Sensor for particle size and number density in an engine plume

An advanced sensor is described which will allow measurement, in flight, of the size and number density of particles in an aircraft exhaust plume using a single frequency lidar system. The ratio of the extinction coefficient to the backscatter coefficient is used to determine the particle size, and the backscatter coefficient is then used to determine absolute number density. Various analytical methods are found in the literature. For example, in the case of a plume containing carbon particles with a mean particle radius covering the range from 0.01 to 10.0 micrometers, the backscatter coefficient varies over several orders of magnitude while the extinction coefficient is relatively constant. In this example we assume a constant volume density for the particles, and the size distributions have small standard deviations. To use this method, it is necessary to assume a value for the refractive index of the particles and to have a rough estimate of the expected range of particle sizes. This instrument could have additional applications such as the remote monitoring of smokestack emissions.