The lidar system– LILAS, operated at ATOLL observatory which is a National Facility affiliated to ACTRIS, has a Nd:YAG laser source emitting at 355, 532 and 1064 nm with corresponding pulse energy of 100, 90 and 100 mJ. The laser has a repetition rate of 20 Hz. The backscattered light is collected with a Newtonian telescope of 40 cm diameter. The receiver includes detection channels for the three elastic wavelengths, each equipped with a pair of cross- and co- polarization channels, and three Raman wavelengths: 387 (vibrational Raman of N₂), 408 (vib-rotational Raman H₂O vapor) and 530 nm (rotational Raman of N₂ and O₂). Additionally, LILAS has a broadband fluorescence channel of 44 nm width centered at 466 nm. The lidar signals are digitized with Licel Transient recorders, allowing for a simultaneous acquisition of an analog and photon-counting signal (except for 1064 nm channel, which has only analog detection), with a range resolution of 7.5 m. This configuration allows the acquisition of vertical profiles of the 2α+3β+3δ (α: extinction coefficient, β: backscatter coefficient, δ: particle linear depolarization ratio) dataset, the spectral fluorescence backscattering coefficient βF,λ and fluorescence capacity GF,λ, as well as the water vapor mixing ratio (WVMR) and the relative humidity (RH). The spectral fluorescence backscattering coefficient is computed in a way similar to the calculation of WVMR and requires a calibration constant, as expressed by Eq. (1): 1βF,λ(z)=1ΔDλCRCF,λPF,λ(z)PR(z)TR(z)TF,λ(z)NR(z)σR, where ΔD represents the full-width-half-maximum (FWHM) of the interference filters in the fluorescence channels, PF(z) and PR(z) represent the lidar signals of fluorescence and Raman channels, respectively. CRCF is the calibration constant, which is determined by the ratio of the instrumental constant of the Raman channel to the fluorescence channel. NR(z) is the vertically distributed number concentration of Raman scatters and σR is the Raman differential scattering cross section in the backward direction.

The calibration constant accounts for the ratio of the opto-electronic efficiency of the nitrogen Raman channel to the fluorescence channel. And it can be determined by swapping the PMTs of the two channels (see Appendix A). The fluorescence capacity – GF,λ, is the ratio of the spectral fluorescence backscattering coefficient to the elastic backscattering coefficient at the fluorescence excitation wavelength (i.e., 355 nm for LILAS system), i.e., βF,λ(z)β355(z). This quantity is influenced by the presence of fluorescent compounds within the particles and reflects their ability to emit fluorescence signals when exposed to radiation. More detailed description about the definition and calculation of fluorescence backscattering and capacity can be found in .

The lidar operated at GPI in Moscow, Russia utilizes a tripled Nd: YAG laser at 355 nm, with pulse energy of 80 mJ and repetition rate of 20 Hz. To avoid the contamination to the fluorescence measurements, the laser radiation at 532 and 1064 nm is redirected by a dichroic mirror and then cleared by an optical dump. The characteristics of the telescope and the data acquisition recorder are the same as LILAS system. The optical receiver consists of an elastic channel at 355 nm, a N2 Raman at 387 nm and five fluorescence channels respectively centered at 438 (ΔD=29), 472 (ΔD=32), 513 (ΔD=29), 560 (ΔD=40), and 614 (ΔD=54) nm. This configuration allows for the detection of the extinction and backscattering coefficient at 355 nm, the spectral fluorescence backscattering coefficients and fluorescence capacities at five channels. The calibration of the 438 nm fluorescence channel is performed by swapping the PMTs, as described in the Appendix. And the relative sensitivity of the rest fluorescence channels with respect to the 438 nm channel is determined using a tungsten–halogen lamp, Thorlabs QTH10/M, with a color temperature of 2800 K. A more detailed description about this lidar system and the calibration procedure is presented in the study of . The spectral fluorescence measurements provide valuable information about the chemical composition of aerosols. In this study, we use the color ratio (CR_560∕472) between the 560 and 472 nm channel to represent the spectral variation of aerosol fluorescence: 2CR560/472(z)=βF,560(z)βF,472((z)

On 14 May 2023, LILAS detected a thick BBA plume beneath a dense cloud layer extending from 6000 to 12 000 m height, as shown in Fig. . According to the back trajectory analysis, the plume originated from wildfire on 5–6 May (shown in Fig. a in Appendix), in Alberta in western Canada. This observation suggests the Canadian fires had intensified, as LILAS had only detected some thin and drifting BBA layers before this date. The structure of the BBA layer is clearly illustrated in the quicklook of fluorescence backscatter coefficient in Fig. b, which shows the base of the BBA layer at 4000 m and the top at 6000–7000 m, in contact with the cloud base. A very thin BBA layer was observed at around 12 000 m height, shown by an enhancement in the fluorescence backscatter. However, the thick mixed phase and cirrus clouds at 6000–12 000 m height resulted in significant noise in the fluorescence backscattering signal.

Figure  exhibits the profiles of BBA properties averaged between 21:00 and 22:00 UTC and relative humidity obtained from both lidar observation and the ERA-5 reanalysis. The cloud base was stable at around 6200 m and the fluorescence backscatter coefficients extended up to 6800 m, indicating the presence of BBA particles inside the cloud base, where the humidity is high. Inside the BBA layer at 4600 to 6000 m, lidar ratios are about 36 and 69 sr at 355 nm and 532 nm and particle linear depolarization ratios at 355, 532 and 1064 nm are respectively 0.08 ± 0.01, 0.05 ± 0.01 and 0.013 ± 0.002, showing a typical spectral dependence of BBAs . The extinction and backscatter related Angström exponents are approximately 0.7 ± 0.2 and 2.2 ± 0.2. The fluorescence signal is strong inside the layer, and spectral fluorescence capacity is approximately 3.4 ± 0.2 ×10-6nm-1 and independent of height, which is an important feature of BBA aerosols due to the abundance of fluorescent molecules formed during the combustion of biomass.

Both ERA-5 and lidar-derived RH profiles show a steep increase from about 30 % at 4000 m to nearly 90 %–100 % at the cloud base. However, the vertical oscillations appearing in the RH profile from lidar measurement were not observed in the ERA-5 data, likely due to the decreasing vertical resolution with increasing height in ERA-5. As RH increases, hygroscopic aerosol particles absorb water from the surrounding air. This water uptake by particles is a rapid process–typically reaching equilibrium within seconds. Therefore, it is often considered as an equilibrium process rather than time-dependent . Water uptake increases particle size and sphericity, which can be detected in lidar measurements through increased elastic backscattering and a reduction in the depolarization ratio . In this case, despite a sharp RH increase above 5000 m, the backscatter coefficients at 355 and 532 nm remained steady or even decreased near the cloud base, showing no enhancement of elastic scattering due to water uptake. Additionally, the particle linear depolarization ratio at 532 nm remained low (around 0.05), and the EAE increased only slightly with height, suggesting no obvious response to the RH. Similarly, the variations in lidar ratios and BAE with increasing RH are more attributable to the vertical variability of BBA properties other than hygroscopic growth. The fluorescence capacity, which is sensitive to hygroscopic growth, as shown by in an urban aerosol layer, exhibited minor changes near the cloud base. These observations suggest that BBA particles did not show significant hygroscopic growth even at RH levels of 90 %–100 %.

The fluorescence quenching – i.e. the suppression of aerosol fluorescence capacity due to environment factors, for instance the humidity–has been reported in several publications . In Case 1, despite the sharp increase in RH to 90 %–100 % near the cloud base, the fluorescence capacity showed only minor variations (Fig. 3b). This observation contrasts with other events during the 2023 fire season, where quenching was clearly detected using a five-channel fluorescence lidar . However, with only a single broadband fluorescence channel at ATOLL, we cannot identify the spectra of the aerosols at low and high humidities, to check if they are from the same origin. And lidar observation cannot prove if aerosol and cloud particles interact or they only spatially coexist. Consequently, the absence of fluorescence quenching under high RH conditions remains an open question and requires further observational and laboratory investigations.

Previous studies have claimed that freshly emitted BBAs contain essentially non-hygroscopic compounds, however, they become increasingly hygroscopic during the aging process, due to the increase of oxidation level . Nevertheless, the assessment of BBA hygroscopicity is difficult because of the complex and variability of organic compounds. Our findings in this case contribute to the relatively limited evidence showing that aged BBA particles originated from Canadian wildfires do not exhibit significant hygroscopicity. Similarly, found that long-range transported smoke layers descending to the marine boundary layer have substantially lower hygroscopicity compared to the background marine aerosol.

It is worth noting that the influence of aerosol fluorescence on lidar water vapor measurement cannot be completely excluded, particularly in BBA layers whose fluorescence spectrum touches the water vapor Raman wavelength at around 407.5 nm. Two strategies can be adopted to mitigate this effect: either performing a correction to subtract the aerosol fluorescence contribution from water vapor signal, or reducing the bandwidth of water vapor interference filter to minimize the fluorescence contamination . However, the correction approach is not feasible for LILAS measurements, as it requires at least two fluorescence channels to characterize the spectral behavior of aerosol fluorescence. Instead, LILAS utilizes a very narrow interference filter of 0.3 nm bandwidth at 407.5 nm, to suppress as much as possible the influence of fluorescence. Considering that the humidity derived from measurements during the fire season in 2023 did not exhibit anomalous values in the tropospheric BBA layers, we can reasonably assume the BBA fluorescence had quite limited impact on the measurements of LILAS, although it could increase, to some extent, the uncertainty of the absolute values. To ensure the robustness of our analysis, RH profiles from ERA-5 reanalysis and radiosonde data were used to validate the lidar-derived humidity measurements.

The BBA layers detected at ATOLL on 27 and 28 May originated from wildfires on 19 and 20 May in Alberta, British Columbia and Saskatchewan, in the western area of Canada. Giant and dense smoke plumes can be observed from MODIS observations in Fig. b. In the night of 27 to 28 May, lidar quicklooks revealed stratified BBA plumes over ATOLL at 2000 to 12 000 m height, as shown in Fig. . One notable feature is that BBA layers at 12 000 m showed higher depolarization ratio at 532 nm than those in the free troposphere, as shown in Fig. c.

BBA optical properties derived from averaged lidar observations between 20:55 and 22:15 UTC are plotted in Fig. . The layers with extinction and backscatter coefficients peaking at 4000 and 12 000 m are identified as BBAs, due to their specific signatures in lidar ratios, depolarization ratios and their capability of producing fluorescence when exposed to laser radiation. Between 7000 and 10 000 m, an optically thin residual layer was observed in the profiles of elastic and fluorescence backscattering, although it was near the detection limit of LILAS. In the planetary boundary layer (PBL), below 2000 m, background aerosols dominated. Additionally, the RH from lidar measurement and ERA-5 dataset both show air mass was dry in these two BBA layers, with RH lower than 40 %, therefore, no hygroscopic effect is observed.

Table summarizes the optical properties of two BBA layers at different vertical levels. The BBA layer at around 12 000 m exhibited mean linear depolarization ratios of 0.20. 0.14 and 0.03 at 355, 532, and 1064 nm, respectively, while the depolarization ratios in the BBA layer at around 4000 m are about 50 % lower at each wavelength. Such a difference of depolarization ratio has been detected in previous lidar observations of transported BBA layers. The EAE (extinction-related Angström exponent) of approximately 0.0 is also a characteristic of aged BBA in the UTLS, whereas, typical BBA in the middle or lower troposphere tend to have slightly higher EAE. Notably, in this case we observed a higher fluorescence capacity in the UTLS BBA layer than the tropospheric BBA layer, adding more evidence that UTLS BBAs differ from those in the free troposphere in microphysical and chemical properties.

In the night of 31 May to 1 June 2023, BBA plumes transported to the GPI site also originated from western Canada, similar to Case 2. The emission of the plumes dated back to 27 May 2023, according to HYSPLIT back trajectory (See Fig. a). The lidar quicklooks in Fig.  show BBA plumes, marked by strong fluorescence signals, were distributed at 4000 to 10 000 m height. The PBL height was at around 2000 m, with a residual layer suspending above it. Two BBA plumes sequentially appeared in the height range of 9000–10 500 m, and the second plume appearing at 01:40 UTC showed a stronger fluorescence signal.

Aerosol properties averaged in two time intervals across the night of 31 May to 1 June are plotted in Fig.  and summarized in Table . The first time interval (T1) is from 22:30 to 23:58 UTC, 31 May 2023, and the second (T2) from 01:30 to 02:30 UTC, 1 June 2023. For clarity, only the spectral fluorescence backscatter coefficient at 513 nm and the elastic backscatter coefficient, as well as the vertically averaged fluorescence capacities in the second time interval are plotted for comparison with the first time interval. The color ratio of fluorescence signals at 560 to 472 nm, CR_560∕472 in Fig. a shows higher values in the UTLS layer than in the tropospheric layer. Similar to Case 2, the lidar ratio in the UTLS layer was about 36 sr, lower than 55 sr in the tropospheric BBA layer. The relative humidity at 4000 to 7000 m was in the range of 10 %–50 %, according to radiosonde and ERA-5 data. In the UTLS layer, radiosonde measurements provided RH values around 50 %, noticeably drier than the prediction of ERA-5, i.e, 60 %–80 %. According to our analysis, higher RH values of ERA-5 reanalysis than radiosonde/lidar measurements are often detected in the UTLS for both ATOLL and GPI station during the wildfire season in 2023. The discrepancy between model and radiosonde data in the UTLS has been reported in several previous studies and may be related to the lack of radiosonde measurements above the upper troposphere or/and the bias in other parameters in the meteorological field, for example, the temperature . The radiosonde sensor, for example, the RS92 sonde, has also well-known bias, which could underestimate RH in daytime measurements, however, at night, it showed much smaller errors . Therefore, in this study we take the radiosonde data as reference when it diverges from ERA-5 data.

The spectra of fluorescence capacity are averaged within three vertical ranges in the PBL, FT and UTLS, and are plotted in Fig. d. In both time intervals, the spectral fluorescence capacities in the PBL decreased with wavelength. In the 5000–6000 m, the fluorescence capacities of BBA particles increased significantly at wavelengths greater than 438 nm and peaked at 513 nm, with the values of approximately 7.3 × 10⁻⁶ nm⁻¹ in the first time interval and 8.1 × 10⁻⁶ nm⁻¹ in the second period. The UTLS layer in the first time interval showed even lower spectral fluorescence capacities than in the FT at wavelengths shorter than 560 nm. From the first to the second time interval, the BBA layer in the FT showed minor changes in the values and the spectral dependence. In contrast, the UTLS layer exhibits significantly stronger fluorescence capacities in the second time interval than in the first interval. Whereas, their normalized spectral fluorescence capacities, shown in Fig. e, are still in good agreement, both showing a red shift in the peak toward 560 nm in UTLS. This red-shift of BBA fluorescence spectrum in UTLS was also detected by at Lindenberg, Germany in transported BBA plumes from Canadian wildfires in 2023. It confirms that this was not a feature detected during specific events, but a recurring feature of transported BBA plumes during the Canadian wildfire season in 2023.

The BBA plumes detected in Case 2 and 3 shared similarities in geographical locations of fire sources and closeness of detection time, which offers a good opportunity for the comparison of fluorescence measurements. In Case 3, the spectral fluorescence capacity in the 472 nm fluorescence channel is in the same order of magnitude with the 466 nm fluorescence channel in Case 2 (see Tables  and ). While Case 2 showed a markedly higher fluorescence capacity of the BBA layer in the UTLS than in the FT, observations in Case 3 suggest that BBA particles in the UTLS do not necessarily always exhibit higher fluorescence capacity. Instead, the redshift of the fluorescence spectrum is a more recurring feature that BBA layers in UTLS differ from those in the FT.

Wildfires in Canada intensified significantly from the beginning of June 2023, with active fires spreading from northern British Columbia, Alberta and Saskachewan to central Alberta and the southern region of Northwest Territories. In the same period, large-scale wildfires broke out in eastern Canada, particularly in Quebec, making substantial contributions to the emission of BBA particles into the atmosphere. MODIS observations (see Fig. b) show an extensive coverage of BBA plumes stretching from western to eastern Canada on 13 June, making the attribution of individual plumes highly uncertain. According to HYSPLIT back-trajectory analysis in Fig. b, the BBA plumes arriving at the GPI station on 20–21 June likely originated from wildfires in Alberta or Quebec, after approximately 6 or 9 d of atmospheric transport, respectively. However, it is difficult to identify the exact source region, as intense wildfire activity occurred simultaneously in both provinces.

Figure presents the lidar observations during the night of 20 to 21 June 2023. The BBA layers extending from 3000 to 10 000 m are more clearly identified in the quicklook of the fluorescence backscattering at 513 nm, compared to the elastic backscattering signal at 355 nm. The BBA layer above 10 000 m is optically denser than the layers below, and marked with high fluorescence and moderate volume linear depolarization ratio. In Fig. c, some data points at around 10 000 m show volume depolarization ratio close to 0.10 between 23:10 and 23:50 UTC. It is likely an indication of ice crystals formed inside or below the BBA layers, which has been quite often observed during BBA observations.

Figure presents the BBA properties derived from averaged lidar observations and the RH profiles from ERA-5 analysis and from radiosonde measurement. The profiles of the spectral fluorescence backscatter coefficients in five channels, in Fig. a, show that aerosol layers from the PBL to the tropopause at around 12 000 m presented different levels of fluorescence. The fluorescence backscatter coefficients, the extinction, and backscatter coefficients at 355 nm peaked at 10 500 m, where a thick BBA layer was detected. The color ratio CR_560∕472, showed a clear increase versus height, from below 0.6 in the PBL to 1.2 near the tropopause. The increasing trend is particularly strong in the thin BBA layer at 7000 to 9000 m.

Lidar ratios at 355 nm calculated for the two BBA layers, 7000–9000 and 10 300–10 800 m, are approximately 38 sr and 32 sr, respectively, which are in good agreement with the observations in Case 2 at ATOLL station, although lower than the values observed in the night of 31 May to 1 June, 2023 (Case 3). Additionally, both Case 2 and Case 3 demonstrate lower lidar ratios (at 355 and 532 nm for ATOLL observations, at 355 nm for GPI observations) in the UTLS layers than in the free tropospheric layer. This signature could be an indicator of different morphology and/or radiative properties of BBA particles in the UTLS and in the troposphere.

Radiosonde measurements at Moscow station indicated RH values generally below 40 % at above 2000 m, whereas ERA-5 reanalysis showed RH increasing above 7000 m to a peak of 90 % near 10 500 m. Despite uncertainties in RH estimates from both radiosonde measurements and model data, we observed no evidence of hygroscopic growth in the BBA layer. Therefore, we can conclude that BBA properties in this case are not significantly influenced by the humidity in the atmosphere.

The spectral fluorescence capacities in five channels and the normalized spectra, averaged in three vertical ranges, are plotted in Fig. d and e. Between 800 and 1200 m, where urban aerosol was the dominant aerosol type, the spectral fluorescence capacity monotonically decreased with wavelength–from 1.50 ± 0.12 × 10⁻⁶ nm⁻¹ at 438 nm to 0.43 ± 0.04 × 10⁻⁶ nm⁻¹ at 614 nm. In contrast, within the BBA layer between 7000 and 8000 m, the spectral fluorescence capacity increased at at wavelengths greater than 438 nm, peaking at approximately 1.7 × 10⁻⁶ nm⁻¹ around 472 and 513 nm. In the higher BBA layer (10 300–10 800 m), the spectral fluorescence capacities across the five channels increased significantly, with the spectral peak shifting further to longer wavelengths. The maximum fluorescence capacity reached 6.0 ± 0.7 × 10⁻⁶ nm⁻¹ at 513 nm, closely followed by a second maximum of 5.8 ± 0.8 × 10⁻⁶ nm⁻¹ at 560 nm. The shift of the fluorescence spectrum is further evidenced by the increase of color ratio, CR_560∕472 in this layer, as shown in Fig. a.

Table presents a summary of the BBA properties observed in Case 3 and Case 4. The fluorescence channel at 472 nm of the GPI lidar is spectrally close to the 466 nm fluorescence channel of LILAS lidar at ATOLL observatory, therefore these two channels are used here for comparison. The spectral fluorescence capacities (at 472 and 466 nm) presented in the four cases are comparable in magnitude. Particularly in Case 2 and Case 4, both the tropospheric and UTLS layer show consistent fluorescence capacities measured by two different lidar systems. Higher fluorescence capacities in the UTLS layer were detected in Case 2, Case 4 and the second time interval of Case 3 at 472 and 466 nm. An exception occurred in the first time interval of Case 3, where the UTLS BBA plume did not exhibit enhanced fluorescence capacity, suggesting that this property is variable and likely influenced by multiple factors. In contrast, a redshift in the fluorescence spectrum within the UTLS was consistently observed in Cases 3 and 4.

The extinction coefficients and Angstöm exponents of 34 BBA layers detected from May to September 2023 at ATOLL observatory are presented in Fig. . These layers were identified as BBA based on their signatures in lidar ratios, particle linear depolarization ratios and fluorescence capacity. Among the 34 BBA layers, 26 layers were classified as free tropospheric layers (layer top < 8000 m), and the other 8 layers were classified as UTLS layers (layer base > 8000 m). The extinction coefficients of most BBA layers observed during this period were below 200 Mm⁻¹. The extinction-related Angström exponent in free tropospheric layers averaged 0.8 ± 0.3, generally bigger than those observed in the UTLS layers, where the values averaged 0.1 ± 0.2. The backscatter-related Angström exponents in the free tropospheric BBA layers averaged 2.0 ± 0.2, slightly bigger than those averaged in the UTLS layers, which is 1.5 ± 0.2. Such difference of Angström exponent between tropospheric and UTLS BBAs has been reported in previous lidar observations .

Light scattering models indicate the EAE of aerosols is strongly correlated with aerosol particle size. Both field campaign and laboratory measurements showed that BBA particles tend to grow during the aging process due to the condensation of gas-phase organic compounds and particle coagulation. Particle size can also vary from fire to fire, influenced by the burning conditions, fuel types and lifting mechanisms, which can all affect the morphology and composition of BBA particles injected into the atmosphere . For example, aircraft measurements showed BBA particles injected into the UTLS by pyro-cumulonimbus (PyroCb) convections were with thicker coatings, thus resulting in bigger size compared with tropospheric BBA particles. One likely explanation is that the strong updrafts in PyroCb convections lift large amounts of aerosols and vapors, promoting the coagulation of aerosol particles and condensation of the organic vapors, which accelerates the growth of BBA particles. The aging environment in the ambient atmosphere may also play a role. For instance, particles with diameters larger than 50–100 nm are more efficient CCN particles and therefore are more likely to be washed out by wet removal, a process that is more frequent in the free troposphere .

In contrast to the EAE, the BAE of aerosol particles shows a stronger dependence on the imaginary part of the refractive index (i.e., wavelength-dependent absorption), in addition to the influence of particle morphology. A detailed investigation on the effect of these factors is beyond the scope of this study. So far, there has been few observations addressing differences in BAE between tropospheric and UTLS BBAs, underscoring the need for additional measurements to better understand this feature.

The lidar ratios and particle linear depolarization ratios observed from May to September 2023 at ATOLL observatory are displayed in Fig. . The spectral dependence of lidar ratios, which correlates with wavelength, aligns with previous observations of BBAs from Canada, the US and Australia. The average lidar ratios in tropospheric BBA layers during this period are respectively 44 ± 9 and 72 ± 11 sr at 355 and 532 nm, while in the UTLS layers they were 36 ± 4 sr and 62 ± 4 sr. The slightly higher lidar ratios in tropospheric layers compared to the UTLS layer are consistent with lidar observation in Moscow, as shown in Case 3 and 4.

The particle linear depolarization ratios in tropospheric BBA layers averaged 0.12 ± 0.08, 0.07 ± 0.03 and 0.02 ± 0.01 at 355, 532 and 1064 nm. In contrast, the corresponding values in the UTLS layers are higher at 355, 532 nm, averaging 0.23 ± 0.08 and 0.14 ± 0.05, respectively, while remaining almost unchanged at 1064 nm with an average of 0.02 ± 0.01. Pronounced depolarization ratios of BBA layers in UTLS have been observed by several lidar systems during the remarkable wildfires in global scale – Canadian wildfire in 2017, Australian wildfire in 2019/2020, Californian wildfire in 2020 . High depolarization ratios are usually associated with particle morphology, i.e., their size and irregular shape. In the UTLS, aged BBA particles were detected with thick organic coating, which may appear semi-solid and glassy in cold and dry conditions in UTLS, thereby enhancing their depolarization ratio .

The lifting process of wildfire plumes may also influence the morphology of BBA particles. Exceptionally high depolarization ratios were observed on 12 July and from 23 to 30 September 2023, with values at 355 nm approaching or exceeding 0.30. According to , these plumes originated from wildfires associated with pyroCb activity, detected in Siberia on 30 June and in Canada on 15 and 22 September. PyroCb-convection can rapidly inject thick clouds of BBA particles, organic vapors, and ice crystals into the UTLS . BBA particles processed by pyroCb events are found to exhibit distinctive characteristics, including larger sizes and thicker organic coatings, compared to those not affected by such convective processes . Depolarization ratios at the end of August were also very high. Although the study by indicates that the lifting mechanism in August 2023 was a WCB (Warm Conveyor Belt) rather than a PyroCb, the strong smoke plume intensity observed at ATOLL (with extinction coefficients about 250 Mm⁻¹ at 355 and 532 nm and an EAE around 0.3–0.4 in the night of 29 August) suggested that the plumes were already very dense at emission, which provided favorable conditions for particle growth. In particular, LILAS detected tropospheric BBA layers with depolarization ratios comparable to those in the UTLS on 29 and 30 September. Lidar observations revealed that these layers gradually descended from the free troposphere to the planetary boundary layer over ATOLL within 2–3 d. HYSPLIT back-trajectory analysis suggests that this descent was associated with a dry intrusion – a strong downward motion driven by cyclonic activity that can rapidly transport air masses from the UTLS into the lower troposphere and boundary layer . Consequently, the observed tropospheric BBA layers with high depolarization ratios were likely of UTLS origin.

Transported BBA plumes originated from Alberta wildfires in late May 2023 have been reported by other lidar stations in Europe, providing an opportunity for the cross-comparison of fluorescence measurements. Figure  presents the spectral fluorescence capacities analyzed in Case 2 and 3 in this study, as well as measurements reported by two German lidar systems–MARTHA at TROPOS in Leipzig and RAMSES at DWD in Lindenberg. MARTHA and LILAS utilize the same interference filter at 466 nm for fluorescence measurements, while the RAMSES lidar employs spectrometers that allow for the detection of the fluorescence spectrum with a spectral resolution of about 12 nm. Consequently, the measurements of RAMSES can reveal more features of the fluorescence spectra and serve as a reference for broadband fluorescence measurements . Another important aspect of this comparison is to assess whether the fluorescence measurements, made by different lidar systems, each calibrated individually by different lidar groups, are consistent. Although the proximity in observational time does not guarantee that the plumes originated from the same wildfire, it can still eliminate the possibility of plumes coming from other regions, since the Alberta wildfires were the dominant fire sources during the second half of May.

Figure demonstrates the fluorescence capacities measured by LILAS and MARTHA are in good agreement in the tropospheric BBA layer. However, in the UTLS layer MARTHA derived much lower values than LILAS. This low fluorescence capacity is likely linked to cloud processing, as MARTHA detected ice cloud formation at the base of the UTLS BBA layer. Nevertheless, other explanations cannot be excluded, such as different smoke plumes arriving at ATOLL and TROPOS–specifically, those originating from the same fire complex but not the same fire, or having experienced different aging conditions. Additionally, changes in smoke properties during transport between ATOLL and TROPOS may also contribute to the observed differences. Observations from Lindenberg, which is closer to Leipzig than Lille, as well as the data from time interval T1 at GPI, show more consistent values with MARTHA observations in the UTLS layer. And the fluorescence capacity from LILAS is in good agreement with GPI data during time interval T2. In the spectral range around 466 nm, the fluorescence capacities by the four lidar systems are consistent in magnitude, although variability should not be overlooked. At wavelengths greater than 450 nm, the spectra of RAMSES also demonstrate a gradual increase of spectral fluorescence capacities versus BBA layer height, except in the layer at 4600 m. Although spectra of the fluorescence capacities measured by GPI lidar and RAMSES show some extent of variability from layer to layer, the values are generally comparable and show consistent features, particularly in terms of the shape of the spectra and the central wavelengths in the troposphere and UTLS. For instance, the two BBA layers detected by RAMSES at 5700 and 10 500 m over Lindenberg, have almost the same central wavelengths and color ratios CR560/472, compared with the layers at 5500 and 9000 m over GPI station (Case 3). A quantitative comparison of BBA fluorescence measurements is presented in Table  in the Appendix. It is important to note that the calibration of single-channel broadband fluorescence in LILAS currently carries high uncertainty (potentially leading to a 30 % underestimation, see details in Appendix A), due to the insufficient characterization of the transmission ratio between the fluorescence and Raman channels. More sophisticated calibration procedures will be required for future cross-comparisons.

Figure a and b show the time series of spectral fluorescence capacities in BBA layers detected by lidar systems at ATOLL observatory and at GPI, respectively, in the period from May to September 2023. The average of the spectral fluorescence capacity in the tropospheric BBA layers was 2.7 ± 0.8×10-6 nm⁻¹ at ATOLL and 3.2 ± 1.3×10-6 nm⁻¹ GPI, while in the UTLS layers, the values were 4.7 ± 1.6×10-6 nm⁻¹ at ATOLL and 4.8 ± 2.0×10-6 nm⁻¹ at GPI. Despite differences in spectral coverage and calibration methods between the two fluorescence channels, the spectral fluorescence capacities at both sites are generally comparable in both the troposphere and the UTLS. This consistency validates the comparability of fluorescence measurements between the two different lidar stations and confirms that the properties of BBA particles arriving at these two lidar stations do not exhibit significant geographical variations. The values of spectral fluorescence capacity detected by lidar at GPI at 472 nm are generally greater than those at ATOLL observatory at 466 nm, it is probably because the 472 nm is closer to the peak of BBA fluorescence spectrum, as have been shown in Fig. .

In Fig. , we can see that enhanced spectral fluorescence capacities of BBA layers in the UTLS were observed at both stations. Although the spectral fluorescence capacity exhibits a substantial variability of approximately 40 %–60 % in both the UTLS and the free troposphere, the highest values were consistently observed in the UTLS, with no comparable peaks detected in the free troposphere. The values of spectral fluorescence capacity of BBAs may depend on multiple factors, such as the geographic location of wildfires, the vegetation, the burning condition, the aging process and so on. However, it is difficult to assess their influences on the fluorescent properties of BBAs, due to the limited information derived from remote sensing observations and uncertainties in the back trajectories of the plumes. The series of CR_560∕472, which is the color ratio of fluorescence capacity or backscatter coefficient between 560 and 472 nm channels, is generally higher in the UTLS, averaging 1.3 ± 0.2, compared to 0.9 ± 0.2 in the troposphere. Higher color ratio in the UTLS than in the troposphere is a recurring feature of BBAs and has been detected consistently by both GPI and RAMSES during the fire season in 2023 .

Figure summarizes the spectra of fluorescence capacity, as well as their normalized forms, in aerosol layers in three vertical ranges–below 3000, from 3000 to 8000, and 8000 to 12 000 m, measured by the lidar system at GPI from May to September in 2023. Urban aerosols, which dominated below 3000 m, showed fluorescence capacities generally lower than 2×10-6 nm⁻¹, with a spectrum monotonically decreasing versus wavelengths. Long-range transported BBAs were the major aerosol type at above 3000 m, while their fluorescence spectra show distinctive characteristics. In the free troposphere at 3000 to 8000 m, the fluorescence capacities varied between 1.5 ×10-6 and 5×10-6 nm⁻¹. In the UTLS at above 8000 m, the mean spectral fluorescence capacities are generally higher than their counterparts in the free troposphere. Additionally, the normalized fluorescence spectra in the UTLS showed a clear shift of the spectrum peak toward longer wavelengths.

Although the 2023 Canadian wildfire season was exceptional in terms of burnt area, fire emissions and the generated detected PyroCb count, the emitted BBA plumes were mostly limited to the upper troposphere and the lowermost stratosphere. The uppermost BBA layer heights in this study is at 12–14 km, in agreement with SAGE III (Stratospheric Aerosol and Gas Experiment) observation presented in . In this analysis, we collect three intensive parameters of BBAs – the EAE_355−532 and particle depolarization ratio at 532 nm from the observations at ATOLL and the color ratio CR_560∕472 from the observations at GPI station, to investigate the vertical variation of BBA properties. Figure  presents the variations of the three parameters with respect to the altitude of the BBA layer base, along with the corresponding linear regressions. The EAE decreases gradually with the increasing layer altitude, yielding r2≈0.61, while the depolarization ratio exhibits an increasing trend with r2≈0.68. The color ratio CR_560∕472 also shows a positive trend with altitude, with r2≈ 0.69. These values of r2 suggest moderate correlations; however, the data sets do not provide sufficient evidence to establish a genuine altitude dependence.

Most data points collected at ATOLL were from the free troposphere, as the calculation of EAE and depolarization ratio requires relatively higher optical thickness and signal-to-noise ratio in BBA layers, which are challenging conditions for UTLS layers. Consequently, the number of measurements in UTLS layers is lower than in the free troposphere, which limits the analysis of the vertical dependence. For the depolarization ratio in the troposphere, most data points are clustered near 0.05, with little variation across the altitude range and no clear trend versus altitude. The apparent positive correlation with altitude emerges only when the UTLS data points were included, as they are at higher altitude and exhibit distinctly higher depolarization ratios. The pattern may be an indication that the statistical correlation is driven primarily by the separation between the two regimes–tropospheric and UTLS, rather than by a continuous, altitude-driven relationship. A similar two-cluster distribution is also evident in the color ratio plot. These features are more consistent with discrete differences in BBA properties than with a vertical dependence. These observations indicate a possibility that altitude itself may not be the ultimate controlling factor of the BBA properties, but rather an intermediate variable linked to other factors, which could be the temperature and/or the injection height of the BBA layers in their source region, and so on. investigated the correlation between BBA properties and factors such as the origin, transport time, in-layer temperature and humidity, using RAMSES lidar data. They reported that the fluorescence spectra of the BBAs from different wildfire sources at different geographical locations showed, to some extent, correlation with the vegetation type (or climate zone) and in-layer temperature, while weak correlation with the transport time. Relative humidity also appeared to influence the fluorescence spectra, although its effect likely depends on the origin of BBA plumes. At present, further investigation of the dependence of BBA properties is constrained by the limited variability in the available BBA measurements and the small number of collocated and high-quality humidity and temperature data in the BBA layers.

RHs within approximately 50 BBA layers observed at each lidar station were determined using a combination of lidar and radiosonde measurements. The RH data at GPI site were obtained from radiosonde measurements at Moscow site, while at ATOLL site, priority was given to RH derived from the lidar water vapor channel. When lidar water vapor measurements were unavailable (10 BBA layers), radiosonde data from Beauvechain (Belgium) station, 100 km from ATOLL, were used.

Figure a shows the frequency distribution of detected BBA layers as a function of their mean RH. All the analyzed layers exhibited RH values below 80 %, with the majority (90 % at Moscow and 85 % at ATOLL) showing mean RH below 50 %. At both sites, observations showed that RHs generally decreased at the altitudes where BBA layers occurred, even in the free troposphere where water vapor is more abundant than in the UTLS. The peak occurrence of RH within the detected BBA layers was in the range of 10 %–20 %.

Figure b presents the distribution of the spectral fluorescence capacity as a function of the mean RH within the BBA layers. The fluorescence capacities at 472 nm (GPI lidar) and at 466 nm (ATOLL lidar) are comparable and distribute mainly in the range of 2.0×10-6 to 4.0×10-6nm-1. Layers with pronounced fluorescence capacity, greater than 6.0×10-6nm-1, are mainly UTLS layers, with RH approximately at 20 % or lower. According to the vertically averaged data across 5 months, plotted in Fig. b, the spectral fluorescence capacities do not show noticeable correlation with RH when air mass is relatively dry, i.e. RH ≤ 60 %. For RH > 60 %, the fluorescence capacity seemingly decreases with RH, however, the number of data points is not sufficient to draw a firm conclusion.

and investigated the relationship between aerosol (not limited to BBA) fluorescence properties and RH using vertically resolved profiles from selected cases, rather than multi-month layer-averaged means. reported no clear correlation between RH and the central wavelength of the fluorescence spectra for the BBA layers likely originating from Western Canada on 26–27 May. While for BBA plumes from Eastern Canada, the results showed a weak to moderate correlation. These findings are in line with our observation in Case 1 where the fluorescence capacity stayed almost unchanged in the BBA layer with high humidity below the cloud base. While this cannot be considered direct, mutually corroborating evidence, it provides indirect support for limited hygroscopicity of aged BBAs. Most BBA layers analyzed in this study are dry and do not exhibit significant RH gradient, affecting the robustness of the correlation analysis. However, this generally low humidity in BBA layers suggests that aged BBAs have limited ability of absorbing water from the environment, implying low hygroscopicity. Fluorescence measurements have potential as a proxy for assessing aerosol hygroscopicity, but current studies are sparse and scattered, highlighting the need for more systematic investigation.

In previous publications, the atomic oxygen-to-carbon ratio (O:C) of the organic compounds are often used to assess the hygroscopicity organic particles, based on semi-empirical relationships between the hygroscopicity and the oxidation level of the organic aggregates . These studies, based on laboratory and field measurements, overall derived a generally positive correlation between the hygroscopicity and O:C ratio of organic aerosols. As a result, BBA are typically expected to become increasingly hygroscopic after being emitted into the atmosphere. However, the measured hygroscopicity of aerosols originated from biomass burning is still very variable, with κ – the hygroscopic parameter, varying from below 0.1 (weakly hygroscopic) to 0.4 (moderately hygroscopic) and showing dependence on multiple factors, such as fuel type and aging time . Recent research reported other factors, such as carbon chain length, organic functionality and water solubility have significant impact on the hygroscopicity of organic aerosols, which explains the widely varying hygroscopicity of organic aerosols. It also points out the oxidation level of organic aerosols is not sufficient for parameterizing their hygroscopicity and the determination of the hygroscopicity of BBAs is a challenging task due to their complex organic composition and aging conditions in the atmosphere .