Multiwavelength fluorescence lidar observations of fresh smoke plumes

Abstract. A five-channel fluorescence lidar was developed for the study of atmospheric aerosol. The fluorescence spectrum induced by 355 nm laser emission is analyzed in five spectral intervals using interference filters. Central wavelengths and the widths of these five interference filters are respectively: 438/29, 472/32, 513/29, 560/40 and 614/54 nm. The relative calibration of these channels has been performed using a tungsten-halogen lamp with color temperature 2800K. This new lidar system was operated during Summer – Autumn 2022, when strong forest fires occurred in the Moscow region and generated a series of smoke plumes analyzed in this study. Our results demonstrate that, for urban aerosol, the maximal fluorescence backscattering is observed in 472 nm channel. For the smoke the maximum is shifted toward longer wavelengths, and the fluorescence backscattering coefficients in 472 nm, 513 nm and 560 nm channels have comparable value. Thus, from the analysis of the ratios of fluorescence backscattering in available channels, we show that it is possible to identify smoke layers. The particle classification based on single channel fluorescence capacity (ratio of the fluorescence backscattering to elastic one), has limitations at high relative humidity (RH). Fluorescence capacity is indeed decreasing when water uptake of particles enhances the elastic scattering. However, the spectral variation of fluorescence backscattering does not evidence any dependence on RH and can be therefore considered for aerosol identification.