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The particle size distributions, morphologies, and chemical composition distributions of 14 coal fly ash (CFA) samples produced by the combustion of four western U.S. coals (two subbituminous, one lignite, and one bituminous) and three eastern U.S. coals (all bituminous) have been examined by computer-controlled scanning electron microscopy (CCSEM). The samples were obtained from a 50-kW combustor using an isokinetic sampler and separated aerodynamically using a cyclone into PM fractions that were nominally <2.5 μm in mean diameter (PM_2.5) and >2.5 μm in mean diameter (PM_2.5+). However, the CCSEM particle size distribution results show that particles with mean diameter <2.5 μm typically constitute more than 80% of the total number of particles and particles with mean diameter >2.5 μm account for over 80% of the total particle volume for both the PM_2.5 and the PM_2.5+ fractions. The samples …