Scanning 6-wavelength 11-channel aerosol lidar

Abstract

A transportable multiple-wavelength lidar is presented, which is used for the profiling of optical and physical aerosol properties. Two Nd:YAG and two dye lasers in combination with frequency-doubling crystals emit simultaneously at 355, 400, 532, 710, 800, and 1064 nm. A beam-combination unit aligns all six laser beams onto one optical axis. Hence the same air volume is observed by all six beams. The combined beam can be directed into the atmosphere from -90 degrees to + 90 degrees zenith angle by means of a turnable mirror unit. From the simultaneous detection of the elastic-backscatter signals and of the Raman signals backscattered by nitrogen molecules at 387 and 607 nm and by water vapor molecules at 660 nm, vertical profiles of the six backscatter coefficients between 355 and 1064 nm, of the extinction coefficients, and of the extinction-to-backscatter ratio at 355 and 532 nm, as well as of the water vapor mixing ratio, are determined. The optical thickness between the lidar and a given height can be retrieved for all six transmitted wavelengths from measurements under two different zenith angles. In contrast to sun-radiometer observations, this option allows the resolution of spectral extinction information of each of the aerosol layers present in the vertical. The profile of the depolarization ratio is determined at 710 nm and used to investigate particle shape. A few measurement cases are presented to demonstrate the capabilities of the new lidar.