Optical properties of birch pollen using a synergy of three lidar instruments

Pollen has important implications for health, but also for the climate as it can act as cloud condensation nuclei or ice nuclei in cloud processing. Active remote sensing instruments equipped with polarization capability can extend the detection of pollen from the surface up to several kilometres in the atmosphere maintaining continuous and high time resolution operation. In this study, we use a synergy of three lidars, namely, a multi-wavelength PollyXT lidar, a Vaisala CL61 ceilometer and a Halo Photonics StreamLine Doppler lidar, to investigate the optical properties of birch pollen particles. All three lidars are equipped with polarization channels enabling the investigation of the wavelength dependence at 355, 532, 910 and 1565 nm. Together with pollen observations from a Hirst-type spore sampler and aerosol in situ observations, we were able to characterize the linear particle depolarization ratio (PDR) and backscatter-related Angstrom exponents of the pollen particles. Both optical properties have been extensively used in aerosol classification algorithms and they are therefore highly desired in the lidar community. We found that birch pollen exhibits a spectral dependence in the PDR, and its classification is feasible when, preferably, two or more polarization wavelengths are available.