Discussion of the spectral slope of the lidar ratio between 355 nm and 1064 nm from multiwavelength Raman lidar observations

Abstract. The extinction-to-backscatter ratio also known as lidar ratio is an important quantity in active remote sensing with lidar. It is a key parameter in aerosol typing and an essential quantity to derive the extinction coefficient from elastic backscatter lidars like the spaceborne Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) or ceilometer observations. The present study discusses the spectral dependence of the lidar ratio from the frequently measured lidar ratios at 355 and 532 nm to the rarely observed lidar ratio at 1064 nm. Therefore, a special focus is put on the rotational Raman lidar method applied to the emission wavelength of 1064 nm. We present a literature review on existing triple-wavelength lidar ratio observations and add new measurements for marine and continental aerosol. For cirrus clouds, marine and continental aerosol the spectral behavior is neutral; for sulfate aerosol, we see a decrease from 532 to 1064 nm. In the case of mineral dust, the lidar ratio slightly increases towards 1064 nm and in the case of aged smoke an increase with wavelength is found. Furthermore, we observe an increase of the lidar ratio with relative humidity for continental aerosol and report hygroscopic growth factors. The findings are compared to the assumptions made in the CALIPSO version 4 automated aerosol classification and lidar ratio selection algorithm (Kim et al., 2018). We could confirm these assumptions for marine and sulfate aerosol. However, we see slight differences for mineral dust and continental aerosol and pronounced differences for elevated smoke.