A new X-ray fluorescence (XRF) device for the analysis of dense particle-bearing aerosols at the worksite has been developed. It provides values of the mass concentration M of each element Z > 22 held in aerosols typical of "smokestack" or acute occupational exposures (mg/m 3-g/m 3) on-site and in-time for corrective action before extensive harm to workers or the environment occurs. In operation the monitor irradiates the dispersed aerosol directly with the output of an interchangeable anode X-ray tube. The energy and number of returning XRF photons are used to identify the respective energies and abundances of the heavier elements contained within the aerosol. To date several specific apparatus configurations have been evaluated, mainly by comparing their relative sensitivities and minimum measurable mass concentrations as derived from data acquired with simulated target aerosols. These results are discussed along with the general principles behind the device, the target simulation, and the measurements themselves.