Dual-wavelength linear depolarization ratio of volcanic aerosols: Lidar measurements of the Eyjafjallajِkull plume over Maisach, Germany

The ash plume of the Eyjafjallajökull eruption in April 2010 offered an exceptional opportunity to assess the potential of advanced lidar systems to characterize the volcanic aerosols. Consequently, the plume was continuously observed in the framework of EARLINET. In this paper we focus on the EARLINET-Raman-depolarization-lidar measurements at Maisach near Munich, Germany. From these data sets the lidar ratio Sp and the particle linear depolarization ratio δp at two wavelengths (355 nm and 532 nm) were retrieved. These quantities can be used to characterize volcanic aerosols and to establish criteria for the discrimination from other aerosol types. In the pure volcanic ash plume, observed until noon of 17 April, wavelength independent values of δp as high as 0.35 < δp < 0.38, indicating non-spherical particles, were found, and lidar ratios of 50 < Sp < 60 sr at 355 nm and 45 < Sp < 55 sr at 532 nm. Later, volcanic aerosols were mixed into the boundary layer. This mixture showed in general lower values of δp as expected from the contribution of boundary layer aerosols. Especially noteworthy is the increase of δp with wavelength, when volcanic ash was mixed with small spherical particles.