Laboratory setup to study the polarization properties of size-resolved aerosol particles in exact back-scattering direction

Abstract. In this study, a new laboratory-based experimental setup, the Optical Lab for Lidar Applications (OLALA), was developed and characterized to measure the polarization properties of size-resolved aerosol particles in the exact backscattering direction. The linear depolarization ratio was measured at 532 nm wavelength at a scattering angle of 180 ± 0.2° to be comparable to lidar observations. To realize exact backscattering measurements, a 50:50 beam splitter was used to establish a coaxial transmit-receive optical configuration. Aerosols were generated using an atomizer and size-selected with a differential mobility analyzer, which allowed to measure particle size distributions in the fine mode (mostly below 1 μm). Polystyrene latex (PSL) beads, ammonium sulfate, sodium chloride, Arizona Test Dust (ATD), and Saxonian soil dust were used as test aerosols to evaluate the performance of the system. PSL particles of diameter 1 μm exhibited the lowest depolarization ratio of 0.017 ± 0.002, which agrees with theoretical values for spherical particles. Arizona Test Dust, which is characterized by an irregular particle shape, produced the highest depolarization ratio of 0.168 ± 0.048 for particles having a median diameter of 766 nm. For the non-spherical particles like sodium chloride, ATD, and Saxonian soil dust, an increase in the depolarization ratio with increasing size was observed.