Advanced insights into biomass burning aerosols during the 2023 Canadian wildfires from dual-site Raman and fluorescence lidar observations

Wildfires, as a major source of aerosols, affect air quality and climate at regional and global scales. Fluorescence lidar is a promising technique for characterizing biomass burning aerosols (BBAs) from wildfires, as it detects signals from fluorescent organic compounds in BBAs in addition to elastic and Raman backscatter. However, published fluorescence lidar studies on BBAs remain largely limited to sparse single-site case studies, hindering the development of the technique and a systematic characterization of BBA properties. This study presents dual-site observations of transported BBAs from the exceptional 2023 Canadian wildfires, recorded between May and September at the ATOLL observatory (France) and the GPI site (Russia). ATOLL operates a multi-wavelength Raman lidar with one fluorescence channel at 466 nm; GPI operates a five-channel broadband fluorescence lidar excited at 355 nm. This dual-site dataset combines elastic, depolarization, and fluorescence observations in free troposphere (FT) and upper troposphere–lower stratosphere (UTLS). Compared with FT layers, UTLS layers exhibit higher depolarization, slightly lower lidar ratios, lower Ångström exponents, and a redshift in fluorescence spectral peaks. Cross-site comparisons reveal consistent fluorescence magnitudes and spectral shapes. Depolarization ratio, Ångström exponent, and fluorescence color ratio are moderately correlated with altitude ( r 2 ≈0.61–0.68), although altitude likely acts as an intermediate variable governed by plume injection height, in-layer temperature, and plume origin. Finally, the near-absence of hygroscopic growth at RH of 90 %–100 % challenges the assumption that aged BBAs are typically hygroscopic, suggesting their water uptake properties may be more complex than currently represented in climate models.