Combined use of Mie-Raman and fluorescence lidar observations for improving aerosol characterization: feasibility experiment

Abstract. To study the feasibility of a fluorescence lidar for aerosol characterization, the fluorescence channel is added to LILAS – multiwavelength Mie-Raman lidar of Lille University, France. A part of fluorescence spectrum is selected by the interference filter of 44 nm bandwidth centered at 466 nm. Such an approach has demonstrated high sensitivity, allowing to detect fluorescence signal from weak aerosol layers (backscattering coefficient at 1064 nm is below 0.02 Mm−1 sr−1) up to a height of 5000 m. Simultaneous detection of nitrogen Raman and fluorescence backscatters allows to quantify the fluorescence backscattering coefficient. Observations were performed during November 2019–February 2020 period. The fluorescence capacity (ratio of fluorescence to elastic backscattering coefficients) varied in a wide range, being the highest for the smoke and the lowest for the dust particles. The fluorescence capacity depends as well strongly on the relative humidity, because the water uptake at the condition of high RH increases the elastic backscattering, without significant modification of the fluorescence. Thus, simultaneous measurements of Mie-Raman and fluorescence lidars open opportunity for the study of the particle hygroscopic growth. The fluorescence technique can be used also for monitoring the aerosol inside the cloud layers. The results presented demonstrate, that aerosol and cloud particles can be mixed both externally and internally. When the cloud is formed at the top or inside the aerosol layer (such scenario can be probably considered as internal mixing) we observed significant (up to factor 5) increase of fluorescence backscattering. Among possible mechanisms of such enhancement we can assume modification of the scattering phase function of the particles embedded in the water microspheres and the lens effect due to the water shell presence.