Depolarization ratio of smoke and volcanic ash aerosol particles at 1565 nm using a HALO Doppler lidar

Abstract. Particle linear depolarization ratio is a widely used parameter in lidar research to distinguish different aerosol types and the thermodynamic phase of water. It is most frequently measured at ultraviolet and visible wavelengths (355 and 532 nm), yet multi-wavelength observations suggest that this parameter can vary substantially with wavelength. In this work, we assessed particle linear depolarization ratios at 1565 nm using Halo Photonics StreamLine Doppler lidars. We examined the depolarization ratio through three case studies featuring extremely fresh and aged smoke, and volcanic ash aerosol particles in the troposphere. Both fresh and aged smoke aerosol particles induced low values. Specifically, aerosol layers dominated by extremely fresh smoke showed a depolarization ratio of 0.017 ± 0.004, whereas aged long-range transported smoke par ticles exhibited marginally higher values. Volcanic aerosol layers induced high depolarization ratios with layer mean values of 0.45 ± 0.01. For the extremely fresh smoke case, we further estimated the smoke mass concentration using the lidar observations at 1565 nm and found good agreement with the in situ observations. These results demonstrate that Halo Doppler lidars operating at 1565 nm wavelength are capable of distinguishing several key aerosol types and can therefore assist in the characterization of atmospheric aerosols.