Lidar Backscatter Properties of Al2O3 Rocket Exhaust Particles

The lidar backscatter cross sections of aluminum-oxide rocket exhaust particles are sensitive to slight deviations from their nominal spherical shape. We have therefore determined the distribution of particle shapes using automated image analysis of scanning electron microscope particle photographs. These shape statistics were used in conjunction with T-matrix calculations for nonspherical particles to estimate the backscatter properties of representative rocket exhaust particles. For the range of particle sizes and nonsphericities a relevant to exhaust plumes, both the backscatter intensity and depolarization scale approximately with the product of nonsphericity a times the particle radius (epsilon(er)). This correlation allows us to formulate a simple model for both the backscatter intensity and the depolarization of typical Al2O3 rocket exhaust particles. This model is applied to a recent particle collection from the exhaust plume of a Titan-IVA rocket obtained from the NASA WB-57F high-altitude research aircraft at an altitude of 19 km.