Large Particle Collection Characteristics of Ambient Aerosol Samplers James 6. Wedding*, Andrew R. McFarland’, and Jack E. Cermak Department of Civil Engineering, Colorado State University, Fort Collins, Colo. 80523
w Large particle sampling effectiveness of commercially available and prototype particle collectors was determined by wind tunnel testing. Included in the tests were the standard 1 CFM Andersen and a specially modified version of it which utilizes a weatherproof, directionally insensitive inlet; the standard Hi-Volume sampler; a Prototype Dichotomous sampler; and a prototype sampler which utilizes the 20 CFM Andersen with a rotating cowl inlet. The tests were performed using particles ranging in size from 5 to 50 pm, approach velocities from 5 to 15 ft/s (1.5-4.6 m/s), turbulence levels of 5 pm) will increase from mining efforts, oil shale usage, and fuel transportation and utilization operations. The environmental impact of energy related phenomena in the form of damage to materials and welfare will necessitate that control considerations also be directed to the larger particles in the future. A t the present, little effort has been devoted to the development of instrumentation with the capability of collecting unbiased samples of the larger fraction of ambient particles. The increased inertia associated with large particles poses significant internal transport problems for any collection apparatus. Particles may also impact on the sampler inlet or upon surfaces of fractionation elements within the sampler. I t was the primary purpose of this study to evaluate the ambient air samplers to determine the capabilities of these I Present address, Civil Engineering Department, Texas A&M University, College Station, Tex. 77843.
devices for effective collections of the larger particles. The resulting data can be used not only for future judgments in aerosol sampler selection, but also for interpretation of previously acquired data. For example, Hi-Volume samplers have been used extensively for sampling in situations in which large wind-blown dust particles are present. The efficiency with which these larger particles are drawn into the samplers has not been known. There are two basic sampling approaches incorporated into commercial instruments that are currently widely used for collection and subsequent physical and chemical analysis of atmospheric particulate matter. The first is exemplified by the standard Hi-Volume sampler (1, 2) which employs an absolute filter and a vacuum source and provides an assessment of total aerosol mass concentration. For the wind tunnel experiments, the sampler tested was made by General Metal Works and represents the standard version adopted by EPA ( 3 ) .The second approach utilizes the impaction principle to introduce a size-fractionation capability. The basic commercial instrument employed in tests was a 1 CFM eight-stage system introduced by Andersen ( 4 ) . Prototype Instruments Tested. Another instrument tested which, utilizes an impaction system is the virtual impactor, first introduced by Houman and Sherwood ( 5 ) .A version of this device, a Prototype Dichotomous sampler ( 6 ) , was examined in the wind tunnel tests. This sampler is designed to permit two size fractions of particles to be collected separately with the cut point being near 3 pm. This point is suggested by the bimodal nature of the mass distribution of urban aerosol (7) with the lower fraction being associated with potential health hazards and the upper size with materials damage. The size segregation is affected by inertial impaction across a virtual surface into a chamber of relatively stagnant air with the large particles penetrating into the chamber and the small particles being diverted around it. Both size fractions are then collected on filters. The particular dichotomous sampler tested has a total inlet flow of 220 lpm, 14 lpm of which are brought through the two-stage virtual impaction fractionator. For more details of the system, see Dzubay and Stevens (6). In addition to the particles collected in the fractionation system, the Dichotomous sampler tested also has accommodations to collect a total particle sample. The system operates in parallel with the fractionation apparatus and consists simply of a 127-mm glass-fiber filter. A second prototype instrument developed and tested was designed to improve the sampling effectiveness of the 1 CFM Andersen. Specifically, two modifications were introduced which consisted of the inclusion of an all weather sampler inlet (AWSI),and the replacement of the original Andersen stages 0 and 1 with revised stages denoted by A and B and the use of a conically shaped connector between the inlet and sampler (Figure 1). Stages A and B had only 36 jets each, with the jet inlets tapered. The collection plates were modified to allow the flow to go through the center of the plates as well as around them. For more details, see McFarland et al. (8).With reference to the inlet housing, for a sampling rate of 0.47 l/s. (1 CFM) the flow enters a t a velocity of 15 ft/min (0.076 m/s). Once inside the unit the flow is decelerated, flows vertically upward through a stilling chamber, and then through a set of 24 holes, each in. (1.27 cm) in diameter, into the internal inlet for transport to the impaction stages. Volume 11, Number 4 , April 1977 387
The other prototype instrument tested was the Sehmel Rotating Cowl and Impactor system (9))which consists of a 20 CFM Andersen equipped with a special housing that orients a 6-in. (15.24 cm) diameter cylindrical inlet directly into the wind. The inlet velocity is 0.52 m/s and upon passing through the cylinder, the particles enter the 20 CFM Andersen head for fractionation and collection.
placed normal to the flow. Second, the expanded particle cloud was passed through a biplane grid to generate turbulence and increase mixing and homogeneity and then into the test section. The aerosol concentration was determined prior to and subsequent to each test of the various samplers using an isokinetic sampling manifold fitted with six isokinetic sampling nozzles attached to absolute filters. The manifold was approximately 90 cm in width with the six isokinetic sampling nozzles spaced a t equal intervals (-15 cm) on the same plane. The sampling effectiveness of the instruments was then determined by comparing the quantity of aerosol deposited on the collection substrates of a particular sampler to that detected by the isokinetic sampling system with appropriate corrections for differences in sampling volumes. The turbulence level of 8% was achieved by introducing a biplane grid approximately 10 mesh diameters upstream of the sampler placement area in the wind tunnel. Note that the wind tunnel tests were primarily a test of inlet effectiveness. Sample Analysis. The particles used in the studies were formed from the atomization of an oleic acid solution tagged with uranine dye, the latter used for increasing mass sensitivity through fluoroscopic analysis. Analysis was performed by washing the collection substrates (or filters) from the particular sampler being tested in pure ethanol. The resulting solution was diluted 1:1with distilled water. One drop of 1 N NaOH was added to a 4-ml aliquot of each sample solution to stabilize and maximize fluorescence. These aliquots were quantified in terms of fluorescent content with the aid of a calibrated Turner Model 111 fluorometer. Special Tests. Certain tests were performed on some instruments in addition to the wind tunnel tests described. The internal wall losses of the original and modified upper stages of the 1 CFM Andersen impactor were examined by direct sampling of the test aerosol of 5-, lo-, and 15-pm aerodynamic diameter. Internal losses were determined by comparing the quantity of material on the collection substrates (Mylar or Teflon surfaces) with the total deposited on all surfaces. Also, tests were performed on the Prototype Dichotomous sampler to determine transport losses. In this case, the total particle mass deposited on the walls and elements of the fractionation stages was divided by that collected by the 127-mm filter (with appropriate correction for flow rate) to obtain the fractional wall losses.
E x p e r i m m t a l Procedure
T e s t Results a n d Discussion
Wind Tunnel Tests. The five samplers-commercially available and prototype-1 CFM Andersen impactors, the General Metal Works Hi-Volume sampler, the Prototype Dichotomous sampler, and the Sehmel Rotating Cowl and Impactor system were placed one a t a time in the Environmental Wind Tunnel facility a t Colorado State University (shown schematically in Figure 2). The tunnel has a 3.65m-wide test section with a roof that can be adjusted up to 2.44 m in height-a feature which allows the tests to be conducted with a zero pressure gradient in the direction of flow. The present series of tests was performed with a 1.83-m ceiling height which was also sufficient to preclude blockage effects. The samplers were tested under a variety of field-realistic conditions using monodisperse aerosols of 5-50 pm in aerodynamic diameter, longitudinal components of approach flow turbulence intensity
