This experimental study investigated the validity of using a single-wavelength infrared transmissometer to measure mass concentration of particulate emissions from stationary sources. Previous data and theory have indicated that an optimum wavelength region (3 to 4 micrometers) exists for relating aerosol extinction to mass concentration, independent of size of the scattering particles. Results show that the extinction-to-mass concentration ratio for a given aerosol type is dependent on particle size at the visible wavelength but nearly independent of particle size at the infrared wavelength. Therefore, the infrared transmissometer is an eminently suitable instrument for monitoring mass concentration of particulate emissions with uncontrolled particle sizes. Moreover, the results obtained for the different particle types indicate that this transmissometer can provide estimates of particulate volume concentration that are nearly independent of particle shape, composition, or size.