[en] This study attempted to differentiate between coal fly ash and other minerals in the atmosphere, using samples which were collected on Nuclepore filters in dichotomous samplers and analyzed by scanning electron microscopy and energy-dispersive x-ray spectrometry. The samples included ambient aerosol from two sites, resuspended soil, and emissions from coal- and oil-fired power plants in the Philadelphia area. Fly ash was distinguished from minerals by morphology. In the fine fraction, estimated concentrations were 6 to 75 ng/cu m for fly ash and 30 to 300 ng/cu m for minerals. In the coarse fraction, minerals were the predominant component; estimated concentrations were 0 to 320 ng/cu m for fly ash and 400 to 5800 ng/cu m for minerals