Sampling and Collection of Particles

The first steps in traditional analysis of the physical and chemical properties of atmospheric particulate matter are sampling, that is, obtaining a representative sample over the desired size range, and collection, that is, separating the particles from air. During sampling and collection, such parameters as humidity, temperature, and particle concentration must be controlled to maintain the sample integrity. 

Because it is particles in the smaller size range, 2.5 /x.m (PM25), that are of greatest interest with respect to health effects, inlet systems are normally used that exclude larger particles. These size exclusion inlets are usually based on filters, cyclone collectors, or impactors, the principle behind which is discussed shortly. Inlet cutoff diameters from 2.2 to 15 /im are achieved in commercial instruments using these techniques (Chow, 1995). 

Collection of particles is based on filtration, gravitational and centrifugal sedimentation, inertial impaction and impingement, diffusion, interception, or electrostatic or thermal precipitation (e.g., see Spurny, 1986, Chapter 3). The choice of method depends on a number of parameters such as the composition and size of the particles, the purpose of the sample, and acceptable sampling rates. Table 11.10 summarizes some of the commonly used methods and the size ranges over which they are effective. 

Filters collect liquid and solid particles by mechanisms including diffusion, impaction, interception, electrostatic attraction, and sedimentation onto the filter while allowing the gas to pass through. The types commonly used in atmospheric particulate collection are membranes, fibrous mats, or porous sheets. Different filter materials are used depending on the particular type of measurement being carried out, including Teflon, quartz fiber, nylon, silver, cellulose filters, glass fibers, and polycarbonate. The characteristics of each are summarized by Chow (1995). 

TABLE 11.10 Some Commonly Used Methods of Collecting Atmospheric Particles 

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