Virtual impactor

Aerosol can be separated by size using devices such as cyclones, classical impactors, virtual impactors, and filters before detection. These same components can also be used for aerosol concentration and collection as described below. Other methods involving electrostatic and ultrasonic effects are being investigated for aerosol concentration and separation, but these methods are less developed and are not available for near-term deployment. 

Cascade impactors are instruments which have been extensively used for sampling and separating airborne particles to determine the aerodynamic size classification of aerosol particles. There are three kinds of cascade impactors inertial impactors, virtual impactors and particle trap impactors. 

B. W. Loo, J. M. JaMevic, and F. S. Goulding, "Dichotomous Virtual Impactors for Large Scale Monitoring of Airborne Particulate Matter," in B. Y. H. Liu, ed., Eine Particles, Aerosol Generation, Measurement, Sampling and Analysis, Academic Press, Inc., New York, 1976, pp. 311—350. 

Figure 4 (a) High-volume dichotomous (virtual) impactor and (b) liquid-filled impinger... 

Virtual Impactors (MesoSystems, Research Inti.) -Bubblers/lmpingers -Variable-Particle-Size Impactors -Microfluidics (MFSI, SNL, LLNL)... 

Figure 4.2 shows the complete CBMS II system. The main unit is comprised of three modules, the Biosampler Module, the Sample Introduction Module (SIM), and the Mass Spectrometer Module. The Biosampler Module houses the virtual impactor air particle concentrator and is only needed for the biological agent monitoring mode. The Sample Introduction Module contains the multiport sampling valve with its three input connections ... 

Better collection methods are needed for stack measurements. Virtual Impactors or other devices that avoid the bounce-off problems of cascade Impactors should be developed. Methods for collection of very large amounts of size-segregated particles suitable for detailed organic analyses are needed. Better low-blank filter and collection-surface materials that can withstand high stack temperatures are needed. Careful attention should be given to the determination of vapor-phase species of volatile elements and compounds. 

Solomon, P. A., J. L. Moyers, and R. A. Fletcher, High-Volume Dichotomous Virtual Impactor for the Fractionation and Collection of Particles According to Aerodynamic Size, Aerosol Sci. Technol., 2, 455-464 (1983). 

There are two overall types of impactors in widespread use cascade and virtual impactors. 

Virtual impactors. The virtual impactor is a modified type of impactor, an example of which is shown in Fig. 11.58 one commonly used type of virtual impactor is known as the dichotomous sampler. The basis of virtual impactors is that the airstream impacts against a mass of relatively still air rather than against a plate. The inertia of the particles carries them into the still air... 

FIGURE 11.58 Schematic diagram of a virtual impactor (adapted from Conner, f966). 

Anderson, T. L., D. S. Covert, and R. J. Charlson, Cloud Droplet Number Studies with a Counterflow Virtual Impactor, J. Geo-phys. Res., 99, 8249-8256 (1994). 

There are methods that collect particles from both the coarse and fine modes simultaneously. A clever way to achieve collection of both modes of particles onto filters is the virtual impactor (VI) (3). The gas-particle mixture is forced to make a sharp bend, and particles above 2.5 xm are ejected into a small portion (10%) of the gas stream onto a filter, so all the coarse particles (typically from the 10 xm limit set by the PM-10 inlet to 2.5 xm) and 10% of the fine particles are collected on a filter. The remainder is then filtered this portion contains no coarse particles but 90% of the fine particles (2.5 to 0 xm). After analysis, the fine admixture in the coarse fraction is removed mathematically. Limitations in the process limit its usefulness below 1 xm, however. 

A second way to achieve collection of coarse- and fine-mode particles onto filters is through tandem filtration through the stacked filter unit (SFU) (4, 5, 6). In these devices, the convenient filtration characteristics of filters (Nuclepore) allow a 2.5- xm cut point on the basis of pore size and the face velocity of the airstream. Such devices are very compact and inexpensive and have been heavily used in remote-area networks (7, 8, 9). Again, however, the limitations of the method limit the number and sharpness of the size cuts so that almost all units are operated at 2.5 xm and give coarse and fine fractions very much like those of the virtual impactor. Examples of the 2.5- xm cut points of VI, SFU, and impactors are shown in Figure 2. However, cyclones, virtual impactors, and stacked filter units cannot give the sharp, multiple cut points of impactors as shown in Figure 1. 

Particle-Particle Interactions. Loss of strong acid content of aerosol particles can also occur because of reactions between co-collected acidic and basic particles impacted together on the collection surface. This phenomenon most frequently occurs as the result of interaction of coarse (>2.5 xm diameter), alkaline, soil-derived particles with fine (<2.5 xm diameter) acidic sulfate particles (66). Particle-particle interactions with net neutralization can be reduced in many cases by sampling with a virtual impactor or a cyclone to remove coarse particles, although this procedure does not prevent the effect if external mixtures of fine particles of different acid contents are sampled. In situ methods with shorter sampling times can be used such that these topochemical reactions are less likely to occur. 

Virtual impaction uses the principle of inertial separation, but the impaction plate is replaced by a zone of relatively stagnant air below the nozzle. The virtual surface formed by deflecting streamlines gives separation conditions similar to those in conventional impactors. Large particles travel straight through into the low-flow region,... 

Instead of using the virtual impactor approach, North American air monitoring programs in the 1980s and later have adopted simpler reference methods that use the weighing of filters in the laboratory. The filters are obtained from samplers equipped with an inlet device that provides for a sharp cut-point in particle entry for samples of particles < 10 xm diameter or <2.5 [im diameter, which are operated over a fixed time period of 24 hours. The inlet fractionation is facilitated either by a carefully designed cyclone or by an impactor. The combination of the two samplers can give estimates of mass concentration for fine-particle and coarse-particle concentrations. 

Aerosol samples were collected on filters of various types (including impactors and virtual impactors) and analyzed for H+,... 

Virtual impactors Eliminates bounce and blow off. Separated particles are left in suspension and may be transported accordingly. Sharp cut points and low losses are possible. Existing designs do not operate well at D < 2.5 fim. 

Impactors accelerate the particles in a jet toward a surface (classical impactors) or toward a nozzle (virtual impactors). Both approaches can be used to remove large particles from the sample airstream and typically have steeper sigmoidal cutoff curves than cyclone separators. The remaining particles can then be collected using a filter, cyclone, or impaction onto a surface or into a liquid. Classical impactors are compact but need to be cleaned frequently. Virtual impactors reduce the cleaning problem but are more expensive to build than classical impactors. Impactors have been configured to collect particles of 0.1 to greater than 10 pm with reasonable efficiency. 

A collection system in which particles are accelerated in a jet toward a surface (classical impactors) or toward a nozzle (virtual impactors). 

Virtual Impactor Spore Sampler High Throughput Jet Spore and Particle Sampler ... 

Quality of measurement When the Harvard high volume cascade impactor was compared to the virtual impactor, fine (PM 2.5 to 0.2 2500 to 200 nm) material and coarse (PM 10 to 2.5 10,000 to 2500 nm) material masses in the respective fractions were within 10% of agreement with values from other methods of collection and particle sizing. Measurements were simpler to make with the Harvard impactor. However, chemical constituents appeared to be sufficiently variable within fractions that activity should be measured on the material used to expose whole animals, or cells (5). 

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