Practical depolarization-ratio-based inversion procedure: lidar measurements of the Eyjafjallajökull ash cloud over...

In this paper we present a technique for estimating optical backscatter and extinction profiles using lidar, which exploits the difference between the observed linear volume depolarization ratio at 355 nm and the corresponding expected aerosol-only depolarization ratio. The technique is specific to situations where a single strongly depolarizing species is present and the associated linear particulate depolarization ratio may be presumed to be known to within a reasonable degree of accuracy (on the order of 10%). The basic principle of the technique is extended to deal with situations where a depolarizing fraction is mixed with nondepolarizing aerosol. In general, since the relative depolarization interchannel calibration is much more stable than the absolute system calibration, the depolarization-based technique is easier to implement than conventional techniques that require a profile-by-profile calibration or, equivalently, an identification of aerosol-free altitude intervals. This in particular allows for unattended data analysis and makes the technique well-suited to be part of a broader (volcanic ash) surveillance system. The technique is demonstrated by applying it to the analysis of aerosol layers resulting from the 2010 eruptions of the Eyjafjallajökull volcano in Iceland. The measurements were made at the Cabauw remote-sensing site in the central Netherlands. By comparing the results of the depolarization-based inversion with a more conventional manual inversion procedure as well as Raman lidar results, it is demonstrated that the technique can be successfully applied to the particular case of 355 nm depolarization lidar volcanic ash soundings, including cases in which the ash is mixed with nondepolarizing aerosol.