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Final Report: Development and Evaluation of a Novel Sampling Method to Determine the Phase Partitioning of Semi-Volatile Organic Compounds
EPA Grant Number: R825270Title: Development and Evaluation of a Novel Sampling Method to Determine the Phase Partitioning of Semi-Volatile Organic Compounds
Investigators: Koutrakis, Petros , Sioutas, Constantinos
Institution: Harvard University
EPA Project Officer: Shapiro, Paul
Project Period: December 1, 1996 through November 30, 1999 (Extended to November 11, 2000)
Project Amount: $409,507
RFA: Air Quality (1996)
Research Category: Air Quality and Air Toxics
Description:
Objective:The originally specified objectives of the research project were to: (1) develop a novel semi-volatile organic compound (SVOC) sampler that is designed to minimize sampling biases; and (2) design and test a SVOC sampler that will collect and size particulate matter and gas phase SVOCs. The detailed features required to achieve these objectives can be summarized as follows: (1) separation of particulate and gas phases; (2) determination of SVOC particulate phase size distribution; (3) gas phase determinations of SVOCs in comparison with currently available sampling methods; (4) comparison of observed particle/gas partitioning with model predictions; and (5) phase distribution of individual SVOCs, as a function of particle size distribution.
Toward the end of the project period, additional research was conducted in the related area of developing technology to allow toxicological testing with atmospheres of concentrated ambient particles. This area of work falls within the Ambient Air Quality research category, and is particularly relevant to ambient air particulate matter, of which, SVOCs are one component. The following added objectives specifically addressed the concerns of minimizing the volatilization losses of both ultrafine and coarse particles during the process of producing concentrated exposure atmospheres of these two particle size ranges to: (1) design and test an ultrafine particle concentrator (UFPC); and (2) design and test a coarse particle concentrator (CPC).
Summary/Accomplishments (Outputs/Outcomes):This project had a range of features addressed to achieve the overall objectives of developing and testing a new sampling method for the determination of phase partitioning of SVOCs. The completed work addressed most of the original fundamental objectives of the project, and left some of the less challenging tasks for future work. In the following discussion, each of these essential features is addressed.
Separation of Particulate and Gas Phases. We attempted to develop and evaluate a novel sampling method to separate the particulate and gas phases of SVOCs. The goal was addressed by the design and construction of a parallel plate diffusion denuder system, which was validated by laboratory tests, and used for a field study to collect and measure certain classes of nonvolatile and semi-volatile organic particulate species. The laboratory evaluation included the determination of collection efficiency, precision, and capacity of a parallel flat multi-plate diffusion denuder for collecting several VOCs, and two SVOCs, thus completing the methodology that could be used for future studies of the originally planned tests of gas-phase SVOCs (specifically, PAHs and PCBs). The related goal to develop diffusion denuders that remove reactive gases (O3, HNO3, HONO, SO2) during sampling was left for future studies, but the methodology of appropriate coatings to remove these gases already has been thoroughly developed and tested, so the practical adaptations for the purposes of this project would not present a significant challenge.
Determination of SVOC Particulate Phase Size Distribution. We worked to develop and evaluate a sampler that would allow characterization of the SVOC particulate phase size distribution. The key challenge was to collect relatively large quantities (mg to g) of particles in different size ranges (i.e., coarse, fine, and ultrafine) so that trace concentrations of specific semi-volatile species, for short enough sample durations, would prove useful in the investigation of acute health effects, and assist in determining relative contributions of different sources. These objectives were achieved by the development and validation of a novel polyurethane foam (PUF) inertial impaction substrate. The PUF substrate largely overcame the limitations of previously used substrates for collection of size-selected ambient air particles by conventional inertial impaction. To take advantage of the new sampling capabilities of the PUF substrate material, a High Volume Cascade Impactor (HVCI) with a flow of 900 liters/minute (LPM) was designed, developed, and validated. The HVCI allows for collection of particles for the typical ranges of coarse (2.5-10 µm), fine (0.15-2.49 µm), and ultrafine (less than 0.1 µm), using circular slit nozzles for acceleration/impaction. For appropriate measurement goals (which require substantially smaller amounts of collected particles), a portable personal cascade impactor sampler with a flow of only 5 LPM was later developed and tested, which allows for a collection of particles with a similar set of size ranges as the HVCI.
Gas Phase Determinations of SVOCs. There were two challenges to overcome to successfully measure the gas phase concentrations of SVOCs. The first challenge was the initial separation of the particulate phase species from the gas phase species. In an ideal case, a diffusion denuder would be used to remove the gas phase species, while allowing the particles to pass through, as well as to be able to measure the gas phase species directly. No practical denuder coating/composition was found that could both remove and allow for quantitative analysis of the collected species. However, the successful development of the parallel plate denuder described above allows for quantitative determination of the gas phase concentrations of the SVOCs by the diffusion denuder difference technique. This technique uses simultaneous sampling with and without the diffusion denuder. For both sample trains, there is collection of both particle and gas phase (the gas phase in the sample with the diffusion denuder comes from vaporization of the semi-volatile species collected on the impaction substrates and a particle after-filter). The original gas phase concentration of a given species is calculated by subtracting the total amount of the same species collected with the denuder from the total amount of this species collected with the denuder.
The second challenge was to validate the method used to collect the gas phase SVOCs downstream of the particle collection stages. Although the originally planned tests of capacity and collection efficiency of the sorbent particles for target PCBs and PAHs for sorbent beds to be used for collecting the particulate and gas phases of SVOCs were not conducted, the methodologies of these tests would be similar to those developed in laboratory tests for gas phase species alone, and thus do not represent a significant challenge for future work. A related goal was to develop analytical procedures for fast and efficient extraction of both particulate and gas phase SVOCs for subsequent chemical analysis. The major limitation imposed by previous methods was the use of a relatively large volume of collection filter/impaction substrate. Use of the PUF material developed in this project significantly reduces this volume, and thus simplifies the extraction process.
Comparison With Currently Available Sampling Methods. We conducted a projected field study that compared the new method with currently available sampling methods such as filter packs, conventional diffusion denuders, and low pressure impactors. In retrospect, the setting of these particular goals may have been overambitious, particularly with respect to suitability and validity of the available techniques. At the same time that this project was being conducted, the conventional filter pack method, which uses two quartz fiber filters in sequence, with the objective of distinguishing between vapor and particle phases of organic species, was found by independent research to be invalid.
Comparison of Observed Particle/Gas Partitioning With Model Predictions. We attempted to make a comparison for the observed particle/gas partitioning for PAHs and PCBs with predictions of previously developed theoretical models. This goal may have been overambitious; however, the limitations for achieving this goal were not in the method itself, but in the availability of human resources and opportunities to conduct the tests, given priorities that were chosen for each of the other objectives. It is clear, nonetheless, that the methodology developed and validated in this work can provide an adequate basis for a valid comparison of the existing theoretical models, with actual partition coefficients of SVOCs in ambient air, in any future studies that may be conducted.
Phase Distribution of Individual SVOCs, as a Function of Particle Size Distribution. We worked to develop a new technology that would provide accurate, sensitive, and unbiased information on the phase distribution of individual SVOCs, as a function of particle size distribution, with negligible bias by sampling artifacts. There are several features of the methodology developed in this project, each of which has been discussed above, that illustrate how this objective has been accomplished reasonably well.
Achievement of sufficient accuracy is based on quantitative determination of individual SVOC species concentrations in each phase, and on adequate separation between particles in each of the selected size ranges. Adequate sensitivity requires both a sufficiently high collection flow, as well as substrate extraction features with minimum solvent and time requirements. The minimization of bias in phase distribution is achieved using a highly efficient diffusion denuder, and by not only having sharp impaction cut-points, but also by minimizing particle bounce and re-entrainment.
The sampling artifacts not avoided by the above-described features of the new methodology include potential volatilization of particle-phase semi-volatile species from the PUF impaction substrates. Although few tests were performed to determine the extent of this volatilization, there are three features of the methodology that act to minimize this process. First, due to the boundary layer above the impaction substrate, there is considerably less volatilization than there would be for particles collected on a filter with sample air passing through. Second, with the relatively high sampling flow, the duration of sampling can be low enough to allow for minimal vaporization. Third, perhaps a bit less conclusively, the PUF impaction substrate material itself is a strong trap for vapor phase organic species, especially for semi-volatiles. The feature of the methodology reported here that especially deserves future investigation is the potential for inadequate downstream trapping of species that volatilize from the filter used to collect ultrafine particles.
The potential for positive sampling artifacts resulting from adsorption of gas phase SVOCs on filter media, which could occur with previously used filter pack methods, is not a feature of the methodology developed here, because the gas phase species are removed prior to particle collection. Also, as described above, sampling artifacts resulting from interaction of collected particulates with reactive gases can be avoided with suitably coated diffusion denuders that remove these gases upstream from particle collection.
Overview of Accomplishments for Additional Objectives
Ultrafine Particle Concentrator. We attempted to design and validate a method that would minimize volatilization of particulate SVOCs and other semi-volatile components of ultrafine particles during the process of enriching particle concentrations (while maintaining the particles airborne) to make it feasible to perform toxicological exposure tests with ambient ultrafine particles. This goal was accomplished, in part, by using a novel technique (steam injection), which, due to the latent heat of vaporization of water, allows the humidity to be increased to near saturation (at room temperature) with an increase in temperature of less than 2°C. With the air containing the ultrafine particles at near saturation, a heat exchanger is used to rapidly cool the air, achieving supersaturation conditions that cause condensational growth of the particles to supermicron sizes. A virtual impactor is then used to increase the particle concentration. Because the pressure drop in the impactor is very low, and the residence time is very short, a negligible amount of vaporization of SVOCs occurs in passage through the impactor. The minor flow of the impactor, containing the concentrated particles, is passed through a thermal dryer, with a maximum temperature of 30°C, to return the particles to their original size distribution. Because this is a relatively small elevation of the temperature of the particles, for only a very brief period of time prior to passage through an exposure chamber for the toxicological tests, only a minimal evaporation of SVOCs and other semi-volatile particle components occurs.
Harvard Coarse Particle Concentrator. We worked to minimize volatilization of particulate SVOCs and other semi-volatile components of coarse particles during the process of enriching particle concentrations (while maintaining the particles airborne) to make it feasible to perform toxicological exposure tests with ambient coarse particles. It was easier to accomplish this goal for coarse particles than for ultrafine particles because for the coarse particles, only a virtual impactor is needed to achieve concentration enrichment. Because there is a minimal pressure drop in the impactor, and because the residence time is very short, only a minimal amount of SVOCs and other semi-volatile species is expected to vaporize during passage through the impactor and during passage to the exposure chamber for toxicological tests.
Conclusions:This research project achieved many of the original objectives associated with measuring gas and particle phase components of SVOCs, and also was able to accomplish other important related goals associated with SVOCs. In achieving these goals, the following issues were addressed successfully: (1) separation of particulate and gas phases using a high-volume novel parallel plate diffusion denuder (indirectly allowing measurement of both gas and particle phase SVOCs); (2) development of high-volume low cut-point conventional inertial impactor technology (slit-shaped acceleration nozzles) to effectively collect fine ambient particle SVOCs using minimum amounts of impaction substrate material; (3) development of an impaction substrate medium (polyurethane foam) most suitable for minimizing sampling errors and volatilization losses of SVOCs; (4) determination of SVOC particulate phase size distribution using a high-volume cascade impactor; (5) measurement of size distribution of particles for personal exposures; and (6) high concentration enrichment of ultrafine and coarse particles with minimal volatilization of particle phase SVOCs for use in toxicological studies of the health effects of ambient air particles.
Journal Articles on this Report: 4 Displayed | Download in RIS Format
| Other project views: | All 11 publications | 7 publications in selected types | All 7 journal articles |
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Demokritou P, Gupta T, Koutrakis P. A high volume apparatus for the condensational growth of ultrafine particles for inhalation toxicological studies. Aerosol Science and Technology 2002;36(11):1061-1072. |
R825270 (2000) R825270 (Final) |
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Demokritou P, Gupta T, Ferguson S, Koutrakis P. Development and laboratory performance evaluation of a personal cascade impactor. Journal of the Air & Waste Management Association 2002;52(10):1230-1237. |
R825270 (2000) R825270 (Final) |
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Demokritou P, Gupta T, Ferguson S, Koutrakis P. Development and laboratory characterization of a prototype coarse particle concentrator for inhalation toxicological studies. Journal of Aerosol Science 2002;33(8):1111-1123. |
R825270 (2000) R825270 (Final) R827353 (Final) R827353C017 (Final) |
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Demokritou P, Kavouras IG, Ferguson ST, Koutrakis P. Development of a high volume cascade impactor for toxicological and chemical characterization studies. Aerosol Science and Technology 2002;36(9):925-933. |
R825270 (2000) R825270 (Final) R827353 (Final) R827353C017 (Final) |
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air, ambient air, atmosphere, tropospheric, exposure, toxics, PAHs, PCBs, environmental chemistry, measurement methods, Northeast, Atlantic coast, EPA Region 1, air, geographic area, RFA, atmospheric sciences, ecological risk assessment, environmental chemistry, state, air toxics, particulate matter, Massachusetts, MA, volatile organic compounds, VOCs, aerosol partitioning, air pollutants, air quality criteria, air sampling, ambient air quality, ambient monitoring, atmospheric chemistry, chemical analysis, chemical composition, collection efficiency, gas phase, measurement methods, particle size, particulates. , Air, Geographic Area, Scientific Discipline, RFA, Ecological Risk Assessment, air toxics, Atmospheric Sciences, particulate matter, Environmental Chemistry, State, aerosol partitioning, air quality criteria, phase partitioning, collection efficiency, ambient air quality, Volatile Organic Compounds (VOCs), chemical composition, measurement methods, air sampling, chemical amalysis, air pollutants, particulates, gas phase, ambient monitoring, atmospheric chemistry, PAH, particle size
Progress and Final Reports:
1999 Progress Report
2000 Progress Report
Original Abstract