Dichotomous sampler, particulate

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. 

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... 

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... 

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. 

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