Atmospheric aerosol depolarization ratio with Doppler lidar - A long-term perspective

Some HALO Photonics Doppler lidars measure the atmospheric volume depolarization ratio at a wavelength of 1565 nm. We inspected 4 years of data from 6 instruments operated in different environments across Finland with the aim of retrieving the atmospheric aerosol depolarization ratio. The long-term performance of these instruments was examined by investigating the stability of the noise floor and the amount of polarizer bleed-through. We further developed the method for correcting the background noise and constructed an algorithm distinguishing aerosol from hydrometeors. We observed that the 4-year averaged aerosol depolarization ratio varies from 0.07 at sub-arctic Sodankylä to 0.13 in the boreal forest in Hyytiälä. At all locations, the aerosol depolarization ratio peaks during spring and early summer, which we attribute to pollen. Overall, our analysis supports the long-term usage of HALO Doppler lidar depolarization ratio measurements, especially the detection of aerosols that may pose a safety risk for aviation.