Grantee Research Project Results

Final Report: A Carrier for Quantitative Shipment of Coarse Particle Filter Samples

EPA Contract Number: EPD04013
Title: A Carrier for Quantitative Shipment of Coarse Particle Filter Samples
Investigators: Hering, Susanne
Small Business: Aerosol Dynamics Inc.
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $69,250
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text |  Recipients Lists
Research Category: Particulate Matter , SBIR - Air Pollution , Small Business Innovation Research (SBIR) , Air Quality and Air Toxics

Description:

Concern for the potential health effects of inhaled particulates led to a federal standard (PM₁₀, for particles smaller than 10 µm), and subsequently to a fine particle standard (PM_2.5, for particles smaller than 2.5 µm). The U.S. Environmental Protection Agency currently is conducting research that could lead to an independent coarse particle standard that would cover the size range 2.5 µm to 10 µm. Coarse particle filters require analysis by gravimetry and chemistry. Inherent with filter sampling is the need for transport of filters from field sites to centralized laboratories for analysis. It has been reported that coarse particle filters can suffer 30-35 percent losses in the U.S. Mail system. Another report found 10-53 percent losses when filters in a padded box were dropped upside-down. The goal of this research project was to reduce this serious source of error by developing a low-loss transport device for coarse particle filters.

Aerosol Dynamics, Inc., investigated the feasibility of clamping particles onto filters by the application of a diverging electrostatic field. According to physics theory, an electric field induces an electric dipole on a particle while the divergence of the field produces a net force on the particle. With the proper geometry, the electrical force would clamp particles to the filter, reducing or eliminating losses. The particles would not be charged or chemically altered by the field. An unpublished experiment in 1981 found that losses of glass bead particles from filters could be reduced by this method.

For testing, laboratory-generated aerosols were sampled onto 47 mm diameter Teflon filters. Up to 12 filters at a time then were placed with the particle deposit facing downward in a “drop test” apparatus developed for this research project. The filters were in plastic petri dishes held on a circular platform mounted on a shaft with bearings, allowing the platform to be dropped onto a surface. This subjected the filters to nearly identical impact accelerations. An accelerometer was used to measure the magnitude and time dependence of the acceleration as a function of drop height. Some of the filters were in diverging electric fields produced by wire screens, while other filters were used as controls under no electric field.

Experiments were conducted with glass bead aerosols to verify the principle of the particle clamp. Most of the measurements were with Arizona Road Dust, which provides more realistic coarse particles. Parameters that were varied included the wire screen mesh; electrode geometry; voltage, which ranged from -10,000 V to +25,000 V; particle loading; and drop height. A total of 133 filter samples were tested.

Summary/Accomplishments (Outputs/Outcomes):

Filters loaded with glass beads and dropped while in a diverging electric field showed approximately 12 percent lower loss compared to controls, verifying the principle of particle clamping. The reduction in loss, however, was much less than in the 1981 unpublished work. Possible reasons for the difference could be humidity of the air used in the aerosol generator for the present work (RH 3 to 7 percent) and possibly a difference in the electrical conductivity of the particles.

Surprisingly, filters loaded with Arizona Road Dust showed increased losses under a diverging electric field. The ratio of losses with field to no field averaged 1.6 + 0.3. After extensive investigation with varied parameters, it was concluded that electrical conductivity of the Arizona Road Dust particles resulted in electrostatic forces tending to separate the particles from the filter. Evidence for this conclusion included an enhancement of the pattern of particle deposits on the lid of the petri dish. When the voltage applied to the screen behind the filter was positive, the loss was much less than with negative voltage, as expected because of the mobility of negative (electron) charges. The glass bead results also suggest a dependence on conductivity of the particles.

Theoretically, an isolated conducting particle would experience a clamping force in a diverging electric field. If the particle is in contact with other particles or the filter surface, however, electrons can flow, inducing a net charge on the particle. As a result, the particle will experience a force from the electric field much greater than the force from field divergence. A conducting particle also can receive electrons from other particles, resulting in much greater charge. Unfortunately, the direction of the force resulting from induced charge is inherently away from the filter, increasing losses. Particles in ambient air will undoubtedly have sufficient conductivity to induce charges in a high electrostatic field. Therefore, particle clamping is not feasible for filter samples of ambient aerosol.

Further experimentation was carried out in an attempt to utilize the strong electrostatic force on conducting particles to redeposit dislodged particles onto the filter. The strategy was to have two electrodes with voltages of opposite sign behind the filter. Upon impact, a particle ejected from an area of the filter above an electrode with positive voltage will carry a positive induced charge. It then will be attracted to an area of the filter above the electrode with negative voltage and be redeposited on the filter. A similar argument applies to particles above the negative electrode. Measurements with such an arrangement were found to cut the particle losses by more than 50 percent.

Conclusions:

Clamping glass bead particles to a filter by the application of a diverging electric field was found to reduce the losses occurring under impact accelerations. Losses of Arizona Road Dust particles, however, were increased by the electric field. This was attributed to electric conductivity of the Arizona Road Dust particles, leading to the induction of net charge on the particles and a strong electric force tending to expel the particles from the filter. It therefore is concluded that the particle clamp approach to reducing particle losses cannot be applied to ambient aerosol because of the expected electrical conductivity of the particles.

Although an arrangement using the electrostatic force on the induced particle charge showed some promise, further development work would be required to optimize the device design to determine whether particle losses could be reduced by more than the 50 percent achieved in the initial trial. Because of the dependence of electrostatic devices on particle conductivity, a definitive evaluation would require testing with actual ambient aerosol samples. At present, there is insufficient evidence upon which to base an assessment of commercial potential of this device.

There are some methods that could be implemented in the near term to reduce transport losses of coarse particle samples. One method involves sandwiching the particle deposit by using a second filter during transport after sampling. The filters would be weighed together, both before and after sampling. This would eliminate particle losses without significant increase in weighing errors and does not require any additional weighing. Another practical measure would involve the development of standard sample packaging for shipping. Protocols for impact and vibration testing of packages have been developed by parcel services. The final evaluation would require sending test samples by mail.

Supplemental Keywords:

Coarse particle filter samples, U.S. Mail, PM₁₀, PM_2.5, diverging electrostatic field, aerosol, electric field, Arizona Road Dust, coarse particles, fine particles, SBIR,, RESEARCH, RFA, Ecosystem Protection/Environmental Exposure & Risk, Scientific Discipline, Air, particulate matter, Air Quality, Environmental Chemistry, Engineering, Chemistry, & Physics, Analytical Chemistry, Monitoring/Modeling, Atmospheric Sciences, Environmental Monitoring, Monitoring, aerosol particles, chemical characteristics, particle sampler, air sampling, monitoring stations, microsensor, atmospheric chemistry, aersol particles, modeling studies, particulate matter mass, ambient emissions, aerosol analyzers, particle transport, air quality models, continuous emissions monitoring, electrostatic particle filter, human health effects, atmospheric measurements, modeling