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 transported smoke during the wildfire season from May to September 2023. The lidar system, based on a tripled Nd:YAG laser, performs fluorescence measurements  at wavelengths of 438, 472, 513, 560, and 614 nm. This configuration enables the assessment of the spectral dependence of fluorescence backscattering from the planetary boundary layer (PBL) to the upper troposphere and lower stratosphere (UTLS). The fluorescence capacity of smoke, defined as the ratio of fluorescence backscattering to aerosol backscattering at the laser wavelength, exhibits significant variation in the UTLS, 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 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 UTLS 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 at the 513 or 560 nm channels. This spectral distinction provides an effective means of separating smoke from urban aerosol. The technique was applied to events where smoke from the upper troposphere descended into the PBL and mixed with urban particles, demonstrating its utility in distinguishing between these aerosol types.