The particles on PM2.5 micrometer air samples are generally considered too small for light microscopy to be of much value. That is not the case. The dominant mass on these filters is often in the large particle range, greater than half a micrometer. These particles are easily seen with light microscopy and can often be identified as to their source. The problem with applying the light microscope is generally the skill of the microscopist and not the limitation of the instrument, but there are important aspects of the analysis that are evident using the light microscope that may not be easily recognized using other instruments.

Much of the particle loading on PM2.5 filters is often not on the surface of the filter. Primary and secondary salts may form inside the filter or on the back surface. These salts may include sodium chloride, ammonium sulfate, sodium nitrate, aragonite (CaCO₃), gypsum (CaSO₄-2H₂O), and others. They may constitute the majority of the mass captured by the filter in some environments.

An examination of the PM10 filter collected at the same time can provide value information on the particle assemblages at the site and help to distribute the submicrometer combustion products mass. This "assemblage analysis" can be a very powerful tool in identifying the cause of an abnormally high mass loading. What follows are a few examples.