The most common analytical tool used to image stack emission particulate is the scanning electron microscope (SEM). For automated particle sizing, the image must contain sufficient contrast between the background and the particles that an image analysis algorithm is capable of differentiating between them. For automated image analysis systems, a “particle” is defined as a set of contiguous pixels all of which are brighter (or more rarely, darker) than the threshold brightness used to define the surrounding “background” pixels. SEM measurement of particle size distributions offers benefits over gravimetric techniques, allowing particle size determination of sample collections as small as 0.25 mg. Scanning electron microscopy methods are able to directly determine the number fraction of particles of a given size. In addition, the particle size bins chosen for data analysis and reporting are entirely customizable, which is important to engineering studies for control equipment design. Cascade impactors and Method 201A cyclones cannot be used in wet stack conditions or conditions of cyclonic flow. It may also be impossible to use cyclones in close conditions where they do not fit or large ports are not available. Many sources are now so clean that it is impractical to collect enough material in the cyclones to produce accurate weights. And some conditions are simply too harsh for cyclones. Sample collection using Method 5 or a modified Method 5 followed by particle sizing using SEM provides an opportunity to determine particle size distributions that cannot be otherwise determined. State agencies have shown a willingness to accept microscopy results when Method 201A cannot be used.
