1999 Progress Report: Development and Evaluation of a Novel Sampling Method to Determine the Phase Partitioning of Semi-Volatile Organic Compounds

EPA Grant Number: R825270
Title: 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 Period Covered by this Report: December 1, 1998 through November 30, 1999
Project Amount: $409,507
RFA: Air Quality (1996) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

The objectives of the research project are to: (1) develop a novel semi-volatile organic compound (SVOCs) sampler that is designed to minimize sampling biases; and (2) design and test a SVOC sampler that will collect and size particulate matter as well as gas phase SVOCs.

Progress Summary:

During the first year of this study, we developed a diffusion denuder for gas-phase SVOCs. This denuder is used indirectly to measure these species, by collecting filter samples with and without denuders, so that the difference in the amount collected without the denuder compared to that collected with the denuder is the amount originally present in the gas phase. In the first year, we also designed and partially characterized the performance of a three-stage inertial high volume impactor suitable for collecting particulate phase SVOCs that minimizes problems of both contamination and sampling artifacts. This impactor classifies and collects coarse, fine and ultrafine particles. We showed that it is possible to collect particles for longer periods than are feasible using previous methods, thereby achieving the sensitivity needed for chemical speciation of SVOCs.

Although the results of the first study year's laboratory tests demonstrated very high collection efficiency and capacity for the parallel plate diffusion denuder constructed using paper filters impregnated with activated carbon to remove gas-phase organics, these tests were performed, by necessity, with only one organic species for each test. In contrast, ambient air typically contains a very large number of different species, with very small amounts of any given species. Consequently, during the second year of this study, we performed tests with ambient air to demonstrate the effectiveness of the gas-phase organics denuder. The results showed that we could remove gas-phase organics with the denuder, collect particles on the Teflon filter, and measure volatilization of particles collected on the Teflon filter, using the PUF cartridge downstream of the Teflon filter.

In the second year of the study, we also completed the design of a three-stage inertial high volume slit impactor system which is particularly suitable for collecting size-fractionated particulate phase SVOC samples that minimizes problems of both contamination and sampling artifacts. The results of characterization studies showed that losses of particles are negligible over a wide range of particle aerodynamic diameters. The size cut-off curves are very steep, indicating that size classification is suitable for distinguishing small ranges of particle diameters. Particle collection efficiency is high enough for essentially quantitative collection of particles for sizes reasonably larger than the cutpoint. Because high sampler efficiency is maintained under conditions of heavy particle loading, it is possible to collect particles for longer periods than are feasible using previous methods.

During the third year of this study, we fully evaluated the use of polyurethane foam (PUF) as an impaction substrate for the SVOC sampler. Open pore PUF has been widely used for the collection of organic vapors downstream of a particle filter. Even though PUF has relatively large pore sizes, it can be used effectively as a substrate for conventional inertial impactors, with both high particle collection efficiency and minimal vaporization of semi-volatile particle components. The collection characteristics of PUF as an impaction substrate were studied as a function of PUF density, Reynolds number, impaction substrate diameter, and nozzle-to-plate distance. The conventional impaction substrate of the PM2.5 Harvard Impactor sampler was replaced with the PUF substrate. The use of PUF resulted in significant changes in the collection efficiency curve, with the 50 percent cut-off size (d50) decreasing from 2.48 to 1.12 µm, corresponding to the square root of the Stokes number ( Stk) = 0.24. While the theory for conventional flat impaction substrates accurately predicts d50 values (at Stk = 0.49), for PUF substrates this same theory predicts d50 values much larger than the experimentally determined values. After the collision of the particles with the PUF, a greater fraction of their excess kinetic energy may be absorbed by the substrate than is absorbed by conventional substrates, reducing the amount of particles that would otherwise bounce-off or be re-entrained. Qualitatively similar results were obtained for PUF densities between 1.9x104 and 5.0x104 g/m3. Results obtained for varying Reynolds numbers also suggest that the difference in collection efficiency curves between PUF and oil-coated substrates is due to different flow patterns. In addition, tests showed that overall impactor performance was better for larger impaction plate diameters for both PUF and conventional substrates. An experiment where the impaction surface was a part of a larger non-porous impaction plate showed lower collection efficiencies and less sharp cut-off curves. Finally, significant distortion of the collection efficiency curve was observed for larger nozzle-to-plate distance.

We also completed the final design of the High-Volume, Low-Cutoff Inertial Impactor for SVOC measurements during the third year of this study. This sampler uses a slit-shaped acceleration jet and operates at 1100 L/min. The impaction substrate is polyurethane foam (PUF). The impactor collection efficiency was characterized using polydisperse particles, and the 50 percent size cut-off point was 0.12 µm. Losses within the sampler were also characterized and were less than 10 percent. The use of polyurethane foam (PUF) as a substrate has the following advantages: (1) PUF has a very high particle collection efficiency over a large range of particle sizes, even under conditions of heavy particle loading, as compared to other impaction substrates, such as flat plates and less porous membranes, which typically are subject to significant bounce-off and re-entrainment; (2) no oil or grease coating is required, so potential interferences of impurities within such coatings are avoided when chemical, biological, and toxicological tests are performed on the collected particles; (3) PUF itself is chemically inert, minimizing interference with any of these tests; (4) because of the high flow rate of 1100 L/min, a large amount of particles can be collected in a short period of time on a relatively small surface of substrate, facilitating recovery of the collected particles for the different tests; and (5) a large amount of particles can be collected on a relatively small collection surface and easily extracted with small amounts of water or organic solvents. This method will be suitable for the collection of large amounts for toxicological studies and analysis of organic aerosols, which is not possible with other high volume samplers that utilize large filtration surfaces.

The last accomplishment of the third year of this study was the initial development of a multi-stage round slit nozzle cascade conventional inertial impactor. This system has all of the capabilities of the High-Volume Low-Cutoff Impactor, with other features which expand the capabilities for measuring SVOCs. Ambient air particles are separated into three or more size ranges, so that differences in properties can be investigated as a function of particle size. A size-selective inlet removes particles larger than 10 µm. The remaining particles can be collected using three or more stages, with respective size ranges of: (1) 2.5 to 10 µm (coarse); (2) 0.12 to 2.5 µm (fine); and (3) > 0.12 µm (ultrafine), with other cutpoints available for 1.0 and 0.2 µm. All sampler components are constructed of anodized aluminum and are supported by a light-weight aluminum rack. In order to obtain a large amount of particulate material in a relatively short time, the system has a flow of 900 liters/minute. There are two different configurations for the sampling system, the first has multiple parallel paths for the collection of ultrafine particles, while the second has a single third stage.

The size-selective inlet is preceded by a raincover, which prevents rain from entering the sampling system. Alternatively, air may be sampled indoors by attaching the first stage directly to an air intake ventilation duct. A round nozzle conventional slit impactor is used to remove particles >10 m on a polyurethane foam (PUF) impaction substrate. The first collection stage is connected with the size-selective inlet by a transition piece to minimize particle losses, and uses a second round nozzle slit impactor to collect coarse (2.5 to 10 m) particles on a second PUF impaction substrate. The second collection stage is connected with the first stage by a another transition piece, again to minimize particle losses, and uses a third round nozzle slit impactor to collect particles >0.12 m on a third PUF impaction substrate. The third collection stage, connected directly downstream of the third stage collects the remaining (ultrafine) particles with size > 0.12 m. The first configuration for this third stage has one or more filter packs connected in parallel. These filter packs can consist of any or all of the following: (1) a quartz fiber filter to collect and measure organic and inorganic carbonaceous particles; (2) a Teflon membrane filter to collect and measure trace metals; (3) a Teflon membrane filter to collect and measure inorganic ions; and (4) any other suitable filter substrate for measurement of particulate composition. The flow through these filter packs is 20 to 100 liters/min. The excess flow from the second stage is parallel to the filter packs, and passes through a high efficiency particle air (HEPA) filter so that the total flow is maintained at 900 liters/min. For the second configuration of the third stage, all of the sample air from the second stage passes directly through a HEPA filter at 900 liters/min.

Future Activities:

We will complete the development and characterization of the multi-stage round slit nozzle cascade conventional inertial impactor. Characterization tests will use both artificial aerosols in laboratory tests and field tests with ambient air particles. Laboratory test aerosols will include single species SVOCs. These tests will evaluate the overall performance of the sampler, including validation of particle losses, collection efficiency, and capacity, as well as ability to measure both gas and particulate phases of SVOC species.


Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other project views: All 14 publications 10 publications in selected types All 10 journal articles
Type Citation Project Document Sources
Journal Article Chang MC, Kim S, Sioutas C. Experimental studies on particle impaction and bounce: effects of substrate design and material. Atmospheric Environment 1999;33(15):2313-2322. R825270 (1999)
  • Abstract: Science Direct Abstract
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  • Journal Article Kavouras IG, Ferguson ST, Wolfson JM, Koutrakis P. Development and validation of a high-volume, low-cutoff inertial impactor. Inhalation Toxicology 2000;12(Suppl 2):35-50. R825270 (1999)
    R825270 (Final)
  • Abstract from PubMed
  • Abstract: Taylor and Francis-Abstract
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  • Journal Article Kavouras IG, Koutrakis P. Use of polyurethane foam as the impaction substrate/collection medium in conventional inertial impactors. Aerosol Science and Technology 2001;34(1):46-56. R825270 (1999)
    R825270 (Final)
  • Full-text: Taylor&Francis-Full Text PDF
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  • Abstract: Taylor&Francis-Abstract
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  • Supplemental Keywords:

    air, ambient air, atmosphere, tropospheric, exposure, toxics, PAHs, PCBs, environmental chemistry, measurement methods, Northeast, Atlantic coast, Massachusetts, MA, EPA Region 1., RFA, Scientific Discipline, Air, Geographic Area, particulate matter, air toxics, Environmental Chemistry, State, Atmospheric Sciences, Ecological Risk Assessment, ambient air quality, particle size, particulates, phase partitioning, air pollutants, collection efficiency, gas phase, aerosol partitioning, air quality criteria, ambient monitoring, chemical composition, PAH, Massachusetts (MA), air sampling, Volatile Organic Compounds (VOCs), chemical amalysis, atmospheric chemistry

    Progress and Final Reports:

    Original Abstract
  • 1997
  • 1998
  • Final Report