when applied to adsorbent passive samplers. The use of the passive flow monitor (PFM) has been introduced for the assessment of site-specific changes in water flow. In the presented study we have demonstrated that the R_s at which both atrazine and prometryn are accumulated within the SDB-RPD-Empore disk is dependent on the flow conditions. Further, the calibration of the measured R_s for chemical uptake by the SDB-RPD-Empore disk to the mass lost from the PFM has shown that the PFM provides an accurate measure of R_s for flow velocities from 0 to 16 cm s⁻¹. Notably, for flow rates >16 cm s⁻¹, a non linear increase in the R_s of both herbicides was observed which indicates that the key resistance to uptake into the SDB-RPD Empore disk is associated with the diffusion through the overlying diffusion limiting membrane. Overall the greatest uncertainty remains at very low flow conditions, which are unlikely to often occur in surface waters. Validation of the PFM use has also been undertaken in a limited field study.     

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.     

Calibration of the Chemcatcher passive sampler for monitoring selected polar and semi-polar pesticides in surface water

Passive sampling is a powerful method for continuous pollution monitoring, but calibration experiments are still needed to generate sampling rates in order to estimate water concentrations for polar compounds. We calibrated the Chemcatcher device with an uncovered SDB-XC Empore disk as receiving phase for 12 polar and semi-polar pesticides in aquatic environments in flow-through tank experiments at two water flow velocities (0.135 m/s and 0.4 m/s). In the 14-day period of exposure the uptake of test substances in the sampler remained linear, and all derived sampling rates R(s) were in the range of 0.1 to 0.5 L/day. By additionally monitoring the release of two preloaded polar pesticides from the SDB-XC disks over time, very high variation in release kinetics was found, which calls into question the applicability of performance reference compounds. Our study expands the applicability of the Chemcatcher for monitoring trace concentrations of pesticides with frequent occurrence in water.     

A multi-variate statistical model integrating passive sampler and meteorology data to predict the frequency distributions of hourly ambient ozone (O3) concentrations

A multi-variate, non-linear statistical model is described to simulate passive O3 sampler data to mimic the hourly frequency distributions of continuous measurements using climatologic O3 indicators and passive sampler measurements. The main meteorological parameters identified by the model were, air temperature, relative humidity, solar radiation and wind speed, although other parameters were also considered. Together, air temperature, relative humidity and passive sampler data by themselves could explain 62.5-67.5% (R(2)) of the corresponding variability of the continuously measured O3 data. The final correlation coefficients (r) between the predicted hourly O3 concentrations from the passive sampler data and the true, continuous measurements were 0.819-0.854, with an accuracy of 92-94% for the predictive capability. With the addition of soil moisture data, the model can lead to the first order approximation of atmospheric O3 flux and plant stomatal uptake. Additionally, if such data are coupled to multi-point plant response measurements, meaningful cause-effect relationships can be derived in the future.