| 18 Nov 2025
Advanced insights into biomass burning aerosols during the 2023 Canadian wildfires from dual-site Raman and fluorescence lidar observations
Abstract. This study presents lidar observations of long-range transported biomass burning aerosol (BBA) plumes from the exceptional 2023 Canadian wildfire season, recorded between May and September at the ATOLL observatory (France) and the GPI site (Russia). ATOLL operates a multi-wavelength Raman lidar with 3 polarization channels (355, 532 and 1064 nm) and a single fluorescence channel at 466 nm. GPI uses a fluorescence lidar with 5 broadband fluorescence channels excited by 355 nm. The dual-site dataset combines multi-wavelength elastic scattering and depolarization measurements with fluorescence observations, enabling a comprehensive characterization of BBA properties in the free troposphere (FT) and upper troposphere–lower stratosphere (UTLS). UTLS layers exhibit higher particle depolarization ratios, slightly lower lidar ratios, lower extinction- and backscatter-related Angström exponents, and a redshift in fluorescence spectral peaks. Cross-site comparisons show consistent fluorescence magnitudes and spectral shapes, highlighting the potential of coordinated multi-lidar fluorescence observations. Correlation analysis indicates that depolarization ratio, extinction-related Angström exponent, and fluorescence color ratio are moderately (r2 ≈ 0.61–0.68) correlated with layer altitude, however, this correlation is not sufficient to confirm a solid altitude dependence. It is likely that altitude is an intermediate variable linked to other controlling factors such as injection height of the plume, in-layer temperature and the plume origin. In addition, we observed BBAs showing no clear hygroscopic growth at RH of 90 %–100 % and statistically low RH values in the detected nearly 100 layers, suggesting aged BBAs, which were typically considered as hygroscopic, may have limited water uptake capability.
Summary: The authors report on measurements of biomass burning aerosol (BBA) layers in the free troposphere and in the UTLS region in the summer of 2023 using two multiparameter lidars at two different sites. Emphasis is put on the fluorescence properties of the BBA which are measured with, respectively, one and five discrete fluorescence detection channels. Firstly, a short description of the instruments is given and the calibration procedure for the fluorescence signals is described. Importantly, to make measurements with different fluorescence lidars more comparable, the spectral fluorescence backscatter coefficient and capacity are used as parameters. Secondly, four BBA cases are discussed in detail. Thirdly, a compilation of BBA measurements at the two sites performed between May and September 2023 is presented and analyzed. One topic is the comparison of spectral fluorescence backscatter capacities, supplemented by data obtained at two other measurement sites. Relations between various elastic and inelastic aerosol properties, height, and relative humidity are also investigated.
General comments: The manuscript provides interesting results. In particular, the comparison of fluorescence measurements with two or even four instruments highlights the potential for future comparative research. Ideally, BBA plumes that pass consecutively over several lidar stations (known as lidar match) should then be investigated. Other findings of the study corroborate the conclusions drawn in previous publications, for example the redshift of the BBA fluorescence spectrum with height. The manuscript is well written. After corrections, the paper can be published.
Specific comments:
l.44: The potential of fluorescence measurements for studying aerosol-cloud interaction was first identified by Reichardt (2014). A citation seems appropriate here.
l.46: The reference list should also include as landmark publications Reichardt et al. (2018) [Reichardt, J., Leinweber, R., and Schwebe, A.: Fluorescing aerosols and clouds: investigations of co-existence, EPJ Web Conf., 176, 05010, https://doi.org/10.1051/epjconf/201817605010, 20189] and Reichardt et al. (2025).
l.51: ‘as a proxy to aerosol chemical composition.’ This is a bold claim. Because the fluorescence spectra of atmospheric aerosols are spectrally broad and without notable features, it is rather difficult to draw conclusions about their chemical composition. However, it does appear possible to distinguish between different aerosol classes. Perhaps the wording should be a little more cautious.
l.109: The text ‘in this study, we redefine… where…’ is quite confusing. There is no redefinition of old but a definition of new parameters. Please, consider writing something like ‘we use in this study the spectral fluorescence backscatter coefficient, B_F,lambda, which is the ratio of the spectrally integrated fluorescence backscatter coefficient, b_F,lambda, and the full-width-half-maximum (FWHM) of the interference filters, DD, in the fluorescence channels:
Equation B_F,lambda.
Furthermore, the spectral fluorescence capacity, G_F,lambda, will be used:
Equation G_F_lambda.’
Unfortunately, the notation is a bit different from the one used in Reichardt et al. (2023) and quite confusing because the meaning of ‘b’ and ‘B’ are switched around. The authors should consider mentioning this problem.
l.128: ‘The spectral…’ See comment on l.51.
l.146: ‘thick mixed… cirrus clouds…’. How do the authors know? You would need at least particle depolarization ratio and lidar ratio for cloud typing, you cannot tell from the current Fig. 2. By the way, is the LILAS lidar tilted? If not, Figure 3 could, for example, reveal a cirrus layer with horizontally oriented particles at 6.7 km which is embedded in a broader ice cloud with randomly oriented particles.
l. 164: ‘a sharp RH increase above 5000 m, … water uptake.’ The apparent rise in RH is probably caused to some degree by fluorescence interference as it coincides with an increase in the spectral fluorescence backscatter coefficient. See comment on l.182.
l.174: ‘The absence of fluorescence quenching…’ This is a question difficult to answer with a single fluorescence detection channel instrument. First, one must rule out that the air masses within the cloud have a different origin than those outside, second, one has to show that the aerosol and cloud particles not merely coexist but actually interact.
l.182: ‘It is worth noting… lidar-derived humidity measurements.’ The issue of RH measurements in the presence of fluorescing aerosols must be discussed in more detail. Even with a bandwidth of 0.3 nm the LILAS instrument is very well affected by fluorescence, as Reichardt et al. (2023) have shown; and the comparison between ERA-5 and LILAS RH profiles in Fig. 3 supports this assessment. Above the base of the aerosol layer, lidar RH is consistently higher than the model data and follows roughly the 466-nm spectral fluorescence backscatter coefficient. Unfortunately, the mixing ratio (MR) profile is not given, but probably its values are low within the aerosol layer, and so only a relatively small fluorescence contribution to the measured water vapor Raman signal makes a big difference in RH. The authors might try a 0-order correction of their MR, and then RH, profile: Reichardt et al. (2023) find for a detector bandwidth of 0.22 nm a fluorescence-induced effect on MR of roughly 0.017 g/kg per mean spectral fluorescence backscatter coefficient between 430 to 450 nm (in Tm^-1sr-1nm-1). A spectral fluorescence backscatter coefficient of 6 Tm^-1sr-1nm-1 would therefore result in an MR error of approx. +0.1 g/kg. The broader bandwidth of LILAS would exacerbate the effect, on the other hand it measures fluorescence at a slightly longer wavelength, which would partially compensate for this.
l.251: ‘during the … in 2023’. The redshift has been consistently observed over the years, it thus seems to be a general feature of transported BBA.
l.271: ‘moderate volume linear depolarization ratio.’ The reviewer does not understand why volume depolarization ratio is used in the discussion. It is not a true particle property. Please, consider using particle depolarization ratio instead. Further, are lidar ratios available? Ice particle formation should reveal itself by a drop in lidar ratio.
l.334: ‘In contrast to EAE, … refractive indices.’ This is not correct. EAE is affected by wavelength dependent absorption. Please, reword.
l.351: ‘High … morphology,…’ It also depends on the size parameter!
l.351: ‘semi-solid and glassy…’ Can the authors provide a reference?
l.381: ‘This decrease… BBA layer.’ This is very speculative! Unless it is unequivocally shown that the air masses carrying the aerosols and the clouds have the same origin, it can also be a transport phenomenon. Please, argue more cautiously.
l.397: ‘This consistency… Figure 12.’ The reviewer would prefer a more cautious wording. The two wavelengths are close to the point in the spectrum of least variability (Figs. 7(e) and 9(e)) and therefore not well suited for assessing the overall consistency.
l.410: ‘rather a recurring feature of BBAs (Reichardt et al., 2025) and …’
l.454: ‘The … were used’. Comment: It is quite intriguing that BBA events at both locations go mostly hand in hand with (very) dry air. So it is no wonder that hygroscopic growth is not readily observed.
Equations:
All: Sub- or superscripts that are not variables must not be in italic.
Figures:
2, 4: + Axis titles are not centered; increase font size for better readability
+ Show the calibrated fluorescence backscatter coefficient in panel (b)
+ Show particle depolarization ration in panel (c) for easier interpretation
+ Caption: Unit must not be in italic
+ Caption: Provide integration time per profile; was a sliding average applied?
+ Caption: Provide information on the tilt angle of the lidar
3: + Correct legend of panel (e)
+ It would be helpful to see the temperature profile along with RH
+ Caption: Provide vertical resolution
5: + It would be helpful to see the temperature profile along with RH
+ Caption: Provide vertical resolution
6, 8: + Homogenize the styles for Figs. 2, 4, 6, and 8
+ Axis titles have different font sizes
+ Show particle depolarization ratio in panel (c) for easier interpretation
+ Caption: Provide information on the tilt angle of the lidar
11: + Have the legends the same font size?
12: + Caption: ‘Lindenberg’
+ Caption: ‘Reichardt et al. (2025), respectively.’
13: + Caption: ‘at 472 nm…’
+ Caption: ‘fluorescence capacity/backscatter’ (?)
+ Caption: Please, indicate symbols for CR
A1: + DM must be rotated 90° clockwise
B1: + The blue trajectories are difficult to see in the maps.
+ Caption: ‘airmass passed through.’
B2: + The dotted orange lines are difficult to see in the maps.
Tables:
C1: + Header: ‘calculated as SFC_2 to SFC_1.’
Text:
l.87: ‘The receiver…’
l.89: ‘(vibrational …’
l.94: The symbols used for the spectral fluorescence properties should be given
l.98: How about using ‘/’ for the ratio?
l.103: ‘channel; and…’
l.103: ‘(see …’
l.120: ‘optical receiver…’
l.155: ‘nm^-1 and independent of height, which …’
l.164: ‘(e.g., Navas-…’
l.179: ‘and variability of organic…’
l.197: ‘Giant and …’ is not a sentence
l.200: ‘Figure 4(b) and (c).’ (?)
l.214: ‘adding more evidence…’
l.240: ‘The spectra of fluorescence capacity are…’
l.267: Meaning not clear, please, change wording
l.268: ‘from 4000 to…’ (?)
l.277: ‘coefficients, and the extinction…’
l.341: But lidar ratio does increase with wavelength, so it is correlated, isn’t it?
l.346: The shorter wavelengths are missing
l.363: ‘with a … downward motion…’, meaning not clear, please, change wording
l.425: ‘three intensive parameters…’
l.450: ‘and the small number of collocated and …’ (there are some)
l.555: ‘by the gain of …’
l.559: How about using ‘/’ for the ratio? (here and in the remainder of the Appendix)
l.559: ‘of optics. In the…’
References:
Veselovskii et al. (2024b) and Veselovskii et al. (2025) refer to the same publication, discard the former.