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A virtual impactor aerosol concentrator has been developed that uses circumferential slots for acceleration of aerosol particles and for collection of the coarse fraction. This allows for accurate and economical machining of small slot widths, which leads to lowpressure losses for the separation process. One important application of the device is in the concentration of bioaerosols, especially for military field applications where minimization of power consumption is necessary.A prototype configuration of the circumferential slot virtual impactor (CSVI), which was designed using numerical methods, was constructed and tested. The device has a curvilinear slit nozzle with a diameter of 150.3 mm (5.918 in), which provides a total slot length of 472 mm. Its slot width was 0.499 mm (0.0197 in). According to Loo and Cork, for circular-jet virtual impactors the misalignment between the axis of the acceleration jet and the receiver nozzle will cause an increase in wall losses of about 1.6% for each 1% of misalignment. Measurements were made of the nozzle dimensions in the critical region of the CSVI that showed 1.8% relative misalignment. When this prototype was operated at a flowrate of 122 l/min and a flow fraction (minor air flowrate/total air-flowrate) of 10%, the cutpoint was 2.2 µm aerodynamic diameter and the corresponding cutpoint Stokes number was 0.58. The collection efficiency was greater than 72% for particle sizes larger than twice the cutpoint, up to the largest particle size tested (≈10 µm aerodynamic diameter). The peak collection efficiency was greater than 95%. For virtual impactors, a critical performance parameter is the loss of particulate matter to the inner walls of the system. For the prototype system, where numerical methods had been used to generate designs that reduced wall losses, the losses at the cutpoint size of 2.2 µm aerodynamic diameter, are approximately 3%. For an operational condition of a total flowrate of 122 l/min and a coarse particle flow fraction of 10%, the pressure drop across the major flow stream (fine particle stream) was 63 Pa (0.25 in of water), with an ideal power consumption of 0.14 watts.

The objective of the present study was to characterize the performance of a federal reference method (FRM) PM 10 sizeselective inlet using analysis methods designed to minimize uncertainty in measured sampling efficiencies for large particles such as those most often emitted from agricultural operations. The performance of an FRM PM 10 inlet was characterized in a wind tunnel at a wind speed of 8 km/h. Data were also collected for 20 and 25 mm particles at wind speeds of 2 and 24 km/h. Results of the present sampler evaluation compared well with those of previous studies for a similar inlet near the cutpoint, and the sampler passed the criteria required for certification as a FRM sampler when tested at 8 km/h. Sampling effectiveness values for particles with nominal diameters of 20 and 25 mm exceeded 3% for 8 and 24 km/h wind speeds in the present study and were statistically higher than both the "ideal" PM 10 sampler (as defined in 40 CFR 53) and the ISO (1995) standard definition of thoracic particles (p < 0.05) for 25 mm particles leading to the potential for significant sampling bias relative to the "ideal" PM 10 sampler when measuring large aerosols. INTRODUCTIONExposure to high concentrations of thoracic aerosols has been linked to significant, negative health effects, including increased incidence of asthma, cardiovascular disease, mortality, and morbidity. The National Ambient Air Quality Standards (NAAQS) were established to protect public health and welfare by limiting the concentrations of ambient pollutants to which the public is exposed. The NAAQS for PM 10 is intended to protect the public against high concentrations of thoracic particles. Compliance with the NAAQS is demonstrated through use of ambient particulate matter (PM) monitors that measure 24-h integrated concentrations of pollutants. Specialuse monitors may also be placed at or near the property lines of some sources to ensure that emissions from that source do not lead to public exposure to concentrations exceeding the NAAQS.When monitoring PM 10 concentrations for regulatory purposes, it is important that measured concentrations of PM 10 are representative of concentrations of particles that have the potential to cause adverse health effects. PM 10 samplers are designed to allow those particles expected to penetrate past the larynx of the human respiratory system to penetrate the sampler precollector and deposit on a filter. The American Conference of Governmental Industrial Hygienists (ACGIH) has defined the "thoracic fraction" of particles, and that definition has been adopted as a standard by International Standards Organization (Figure 1; ISO 1995).In the Code of Federal Regulations (CFR), the U.S. Environmental Protection Agency (USEPA) has defined the performance of an "ideal" PM 10 sampler (Figure 1; 40 CFR 53.43) that "approximates the penetration of particles into the human respiratory tract" (40 CFR 53.40(d)). The fractional penetration, or sampling efficiency, of the "ideal" PM 10 sampler closely matches that of the ACGIH/IS...

Experimental data for velocity and tracer gas concentration profiles were collected at several downstream locations of an L-shaped configuration with different mixing elements. Results were presented in the coefficients of variation (COVs) to help determine appropriate locations for single-point sampling downstream of each duct configuration. Comparisons between experimental data and numerical simulations for velocity COVs were also presented.Different mixing elements were applied to the L-shaped configuration to create additional mixing to help provide an acceptable sampling location with low pressure drop for single-point sampling in a duct at less than four duct diameters from the mixing element. The mixing elements included a 90 o elbow, a tee, a horizontal generic-tee-plenum (H-GTP) system, and a vertical generic-tee-plenum (V-GTP) system. The COVs for velocity and gas concentration for the two GTP systems were all determined to be less than 13% as compared to a range of 6% to 89% for the 90 o elbow and a tee at four duct diameters in round and square ducts. Tests with two different sizes of GTPs were conducted and the results showed the performance of the GTPs to be relatively unaffected by either size or velocity as reflected by the Reynolds number. The pressure coefficient was approximately 0.82 for the H-GTPs, as compared to 5.2 for a previously designed generic mixing system (GMS). The GTPs can be useful in the design of biological and chemical sampling systems in air-conditioning ducts and for mixing in aerosol wind tunnels where uniform aerosol concentrations are needed at the test section.

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