Dichotomous samplers

Because a filter sample includes particles both larger and smaller than those retained in the human respiratory system (see Chapter 7, Section III), other types of samplers are used which allow measurement of the size ranges of particles retained in the respiratory system. Some of these are called dichotomous samplers because they allow separate measurement of the respirable and nonrespirable fractions of the total. Size-selective samplers rely on impactors, miniature cyclones, and other means. The United States has selected the size fraction below an aerodynamic diameter of 10 /xm (PMiq) for compliance with the air quality standard for airborne particulate matter. 

The particles most likely to cause adverse health effects are the fine particulates, in particular, particles smaller than 10 p and 2.5 mm in aerodynamic diameter, respectively. They are sampled using (a) a high-volume sampler with a size-selective inlet using a quartz filter or (b) a dichotomous sampler that operates at a slower flow rate, separating on a Teflon filter particles smaller than 2.5 mm and sizes between 2.5 mm and 10 mm. No generally accepted conversion method exists between TSP and PM,o, which may constitute between 40% and 70% of TSP. In 1987, the USEPA switched its air quality standards from TSP to PMk,. PM,q standards have also been adopted in, for example, Brazil, Japan, and the Philippines. In light of the emerging evidence on the health impacts of fine particulates, the USEPA has proposed that U.S. ambient standards for airborne particulates be defined in terms of fine particulate matter. 

Dzubay, T. G. Chemical Element Balance Method Applied to Dichotomous Sampler Data, Annals N. Y. Acad. Sci., 1980, 338. 126. 

A dichotomous sampler with a fine to coarse cut point of about 2 pm is preferred because of the bimodal nature of the ambient aerosol and its sources. This type of physical separation according to particle size, is the first stage of source resolution since it separates the particles as well as sources into two broad classes. 

Dzubay, T.G. Stevens, R.K. "Ambient Air Analysis with Dichotomous Sampler and X-ray Fluorescence Spectrometer" Environ. Sci. Technol., 1975, 9, 663. 

Lodge, J. P., Jr., A Comparison of Urban and Rural Aerosol Composition Using Dichotomous Samplers, Atmos. Environ., 19, 840 (1985). 

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... 

Tsai, C.-J., and S.-N. Perng, Artifacts of Ionic Species for Hi-Vol PM10 and PM10 Dichotomous Samplers, Atmos. Environ., 32, 1605-1613 (1998). 

Because of the labor-intensive nature of annular denuder sampling, alternative techniques deserve further scrutiny. The DDM, relying on Teflon-nylon filter packs (or nylon filters alone) and an anodized aluminum annular denuder ahead of one of a pair of modified dichotomous samplers (46), merits evaluation for the influence of potential interferents (e.g., HONO). The DDM with NaCl-coated denuders would probably be nearly free of interference from HONO, but this conclusion requires evaluation. Because coarse particulate N03 must be excluded to achieve improved accuracy for HN03 and fine particulate N03, supplementary samplers are needed to obtain improved estimates of total as well as coarse particulate... 

Chemical Composition Aerosol composition measurements have most frequently been made with little or no size resolution, most often by analysis of filter samples of the aggregate aerosol. Sample fractionation into coarse and fine fractions is achieved with a variety of dichotomous samplers. These instruments spread the collected sample over a relatively large area on a filter that can be analyzed directly or after extraction Time resolution is determined by the sample flow rate and the detection limits of the analytical techniques, but sampling times less than 1 h are rarely used even when the analytical techniques would permit them. These longer times are the result of experiment design rather than feasibility. Measurements of the distribution of chemical composition with respect to particle size have, until recently, been limited to particles larger than a few tenths of a micrometer in diameter and relatively low time resolution. One of the primary tools for composition-size distribution measurements is the cascade impactor. 

DzubayTG. 1980. Chemical element balance method applied to dichotomous sampler data. Ann NY Acad Sci 338 126-144. 

For these studies, we entered separately the compositions of "fine", "coarse", and "total" particles. "Fine" particles are defined as those of diam <2.5 pm, the cut-point of most dichotomous samplers. "Coarse" particles are those with diameters between 2.5 pm and the maximum collected (usually 15-20 pm). Many of the data were taken with different size cuts, so it was necessary to group the data in different ways and interpolate around the 2.5-pm region. "Total" particles are either those collected without size segregation, or else the sum of the collected size fractions weighted by the relative mass loadings of each. 

Jaklevic. J. M., Loo. B, W., and Goulding, F. S. (1977) Photon-Induced X-Ray Fluorescence Analysis Using Energy-Dispersive Detector and Dichotomous Sampler, in Dzubay. T. G. (Ed.), X-Ray Fluorescence Analysis of Environmental Samples, Ann Arbor Science Publishers, Ann Arbor, MI. 

Dzubay TG, Stevens RK. 1975. Ambient air analysis with dichotomous sampler and x-ray fluorescence spectrometer. Environ Sci Technol 9(7) 663-668. 

The equipment most commonly used for separating fine and coarse particles is a dichotomous sampler. Again, particles are collected on filter papers and the analysis is accomplished in the same way. The typical sampler operates at high airflow velocity (about 17 L/min, depending on the design) so that sufficient Scimple ctm be collected in hours to days, compared to days or weeks for the TSP sampler. The drawback of this method is that the sampling equipment is too large to conveniently ship as an ordinary parcel and requires a trained operator to set it up properly. 

The dichotomous sampler is a virtual impactor with two emerging aerosol flows. The air beyond the virtual interface is withdrawn at a slow rate, thus rendering this impactor a two-flow or dichotomous impactor. In both flows aerosol particles can be collected on a filter or analysed in a real-time aerosol instrument. A continued suspension of the size-fractionated particles in the effluent air flows is the principal advantage of these virtual or dichotomous impactors (Chen et al., 1985). 

The Andersen PM 10/2.5 dichotomous sampler accurately monitors ambient air for coarse and fine particulates (Figure 6.26 Andersen Samplers Inc., 4215 Wendell Drive, Atlanta, GA 30336). The dichotomous (dichot) sampler separates aerosol particles into two distinct size fractions, 2.5 and 10 pm ( coarse particles) and less than 2.5 pm ( fine particles). The virtual... 

Fig. 6.26. An Andersen dichotomous sampler and a diagram of the inertial collection of aerosol particles.

The dichotomous sampler operates at a flow rate of 1 cm h (16.7 Lrnin" ). The coarse (PM 10 to 2.5) particle receiver tube has a flow rate of 0.1 cm h (1.67 Lmin ) for collection of the coarse aerosol particles, while the fine particles, less than 2.5 pm, follow a flow rate of 0.9 cm h (15 L min ) to the fine particle filter. 

Stevens, R.K., McFarland, A.R., Wedding, J.J. (1976). Comparison of the virtual dichotomous sampler with the high-volume sampler. Proc. Am. Chem. Soc. 16 (1), 84—85. 

Loo, B.W., Adachi, R.S., Cork, C.P., Goulding, S.S., Jaklevic, J.N., Landis, D.A., et al., 1979. A second generation dichotomous sampler for large-scale monitoring of airborne particulate matter. Lawrence Berkeley Laboratory. Report No. 8725. As cited in U.S. EPA 1986a, Ch. 4. 

Figure 5.14. The Dichotomous sampler uses a virtual surface with a small airflow through the collection chamber, to help separate respirable and non-respirable flneparticles onto separate collection filters.

T. G. Dzubay, R. K. Stevens, C. M. Peterson, X-ray Fluorescence analysis of environmental Samples , in Application of the Dichotomous Sampler to the Characterization of Ambient Aerosols, T. Dzubay (ed.). Atm Arbor Science Publishers, Ann Arbor, Michigan, 1978. 

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