Abstract: Method and apparatus for monitoring particulate mass concentration of emissions, such as fly ash, from a stationary source are disclosed which employ a single-wavelength infrared transmissometer. Emissions from a source, such as a fossil fueled power plant, generally contain particles of known material of a wide range of sizes. Often the particle size distribution varies during emission. In accordance with the present invention a single-wavelength transmissometer is employed which operates at a wavelength at which the extinction-to-mass concentration ratio as a function of particle size is substantially constant. Consequently, the transmissometer output provides a measure of the mass concentration of particulate material emitted from the source independent of the size of the particles. Logarithmic amplification of the transmissometer detector output provides a direct measure of the mass concentration.