Derivation of aerosol fluorescence and water vapor Raman depolarization ratios from lidar measurements

Abstract. Polarization properties of the fluorescence induced by polarized laser radiation are widely considered in laboratory studies. In lidar observations, however, only the total scattered power of fluorescence is analyzed. In this paper we present results obtained with a modified Mie-Raman-Fluorescence lidar operated at the ATOLL observatory, Laboratoire d’Optique Atmosphérique, University of Lille, France, allowing to measure depolarization ratios of fluorescence at 466 nm (δ_F) and of water vapor Raman backscatter. Measurements were performed in May–June 2023 during Alberta forest fires season when smoke plumes were almost continuously transported over the Atlantic Ocean towards Europe. During the same period, smoke plumes from the same sources were also detected and analyzed in Moscow, at General Physics Institute (GPI), with a 5-channel fluorescence lidar able to measure fluorescence backscattering at 438, 472, 513, 560 and 614 nm. Results demonstrate that, inside the boundary layer (BL), urban aerosol fluorescence is maximal at 438 nm, then it gradually decreases with wavelength. Results also show that the maximum of the smoke fluorescence spectrum shifted towards longer wavelengths. The smoke layers observed within 4–6 km present a maximum of fluorescence at 513 nm while, in the upper troposphere (UT), the maximum shifts to 560 nm. Regarding fluorescence depolarization, its value typically varies inside the 45–55 % range, however several smoke plume layers detected above 10 km were characterized by a δ_F increasing up to 70 %. Inside the BL, the fluorescence depolarization ratio was higher than that of smoke and varied inside the 50–70 % range. Moreover, in the BL, δ_F appears to vary with atmospheric relative humidity (RH) and, in contrast to the elastic scattering, fluorescence depolarization increases with RH.

The depolarization ratio of the water vapor Raman backscattering is shown to be quite low (2±0.5 %) in the absence of fluorescence, because the narrowband interference filter in the water vapor channel selects only strongest vibrational lines of the Raman spectrum. As a result, depolarization of the water vapor Raman backscattering is sensitive to the presence of strongly depolarized fluorescence backscattering. The fluorescence contamination into the water vapor Raman channel can be calculated from the water vapor Raman depolarization ratio with the only assumption that δ_F remains constant within the 408–466 nm range.