Physical Characteristics Mass and Size

The total mass of particles per unit volume of air is one of the major parameters used to characterize particles in air and, along with size, is the basis of air quality standards for particulate matter (see Chapter 2). Methods of mass measurement include gravimetric methods, /3-ray attenuation, piezoelectric devices, and the oscillating microbalance. 

FIGURE 11.60 Schematic diagram of a typical /3-ray attenuation device for measuring particulate mass. 

70 and I are the intensities of the 8-rays that have passed through the clean and loaded portions of the filter, respectively, X is the thickness of the deposit, and fj. is an attenuation constant that is approximately proportional to the density (p) of the material deposited. The mass per unit area deposited on the filter, given by pX, is the parameter desired in this measurement. Rearranging Eq. (S), one obtains 

The parameter p,/p is a constant known as the mass absorption coefficient with the assumption that this is independent of the type of absorbing particles (an assumption that generally holds well enough to cause 10% uncertainty), the value of In(/ //) is directly related to the parameter of interest, pX = mass per unit area. 

Such measurements can be carried out on filters with different cutoff sizes to obtain size resolution as well (e.g., see Spagnolo and Paoletti, 1994). 

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