Fluorescence properties of long-range-transported smoke: insights from five-channel lidar observations over Moscow during the 2023 wildfire season

Abstract. The fluorescence lidar at the Prokhorov General Physics Institute (Moscow) was utilized to study smoke transported over the Atlantic during the wildfire season from May to September 2023. The lidar system, which is based on a tripled Nd : YAG laser, performs fluorescence measurements across five spectral intervals centered at wavelengths of 438, 472, 513, 560 and 614 nm. This configuration enables the assessment of the spectral dependence of fluorescence backscattering over a broad range of altitudes, from the planetary boundary layer (PBL) to the middle and upper troposphere (MUT). The fluorescence capacity of smoke, defined as the ratio of fluorescence backscattering to aerosol backscattering at the laser wavelength, exhibits significant variation in the MUT, with changes of up to a factor of 3. This variation is likely indicative of differences in the relative concentration of organic compounds within the smoke. Analysis of more than 40 smoke episodes has enabled an evaluation of the height dependence of smoke fluorescence properties. Observations reveal that the fluorescence capacity generally increases with altitude, suggesting a higher concentration of organic compounds in the MUT compared to the lower troposphere. Additionally, the measurements consistently show differences in the fluorescence spectra of smoke and urban aerosol. Urban aerosol fluorescence tends to decrease gradually with wavelength, whereas the peak of smoke fluorescence is observed in the 513 and 560 nm channels. This spectral distinction provides an effective means of separating smoke from urban aerosol. The technique was applied the analysis of events where smoke from the upper troposphere descended into the PBL, demonstrating its utility in distinguishing between these aerosol types.