Interpretation of lidar depolarization measurements of the Pinatubo stratospheric aerosol layer during EASOE

Abstract : During the EASOE campaign in the Arctic, all lidar measurements showed total depolarization values for the Pinatubo stratospheric aerosol layer varying between 2% and 4%. If multiple scattering is negligible, the radiation backscattered by a polydispersion of spheres has the same polarization as the incident light. There is always some depolarization of a lidar return because of the anisotropy of the polarizability of the air molecules, but depolarization in the signal returned from aerosols indicates the presence of non‐spherical particles. In this paper, we utilize scattering calculations for non‐spherical particles to explore interpretations of lidar signals returned by Pinatubo aerosols. Taking into account several particle shapes and lognormal size distributions, calculations show that only slightly‐to‐moderate deformed spheres with equivalent‐mean‐radius less then 0.2 µm yield depolarization values in the range given by the lidar measurements, if such particles form a substantial fraction of the population. The implications of particle non‐sphericity on the retrieval of the aerosol size distribution are discussed.