A New Approach to Improve the Near-Range Response of a Lidar System at 1064 Nm

Abstract

The lidar technique is a very useful tool to perform aerosol- and cloud-related studies, both in the troposphere and the stratosphere, due to their high vertical spatial and temporal resolution with multi-wavelength capabilities. Nevertheless, some studies based on lidar (among them the air pollution studies) are often not accurately performed, because of the incomplete knowledge of the lidar performance in the near range. To interpret the near field lidar observations properly, lidar data must be corrected with the so-called overlap function. The lidar overlap function is defined as the overlap between the laser beam and the receiver field of view. This study presents a modification of Raman method, which is widely used in EARLINET (European Aerosol Research Lidar NETwork). The approach is based on the comparison of attenuated backscatter profiles derived from a lidar as well as from a ceilometer set up close to the lidar, to retrieve the overlap function for the lidar at 1064 nm. Similarly to the Raman overlap method, our approach allows for computing the overlap correction without an explicit knowledge of all system parameters. This work complements the Raman overlap method for Raman lidars and it is also applicable to elastics lidar systems. The fundamental idea that motivates our approach is coincident with that proposed in the Raman method, that is, the deviation between the elastic lidar profile and other profiles unaffected by incomplete overlap allows for retrieving the lidar overlap function. Whereas the Raman overlap method assumes that the aerosol backscatter profile derived from the lidar is unaffected by overlap artifacts and is considered as a reference profile, our approach assumes that the ceilometer profile is the reference. Two distinct procedures (iteratively or directly) were investigated for the determination of the overlap function. The application of the proposed methodology will improve the potential of Raman lidars to investigate the aerosol microphysical properties in the planetary boundary layer, by extending the information of 1064 nm backscatter profiles to the ground.