Lidars are also know as Laser-Radar systems. They transmit pulses of light into the atmosphere and measure the return signal as a function of time (altitude) since the launch of the pulse. Lidars can employ different wavelengths of light and, not uncommonly, transmit polarized light and separate the return signal by polarization state. Example single wavelength (355 nm) lidar backscatter signals and the corresponding depolarization signal are shown in Figure 1. Here elevated signal levels between 2 and 6 km are only somewhat visible in the top panel. However, the areas of enhanced depolarization ratio are clearly present in the bottom panel. In general, (neglecting multiplescattering which may be important in clouds) spherical particles will have a depolarization ratio of zero while non-spherical particles (such as ice crystals or ash particles) will have associated depolarization ratios as high as 0.3-0.4 (in the case where linear polarization is used). Normal aerosol have depol ratios below 0.1 or so.

Example results for the UV-lidar system at Cabauw are shown in Figure 2 for 05/17/2010 between 19:00 and 21:00 Hrs. The retrieval shown depends on being able to sense relatively aerosol free altitude regions (R=1) above the aerosol layer. With two “calibration regions” it is then possible to invert the lidar signal to estimate both the extinction (total scattering + absorption) coefficient and the backscatter (scattering back towards the lidar receiver) coefficient. With these two quantities it is possible to then estimate the conversion factor between the optical extinction and the mass loading. This relationship is sensitive to the scattering model used and to the assumed density of the ash particles.