A lidar with five discrete fluorescence channels centered at 438, 472, 513, 560 and 614 nm wavelengths has been operational at the Prokhorov General Physics Institute since 2022 (Veselovskii et al., 2023). The lidar is based on a tripled Nd:YAG laser with pulse energy of 80 mJ at 355 nm and repetition rate of 20 Hz. Backscattered light is collected by a 40 cm aperture telescope and the lidar signals are digitized using Licel transient recorders with 7.5 m range resolution. Measurements were performed through a laboratory window at an angle of 48° to the horizon. This configuration enables the retrieval of the aerosol backscattering, β355, and extinction, α355, coefficients along with five fluorescence backscattering coefficients. Additional atmospheric parameters were obtained from radiosonde measurements at the Dolgoprudny meteorological station, located about 50 km from the observation site. The calibration of the fluorescence channels and the calculation of Bλ follow the procedure detailed in Veselovskii et al. (2020, 2023). As the fluorescence channels in our lidar have different bandwidths, Bλ is determined as the total fluorescence backscattering for a given channel, normalized by its spectral width. Throughout this paper, all reported Bλ are given in units of Tm-1sr-1nm-1.

Urban aerosol comprises a wide range of particle types, including sulfates, nitrates, secondary organic components, black carbon, and others. Among these, the organic components are the primary contributors to fluorescence. Smoke and urban particles exhibit distinct fluorescence spectra, providing a basis for their discrimination (Veselovskii et al., 2025a). In an external aerosol mixture where these two components predominate, the total measured fluorescence spectrum can be approximated as a linear combination of their individual contributions, BλU and BλS: 1Bλ=BλU+BλS=aBλUref+bBλSref This is the system of five equations (one for each fluorescence channel) with two unknown coefficients, a and b. The terms BλUref and BλSref represent the reference fluorescence spectra for pure urban aerosol and pure smoke, respectively. Once the fluorescence contributions of urban aerosol, BλU, and smoke, BλS, are separated, the aerosol backscattering coefficients, attributed to each particle type are calculated as: 2β355U=BλUGλUandβ355S=BλSGλS assuming the fluorescence capacities remain constant within the spatiotemporal intervals considered.

We solve the system of Eq. (1) using the least squares method, minimizing the difference between the measured and reconstructed fluorescence backscatter across all five channels. For most cases, the corresponding discrepancy is within a few percent. Consequently, any fluorescence channel can be used to calculate the backscattering coefficients via Eq. (2). In this study, we used the 513 nm channel as it lies near the center of the measured fluorescence spectrum. For selected G513U and G513S, the fluorescence capacities at other wavelengths are recalculated using the reference fluorescence spectra BλUref and BλSref normalized to their values at 513 nm. As an additional verification of the retrieval, we check that β355U+β355S≈β355.

One challenge in applying Eq. (2) is particle hygroscopic growth at high relative humidity. An increase in particle size enhances the aerosol backscattering coefficient, leading to decrease in the fluorescence capacity. Therefore, if G513U and G513S are derived at low RH, the retrieved β355U and β355S in high RH regions will be underestimated. This effect can be accounted for by interpreting the results as representing the backscattering of dry particles (Miri et al., 2024). However, water uptake can also cause the fluorescence quenching (Veselovskii et al., 2025b). At present we do not correct for this quenching and do not consider β355U and β355S derived within spatiotemporal intervals with high RH. It is important to note that the spectral shape of fluorescence (i.e., the ratios between channels) is not affected by hygroscopic growth (Veselovskii et al., 2025a), allowing the separation of fluorescence contributions via Eq. (1) even at high RH.

As noted, the fluorescence spectrum of urban aerosol decreases monotonically with wavelength, while that of smoke exhibits distinct maxima at the 513 and 560 nm channels. Our 2024 observations corroborate the main findings from 2023 reported by Veselovskii et al. (2025a). Figure 1 shows representative fluorescence spectra of urban particles within the PBL and of smoke in the middle troposphere from several 2024 measurement episodes. The urban aerosol fluorescence spectrum exhibits notable seasonal variation. To quantify the spectral slope, we use the ratio R560/438=B560B438, as this parameter provides the strongest distinction between smoke and urban aerosol signatures. This ratio is about 0.6 for the period from April to the middle of May, and it decreased to a minimum of 0.28 in July before rising to ∼0.35 in September 2024. A similar seasonal pattern for R560/438 was observed in 2023. The monthly averages for May through September 2024 were 0.56, 0.42, 0.35, 0.40, and 0.47, respectively. Thus, the decrease in fluorescence with wavelength becomes steeper in summer. This trend likely results from increased vehicular emissions and enhanced secondary organic aerosol formation under higher summer temperatures. In contrast, the fluorescence spectra of smoke showed no clear seasonal dependence.

In Veselovskii et al. (2025a), the reference spectra BλUref and BλSref were defined as averages over all 2023 observations. Given the high variability in fluorescence spectra, at present study for each analyzed episode, we selected spatiotemporal intervals expected to represent pure smoke or urban aerosol. When such an interval was unavailable for a given episode, we used appropriate spectra from temporally proximate measurements.

To calculate particle volume concentration from the retrieved backscattering coefficients, we use extinction-to-volume conversion factors at 355 nm specific to urban aerosol, C355U, and smoke, C355S. This requires first converting the backscattering coefficients to extinction coefficients: 3α355U=β355U×S355Uandα355S=β355S×S355S where S355U and S355S are the corresponding lidar ratios at 355 nm wavelength. The volume concentration for smoke is then: 4VS=C355S×α355S and the mass concentration is given by: 5Ms=ρSVS=ρSC355Sα355S=ρSC355SS355Sβ355S=ρSC355SS355SBλSGλS=KλSBλS where ρS is the smoke density. Thus, factor KλS relates the fluorescence backscattering to the smoke mass concentration. The mass concentration of urban aerosol is calculated analogously using its respective density ρU, conversion factor C355U and lidar ratio S355U. Based on values reported by Li et al. (2016) and Ansmann et al. (2021), the smoke particle density typically falls within the range of 1.0–1.3 gcm-3. In this study we use the mean value ρS=1.15 gcm-3.

The extinction-to-volume conversion factor at 532 nm for aged smoke, C532S has been derived by Ansmann et al. (2021) from AERONET measurements and by Veselovskii et al. (2025b) from multiwavelength Mie–Raman lidar observations. Both methods yielded a value of C532S≈0.13 µm3cm-3Mm. For the conversion of our 355 nm lidar measurements we adopt the value C355S=0.085±0.015 µm3cm-3Mm, reported by Veselovskii et al. (2025b). For urban aerosol a lidar-derived value of C355U=0.08±0.03 µm3cm-3Mm was obtained for Lille, France (Veselovskii et al., 2025b). However, urban aerosol composition, and thus its conversion factor, can vary significantly by region. For instance, the C532U value from Lille is nearly half that reported by Mamouri and Ansmann (2017) for Leipzig. Consequently, applying the Lille-derived C355U to the urban aerosol in Moscow could introduce substantial uncertainty. In this study we calculate and report mass concentration only for aged smoke. The overall uncertainty in quantifying smoke mass concentration mixed with urban aerosol depends on the combined uncertainties of the parameters used: the smoke density, the lidar ratio, the fluorescence capacity, and the conversion factor. We estimate the total uncertainty of this procedure to be below 40 %. In contrast, the relative spatiotemporal variations in smoke concentration within a single measurement episode can be derived with significantly lower uncertainty, which we estimate to be below 10 %.

As a first step in the analysis of the measurements, it is important to estimate expected sensitivity of fluorescence lidar to the presence of smoke, within the PBL. For an external mixture of urban and smoke particles the total measured fluorescence backscatter is Bλ=BλU+BλS.

As mentioned, for the lidar used, a convenient indicator of smoke intrusion to the PBL is an increase of the spectral ratio R560/438. Pure urban aerosol is characterized by the spectral ratio R560/438U. The intrusion of smoke increases this ratio by factor A, such that R560/438=AR560/438U. Thus, measured spectral ratio is: 6R560/438=B560U+B560SB438U+B438S=R560/438UB438U+R560/438SB438SB438U+B438S And consequently: 7B438S=B438UR560/438U(1-A)(AR560/438U-R560/438S) Our instrument can reliably detect a 10 % change in the spectral ratio (i.e. A=1.1). Using the characteristic values of R560/438U=0.5, R560/438S=1.5 and a typical urban fluorescence backscatter B438U=2 Tm-1sr-1nm-1, the Eq. (7) yields a detectable smoke fluorescence backscatter of B438S≈0.1 Tm-1sr-1nm-1. The smoke mass concentration corresponding to this fluorescence backscatter can be calculated from Eq. (5) as: 8MS=B438SρSC355SS355SG438S Substituting typical aged smoke properties, such as S355S=40 sr (Haarig et al., 2018) and G438S=4×10-6 nm-1 we obtain a detectable smoke mass concentration MS≈0.1 µgm-3. This is a rough estimation, but it reveals high sensitivity of fluorescence technique, which is a direct result of the pronounced difference between the fluorescence spectra of smoke and urban particles. For more accurate evaluation of smoke concentration, it is necessary to solve the system of Eq. (), as will be done in the following section.

On the night of 26–27 September 2023, air masses originating from North America descended from approximately 7000 m to 2000–3500 m height range, as indicated by HYSPLIT backward trajectory analysis (Stein et al., 2015) shown in Fig. 2. This enabled the mixing of transported smoke with urban particles within the PBL. The spatiotemporal distributions of aerosol parameters, such as β355, B513, G513, and R560/438 are shown in Fig. 3. Two distinct aerosol layers are evident. A lower layer extends from the surface to ∼1250 m altitude. It is characterized by a low fluorescence capacity (G513<1.5×10-6 nm-1) and a low spectral ratio (R560/438<0.7), indicating the predominance of urban aerosol. Above 1250 m both G513 and R560/438 increase markedly, with G513>4×10-6 nm-1 and R560/438 up to 1.5. These values are characteristic of smoke particles. Radiosonde measurements taken at 00:00 UTC on 27 September 2023 reveal a temperature inversion at , confirming this altitude as the PBL top. The smoke layer is thus situated directly atop the PBL. Notably, within the PBL itself, the spectral ratio R560/438 for the period 17:00–22:00 UTC is higher than that observed during 23:00–00:00 UTC. This suggests that smoke penetrated into the PBL up to approximately 22:00 UTC.

To separate the fluorescence backscatter contributions of smoke and urban particles using the method described in Sect. 2, reference spectrum of pure smoke, BλSref, was derived from the elevated layer at ∼2800 m, where the fluorescence capacity G513 exceeded 6×10-6 nm-1. The pure urban aerosol spectrum, BλUref, was taken from measurements the previous night (25–26 September), when smoke content within the PBL was below detection limit. The particle parameters used in the analysis of this episode are given in Table A1.

Recall that this method relies on the spectral shape, i.e., the relative fluorescence backscatter across wavelengths. The absolute magnitude is not used directly in the separation algorithm. Formally, the algorithm contains no threshold. However, for low signals, the ratios of fluorescence backscattering in different channels become oscillatory. In data analysis, this manifests as oscillations in the retrieved fluorescence components. Normally, in the lower troposphere, we limit our consideration to values where B513>0.5 Tm-1sr-1nm-1.

The resulting spatiotemporal distributions of the separated fluorescence backscattering coefficients for urban aerosol, B513U, and smoke, B513S, are shown in Fig. 4. Urban aerosol is confined primarily within the PBL. While the main smoke plume resides above the PBL, a significant amount of smoke (B513S up to ∼1.0 Tm-1sr-1nm-1) is present within the PBL.

Vertical profiles of particle properties averaged from 17:15–23:00 UTC are presented in Fig. 5. Due to incomplete geometrical overlap below ∼1000 m, the lidar ratio within the PBL is not shown. Radiosonde measurements indicate that relative humidity increases with height, reaching 83 % at 1150 m. The observed increase in the aerosol backscattering coefficient β355 near the PBL top is likely due to particle hygroscopic growth. Within the PBL, the fluorescence spectral ratio R560/438 is elevated (∼0.65), indicating a mixture of smoke and urban particles. In contrast, above the PBL, where smoke predominates, this ratio increases to ∼1.5. The spectral shape also differs between layers. Within the PBL, fluorescence decreases monotonically with wavelength, which is characteristic of urban aerosol predominance. Above the PBL, the spectrum exhibits a distinct maximum at 513 nm, a known signature of biomass burning smoke. The potential temperature measured by radiosonde (not shown) is constant up to ∼1200 m, confirming a well-mixed PBL. Therefore, we would expect fluorescence backscatter to be relatively uniform with height within this layer. However, Fig. 5b shows a clear decrease in B513 between 500 and 1000 m. This reduction is consistent with fluorescence quenching caused by water uptake at elevated RH, as reported by Veselovskii et al. (2025b).

The vertical profiles of the retrieved fluorescence backscattering coefficients B513U and B513S along with the total fluorescence backscatter B513 are shown in Fig. 5b. The reference spectra, used in the retrieval are presented in Fig. 6a. The reference spectrum of smoke is an average taken overnight within the 2600–2800 m height interval. The reference spectrum of urban aerosol was obtained from measurements in the PBL on 25 September 2023, when smoke contamination was minimal.

The measured B513 is accurately reconstructed by the sum B513U+B513S. To convert B513U and B513S into corresponding aerosol backscattering coefficients, β355U and β355S, accurate values for the fluorescence capacities G513U and G513S are required. For this case, we used G513S=6.0×10-6 and G513U=1.0×10-6 nm-1. The aerosol backscattering coefficient β355 is well reconstructed by the sum β513U+β513S within the elevated smoke layers. However, within the PBL, the sum β513U+β513S is systematically less than measured β355. This discrepancy is attributed to particle hygroscopic growth occurring at high relative humidity.

The smoke mass concentration, MS can be derived from Eq. (). For the elevated smoke layer, the lidar ratio is S355S=38±6 sr and G513S=6.0×10-6 nm-1. For these values, coefficient K513 in Eq. () is of 0.63 µgsr, meaning that a fluorescence backscatter B513S=1.0 Tm-1sr-1nm-1 corresponds approximately to a smoke mass concentration of MS≈0.63 µgm-3. From spatiotemporal distribution in Fig. 4b we estimate that the smoke mass concentration exceeds 30±12 µgm-3 within the main elevated smoke layer. As noted, smoke mass concentrations within the PBL can be underestimated due to fluorescence quenching during hygroscopic growth. Therefore, to analyze relative trends less affected by this bias, Fig. 6b presents the temporal evolution of the mean smoke mass concentration within the 500–550 m altitude range. This layer was selected because the relative humidity remains below 70 % and the total aerosol backscattering β355 is accurately reconstructed by the sum β513U+β513S. Within this layer, the derived mass concentration decreased from approximately 0.8±0.3 to 0.2±0.08 µgm-3 over the course of the night.

As discussed, the retrieved smoke concentration depends on the choice of reference fluorescence spectra. However, because smoke and urban aerosol have fundamentally different spectral shapes, the retrieval is not overly sensitive to minor spectral variations. Figure 6a compares the reference spectra used in our retrieval (solid lines) with the 2023 annual mean spectra (dashed lines). While the urban aerosol spectrum closely matches BλUref, the annual mean spectrum for smoke deviates noticeably from the reference spectrum BλSref used in the retrieval. The impact of this spectral difference on the retrieved smoke mass concentration is shown in Fig. 6b. Using the annual mean spectra increases the derived concentration by approximately 40 %, although the relative temporal trend is mostly preserved. From these results, we conclude that selecting episode-specific reference spectra is essential for accurate concentration estimates. At the same time, it is reasonable to expect that the fluorescence spectra of pure smoke and urban aerosol do not vary significantly within the PBL during a single episode. Consequently, the retrieval method should reliably capture the relative temporal and vertical variations in smoke mass concentration, particularly under conditions where hygroscopic effects are minimal.

Between August and October 2024, intensive wildfires in southern Russia generated multiple episodes of smoke transport to Moscow. In this section, we analyze three representative events: 25–26 September, 1–2 October, and 3–4 October. The corresponding five-day HYSPLIT backward trajectories, shown in Fig. 7, confirm that the sampled air masses passed over active fire regions. These cases exemplify distinct scenarios of smoke interaction with the urban environment: (i) a strong smoke plume directly invades the PBL; (ii) relatively weak smoke plumes are observed against a background of urban aerosol; and (iii) a smoke layer rests atop the PBL. For all three episodes, the relative humidity remained below 60 %. These dry conditions minimized the effects of hygroscopic growth and fluorescence quenching, allowing for a more reliable reconstruction of smoke mass concentration.

The spatiotemporal distributions of particle parameters (identical to those in Fig. 3) for the night of 25–26 September are presented in Fig. 8. A strong aerosol plume, centered at approximately 1400 m altitude, is observed between 22:00 and 01:00 UTC. Within this plume, the aerosol backscattering coefficient, β355, exceeds 10 Mm-1sr-1. The plume is characterized by an enhanced fluorescence capacity (G513>5.0×10-6 nm-1) and high spectral ratio (R560/438>1.7). Figure 7a indicates that the air mass associated with this plume was transported at low altitudes over fire regions near the Black Sea.

For the separation B513U and B513S, the reference smoke spectrum was obtained near the top of the plume (1600–1800 m altitude), where the fluorescence capacity G513 was the highest. The reference urban aerosol spectrum was derived from measurements on 24 August 2024 within the 750–1000 m interval (as will be shown in Fig. 21d), with no smoke in the PBL. The values of G513U=0.8×10-6 and G513S=5.5×10-6 nm-1 were used for analysis. The separated fluorescence backscattering coefficients are shown in Fig. 9. The aerosol plume is dominated by smoke, though a moderate increase in urban aerosol concentration within the plume is also observed (B513U up to 5 Tm-1sr-1nm-1). Outside of plume (17:00–19:00 UTC) the fluorescence backscatter of urban aerosol and smoke is relatively weak (B513U<2 and B513S<5 Tm-1sr-1nm-1).

Vertical profiles of particle parameters within the plume are shown in Fig. 10. The spectral ratio R560/438 and the fluorescence capacity G513 increase with altitude, reaching peak values of approximately 1.75 and 5.5×10-6 nm-1 respectively, near the plume top (∼1750 m). The lidar ratio also increases slightly from 45 to 48 sr. This vertical gradient likely results from the mixing of the smoke with local background pollution during transport. Additionally, atmospheric aging processes, which can alter particle composition during transport, may further contribute to the observed gradient. The measured B513 and β355 are well reconstructed by the sums B513U+B513S and β513U+β513S respectively. Using the G513 and S355 values from 1700 m we calculate that K513S=0.87 µgsr. As follows from Fig. 10b, the maximum smoke mass concentration within the plume is approximately 50±20 µgm-3.

On the night of 1–2 October several intense aerosol plumes were observed within 1000–2000 m altitude range. The air mass trajectory (Fig. 7b) did not intersect the main fire zone but instead skirted its northern periphery. Figure 11 presents the spatiotemporal distributions of the particle parameters (the same as in Fig. 4). Within the plumes the fluorescence capacity G513 and the spectral ratio R560/438 increase to approximately 3×10-6 nm-1 and 1.2 respectively, which are the values characteristic of smoke. However, these values are lower than in the previous episode, as background urban aerosol contributes significantly to the total fluorescence backscatter in this case.

Urban aerosol and smoke were mixed throughout the PBL. To separate their contributions, we used the reference spectrum for urban aerosol measured on 24 August 2024, and the reference spectrum for smoke obtained on 3 October 2024, as detailed in the following section. The results of separation B513U and B513S are shown in Fig. 12. The distribution of background urban aerosol is relatively uniform, confirming that the observed aerosol plumes consist predominantly of smoke.

Vertical profiles of particle parameters, averaged over the period 00:15–01:15 UTC and encompassing a strong smoke plume, are displayed in Fig. 13. Both the fluorescence capacity G513 and the spectral ratio R560/438, increase with altitude, reaching maximum values of 3.2×10-6 nm-1 and 1.3 respectively, near the top of the plume. The lidar ratio also increases with height, rising from 50±8 to 60±9 sr. For reference, the lidar ratio for pure smoke measured on 3 October 2024 is 75±10 sr. The observed increase of S355 with height is thus consistent with an increasing proportion of smoke in the aerosol mixture. The fluorescence capacity spectra in Fig. 13d differ from those presented for 26 September 2024 (Fig. 10). Specifically, the enhancement in G513 and G560 near the plume top is less pronounced, which can again be attributed to a greater degree of mixing between smoke and background urban aerosol.

For calculation the aerosol backscattering coefficients, we used G513U=0.8×10-6 nm-1 (from 24 August 2024) and G513S=5.3×10-6 nm-1 (from 3 October 2024). As shown in Fig. 13c, the measured β355 is accurately reconstructed by the sum β513U+β513S, corroborating the assumption, that fluorescence capacity of smoke and urban particles remained constant with altitude. Using Eq. (5) with S355=75 sr we find that K513S≈1.4 µgsr. Consequently, the maximum smoke mass concentration within the plume is approximately 9.0±3.6 µgm-3.

On 3–4 October air mass passed again over region of intense fires (Fig. 7c). As shown in Fig. 14, a smoke layer with high fluorescence capacity (G513S>5×10-6 nm-1) and a high spectral ratio (R560/438>1.4) is located at the top of the PBL between 17:00 and 22:00 UTC. To separate the fluorescence backscattering contributions of smoke and urban aerosol, the smoke reference spectrum was obtained from the 2600–2800 m altitude range, while the urban aerosol reference spectrum was again taken from measurements on 24 August 2024. The resulting spatiotemporal distribution of background urban aerosol, shown in Fig. 15, is relatively uniform, with B513U<1.5 Tm-1sr-1nm-1. In contrast, the smoke layer atop the PBL exhibited strong fluorescence backscatter B513S>18 Tm-1sr-1nm-1, while within the PBL B513S remained low (below 2 Tm-1sr-1nm-1). Thus, smoke was primarily localized above, rather than within, the PBL.

Vertical profiles of particle parameters averaged over the 16:50–22:00 UTC period are shown in Fig. 16. A smoke layer atop the PBL is centered at approximately 2700 m, with a peak aerosol backscattering coefficient β355 of 3.5 Mm-1sr-1. Within this layer both the spectral ratio R560/438 and the fluorescence capacity G513 increase sharply to values of 1.4 and 5.3×10-6 nm-1 respectively. Below 2250 m the lidar ratio is 45±7 sr, but within the smoke layer S355 increases to 75±10 sr, a value typical for fresh smoke (Haarig et al., 2016).

For the calculation of aerosol backscattering coefficients β513U and β513S we used fluorescence capacities G513U=0.8×10-6 and G513S=5.3×10-6 nm-1. The sum B513U+B513S accurate reconstructs the measured fluorescence profile B513 at all altitudes. However, for the aerosol backscattering coefficient, the sum β513U+β513S falls below the measured β355 at altitudes below 1000 m. This discrepancy may be due to a change in the composition of urban aerosol at these lower altitudes. The factor K513S from Eq. (5) is approximately 1.4 µgsr, therefore, the maximum smoke mass concentration within the smoke layer is 23±9 µgm-3, while within the PBL this value is below 2±0.8 µgm-3.

The fluorescence measurements allow to evaluate the temporal evolution of the smoke mass concentration mixed with the background urban aerosol. Figure 17 shows the mean smoke mass concentration for the three episodes discussed in this section. Values are averaged within the altitude intervals indicated on the plots. On 25–26 September, the concentration was highest, reaching ∼50 µgm-3. On 1–2 October, concentrations were lower, peaking near 10 µgm-3. On 3–4 October, the mean concentration was enhanced at the beginning of the measurements (MS>20 µgm-3) due to the presence of the elevated smoke layer and then it decreased to ∼4 µgm-3. The results presented in this section demonstrate that smoke from regional fires leads to significantly higher concentrations within the PBL compared to long-range transported North American smoke.

While the dominant aerosol layers observed in our 2024 measurements originated from fires, and the technique described in this paper reliably separates smoke from background urban aerosol, we also identified episodes involving transported layers with high optical depth (OD) that were attributed to anthropogenic pollution. This section presents one such episode from 24–25 August 2024.

HYSPLIT seven-day backward trajectories for air masses over Moscow at 1000, 2000, and 4300 m are shown in Fig. 18. The episode featured a complex multi-layered aerosol structure. Relative humidity from the Global Data Assimilation System (GDAS) remained below 60 % thus, minimizing interference of particle hygroscopic growth on data analysis. The spatiotemporal distributions of particle parameters are presented in Fig. 19. At the start of the measurements a layer at ∼5000 m descended overnight to ∼4000 m. This layer exhibited a high fluorescence capacity G513>5×10-6 nm-1 and spectral ratio R560/438>1.0, identifying it as smoke from Canadian wildfires, consistent with the backward trajectories in Fig. 18.

A second strong layer with the aerosol backscattering coefficient β355>10 Mm-1sr-1 was observed initially at ∼2500 m and descended to ∼2000 m. Its fluorescence characteristics, G513∼2×10-6 nm-1 and R560/438 ∼0.5, are indicative of urban/industrial particles. The corresponding backward trajectories show transport at low altitudes across Europe, likely passing over a strong pollution source, though we cannot presently identify its exact origin. To separate the contributions of urban aerosol and smoke we used the reference fluorescence spectra, measured within 750–1000 m range for urban particles, within 4100–4400 m for smoke and averaged from 21:00 to 24:00 UTC. The results of this separation are shown in Fig. 20. The smoke is predominantly contained in the upper layer, while aerosol below 4000 m consists mainly of urban particles.

Vertical profiles of particle parameters are shown in Fig. 21. The aerosol backscattering coefficient in the layer centered at 2000 m peaks near 10 Mm-1sr-1. The lidar ratio in this layer is S355=30±5 sr and it is even lower (S355∼25 ±4 sr) between 2750 and 3750 m, values that are too low for aged smoke. The characteristic lidar ratio for long-range transported Canadian smoke S355=38±6 sr, typical for long transported Canadian smoke, is observed only in the layer near 4300 m.

The spectrum of fluorescence capacity within the 750–1000 m range (Fig. 21d) and the value G513=0.8×10-6 nm-1 are typical for background urban aerosol. In contrast, within the layers at 2000 and 3000 m, G513 increases to 2.3×10-6 nm-1, while the spectral shape remains similar (R560/438∼0.5). This indicates that the urban aerosol above 1400 m has a different composition. A likely explanation is an elevated proportion of organic components (relative to inorganic species like sulfates and nitrates) in the transported pollution layer.

Results of separation the fluorescence backscatters B513U and B513S are shown in Fig. 21b. The sum B513U+B513S accurately reconstructs the measured total fluorescence profile B513 at all altitudes. Urban aerosol predominates in the layers at 2000 and 3000 m, though a minor smoke component is also present. To calculate the aerosol backscattering coefficients β513U and β513S via Eq. (2), we used fluorescence capacities G513U=1.9×10-6 and G513S=5.8×10-6 nm-1. Since the urban aerosol composition differs below 1400 m, G513U was taken from the 2600 m altitude, where the smoke contribution was minimal. The sum β513U+β513S reconstructs the measured β355 well above 1400 m and strongly underestimates it below that height, due to the difference in urban aerosol properties.

Converting the retrieved smoke fluorescence backscatter to mass concentration, shows that K513S≈0.64 µgsr. Consequently, the smoke mass concentration within the layer at is estimated as 3±1.2 µgm-3. This episode demonstrates that transported pollution layers can have a composition distinct from the local background urban aerosol. While the source of the pollution layer remains unidentified, the method provides fluorescent signatures for distinguishing such events.