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In the atmosphere, aerosols can originate from numerous sources, leading to the mixing of different particle types. This paper introduces an approach to the partitioning of aerosol mixtures in terms of backscattering coefficients. The method utilizes data collected from the Mie–Raman–fluorescence lidar, with the primary input information being the aerosol backscattering coefficient (β), particle depolarization ratio (δ), and fluorescence capacity (GF). The fluorescence capacity is defined as the ratio of the fluorescence backscattering coefficient to the particle backscattering coefficient at the laser wavelength. By solving a system of equations that model these three properties (β , δ and GF), it is possible to characterize a three-component aerosol mixture. Specifically, the paper assesses the contributions of smoke, urban, and dust aerosols to the overall backscattering coefficient at 532 nm. It is important to note that aerosol properties (δ and GF) may exhibit variations even within a specified aerosol type. To estimate the associated uncertainty, we employ the Monte Carlo technique, which assumes that GF and δ are random values uniformly distributed within predefined intervals. In each Monte Carlo run, a solution is obtained. Rather than relying on a singular solution, an average is computed across the whole set of solutions, and their dispersion serves as a metric for method uncertainty. This methodology was tested using observations conducted at the ATOLL (ATmospheric Observation at liLLe) observatory, Laboratoire d'Optique Atmosphérique, University of Lille, France. [ABSTRACT FROM AUTHOR]

Aerosol microphysical properties, including aerosol particle size distribution, complex refractive index and concentration properties, are key parameters evaluating the impact of aerosols on climate, meteorology, and human health. High Spectral Resolution Lidar (HSRL) is an efficient tool for probing the vertical optical properties of aerosol particles, including the aerosol backscatter coefficient (β) and extinction coefficient (α), at multiple wavelengths. To swiftly process vast data volumes, address the ill-posedness of retrieval problems, and suit simpler lidar systems, this study proposes an algorithm (modified algorithm) for retrieving microphysical property profiles from the HSRL optical data targeting fine-mode aerosols, building upon a previous algorithm (basic algorithm). The modified algorithm is based on a look-up table (LUT) approach, combined with the k-nearest neighbor (k-NN) and random forest (RF) algorithms, and it optimizes the decision tree generation strategy, incorporating a self-posed scheme. In numerical simulation tests for different lidar configurations, the modified algorithm reduced retrieval errors by 41%, 30%, and 32% compared to the basic algorithm for 3β + 2α, 3β + 1α, and 2β + 1α, respectively, with a remarkable improvement of stability. In two observation scenes of a field campaign, the median relative errors of the effective radius for 3β + 2α were 6% and −3%, and the median absolute errors of single-scattering albedo were 0.012 and 0.005. This method represents a further step toward the use of the LUT approach, with the potential to provide effective and efficient aerosol microphysical retrieval for simpler lidar systems, which could advance our understanding of aerosols' climatic, meteorological, and health impacts. [ABSTRACT FROM AUTHOR]

Based on satellite observations in the Arctic stratosphere at latitudes from 61° to 66°N in the second half of 2019, Boone et al. (2022, https://doi.org/10.1029/2022jd036600) provide the impression that the aerosol in the upper troposphere and lower stratosphere (UTLS) over the entire Arctic consisted of sulfate aerosol originating from the Raikoke volcanic eruption in the summer of 2019. Here, we show that this was most probably not the case and the aerosol layering conditions were much more complex. By combining the stratospheric aerosol typing results of Boone et al. (2022, https://doi.org/10.1029/2022jd036600) with lidar observations at 85°–86°N of Ohneiser et al. (2021, https://doi.org/10.5194/acp‐21‐15783‐2021) of a dominating wildfire smoke layer in the UTLS height range, we demonstrate that the Arctic UTLS aerosol most likely consisted of Siberian wildfire smoke in the lower part and sulfate aerosol in the upper part of the aerosol layer which extended from 7 to 19 km height and was well observable until May 2020. The smoke‐ and sulfate‐related aerosol optical thickness (AOT) fractions were about 0.7–0.8 and 0.2–0.3, respectively, according to our analysis. The sulfate AOT is in good agreement with model‐based predictions of the Raikoke sulfate AOT. Plain Language Summary: In the summer of 2019, the Raikoke volcano erupted. A stratospheric aerosol layer formed from the SO2 emitted into the lower stratosphere. At the same time, intense and long‐lasting Siberian wildfires occurred in July and August 2019 and led to an extremely high level of smoke pollution in the Arctic. The smoke filled the entire troposphere. Boone et al. (2022, https://doi.org/10.1029/2022jd036600) analyzed satellite observation between 61° and 66°N in the second half of 2019 and suggested that the aerosol in the upper troposphere and lower stratosphere (UTLS) over the entire Arctic consisted of volcanic sulfate aerosol, while Ohneiser et al. (2021, https://doi.org/10.5194/acp‐21‐15783‐2021) showed, based on lidar observations at latitudes of 85°–86°N in the fall of 2019, that wildfire smoke dominated in the UTLS aerosol layer, at least up to 12 km height in the central Arctic. In this commentary, we show that the UTLS aerosol over the Arctic most probably consisted of both, smoke particles in the lower part and sulfate aerosol in the upper part of the layer extending from 7 to 19 km height. The smoke and sulfate particles contributed to the overall Arctic UTLS aerosol optical thickness by about 70%–80% and 20%–30%, respectively. Key Points: The impression given by Boone et al. that the 2019 Arctic upper troposphere and lower stratosphere (UTLS) aerosol was composed of sulfate only was not supported by lidar dataThe UTLS aerosol layer most likely contained Siberian wildfire smoke in the lower part and Raikoke sulfate aerosol in the upper part in the autumn of 2019UTLS aerosol optical thickness fractions were about 0.7–0.8 (smoke) and 0.2–0.3 (sulfate) [ABSTRACT FROM AUTHOR]

This study focuses on the characterization of aerosol hygroscopicity using remote sensing techniques. We employ a Mie–Raman–fluorescence lidar (Lille Lidar for Atmospheric Study, LILAS), developed at the ATOLL platform, Laboratoire d'Optique Atmosphérique, Lille, France, in combination with the RPG-HATPRO-G5 microwave radiometer to enable continuous aerosol and water vapor monitoring. We identify hygroscopic growth cases when an aerosol layer exhibits an increase in both aerosol backscattering coefficient and relative humidity. By examining the fluorescence backscattering coefficient, which remains unaffected by the presence of water vapor, the potential temperature, and the absolute humidity, we verify the homogeneity of the aerosol layer. Consequently, the change in the backscattering coefficient is solely attributed to water uptake. The Hänel theory is employed to describe the evolution of the backscattering coefficient with relative humidity and introduces a hygroscopic coefficient, γ , which depends on the aerosol type. The particularity of this method revolves around the use of the fluorescence which is employed to take into account and correct the aerosol concentration variations in the layer. Case studies conducted on 29 July and 9 March 2021 examine, respectively, an urban and a smoke aerosol layer. For the urban case, γ is estimated as 0.47 ± 0.03 at 532 nm; as for the smoke case, the estimation of γ is 0.5 ± 0.3. These values align with those reported in the literature for urban and smoke particles. Our findings highlight the efficiency of the Mie–Raman–fluorescence lidar and microwave radiometer synergy in characterizing aerosol hygroscopicity. The results contribute to advance our understanding of atmospheric processes, aerosol–cloud interactions, and climate modeling. [ABSTRACT FROM AUTHOR]

Polarization properties of the fluorescence induced by polarized laser radiation are widely considered in laboratory studies. In lidar observations, however, only the total backscattered 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 us to measure depolarization ratios of fluorescence at 466 nm (δF) and of water vapor Raman backscatter. Measurements were performed in May–June 2023 during the 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 the General Physics Institute (GPI), with a five-channel fluorescence lidar able to measure fluorescence backscattering at 438, 472, 513, 560 and 614 nm. Results demonstrate that, inside the planetary boundary layer (PBL), the urban aerosol fluorescence is maximal at 438 nm, and then it gradually decreases with the increase in wavelength. The smoke layers observed within 4–6 km height present a maximum fluorescence at 513 nm, while in the upper troposphere, fluorescence maximum shifts to 560 nm. Regarding the fluorescence depolarization ratio, for smoke its value typically varies within the 45 %–55 % range. The depolarization ratio of the water vapor Raman backscattering at 408 nm is shown to be quite low (2±0.5 %) in the absence of fluorescence because the narrowband interference filter (0.3 nm) in the water vapor channel selects only the strongest vibrational lines of the Raman spectrum. As a result, the depolarization ratio at the water vapor Raman channel is sensitive to the presence of strongly depolarized fluorescence backscattering and can be used for the evaluation of the aerosol fluorescence contribution to measured water vapor mixing ratio. [ABSTRACT FROM AUTHOR]

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 (δF) 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 δF 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, δF appears to vary with atmospheric relative humidity (R H) 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 δF remains constant within the 408-466 nm range. [ABSTRACT FROM AUTHOR]

Data obtained from HSRL-2 observations carried out on 20 September 2016 during the ORACLES campaign are publicly accessible. In our presentation we invert 3β+2α data into (1) particle size distributions with a regularization algorithm, and subsequently compute (2) single scattering albedo. We carry out a first comparison to the same particle characteristics measured with airborne in-situ instruments. We find good agreement of the data products. However, a more detailed study is needed as correction factors and sources of retrieval and measurement uncertainties need to be tested.

We derive analytical relationships between bulk microphysical parameters of nucleation, Aitken, accumulation and coarse mode particles and extinction and backscatter coefficients measured at wavelengths 355, 532 and 1064 nm. The bulk parameters are represented by number concentration, mean (effective) radius, variance and complex refractive index. Analytical relationships hold true for arbitrarily shaped particles and complex refractive indices m=mR-imI, where mR>>mI. The accuracy is above 60%.

The lidar ratios of Saharan dust at 355 and 532 nm (LR355 and LR532) measured over West Africa during SHADOW field campaign are analyzed. Results demonstrate that even for pure dust, the lidar ratio may present strong height dependence. The possible reasons of height dependence of lidar ratios during strong dust events are considered.

The results of methane profiling in the lower troposphere by Raman lidar from Lille University observatory platform (France), are presented. The use of powerful DPSS tripled Nd:YAG laser allowed profiling of methane background mixing ratio of 2 ppm in the night time up to 4000 m with 100 m height and 1 hour temporal resolution. Enhancement of CH4 mixing ratio inside the boundary layer comparing to the free troposphere values was observed.

A five-channel fluorescence lidar was developed for the study of atmospheric aerosol. The fluorescence spectrum induced by 355 nm laser emission is analyzed in five spectral intervals using interference filters. Central wavelengths and the widths of these five interference filters are, respectively, as follows: 438 and 29, 472 and 32, 513 and 29, 560 and 40, and 614 and 54 nm. The relative calibration of these channels has been performed using a tungsten–halogen lamp with a color temperature of 2800 K. This new lidar system was operated during summer–autumn 2022, when strong forest fires occurred in the Moscow region and generated a series of smoke plumes analyzed in this study. Our results demonstrate that, for urban aerosol, the maximal fluorescence backscattering is observed in a 472 nm channel. For the smoke, the maximum is shifted toward longer wavelengths, and the fluorescence backscattering coefficients in 472, 513 and 560 nm channels have comparable values. Thus, from the analysis of the ratios of fluorescence backscattering in available channels, we show that it is possible to identify smoke layers. The particle classification based on single-channel fluorescence capacity (ratio of the fluorescence backscattering to the elastic one) has limitations at high relative humidity (RH). The fluorescence capacity indeed decreases when water uptake of particles enhances the elastic scattering. However, the spectral variation of fluorescence backscattering does not exhibit any dependence on RH and can be therefore applied to aerosol identification. [ABSTRACT FROM AUTHOR]

A five-channel fluorescence lidar was developed for the study of atmospheric aerosol. The fluorescence spectrum induced by 355 nm laser emission is analyzed in five spectral intervals using interference filters. Central wavelengths and the widths of these five interference filters are respectively: 438/29, 472/32, 513/29, 560/40 and 614/54 nm. The relative calibration of these channels has been performed using a tungsten-halogen lamp with color temperature 2800K. This new lidar system was operated during Summer -- Autumn 2022, when strong forest fires occurred in the Moscow region and generated a series of smoke plumes analyzed in this study. Our results demonstrate that, for urban aerosol, the maximal fluorescence backscattering is observed in 472 nm channel. For the smoke the maximum is shifted toward longer wavelengths, and the fluorescence backscattering coefficients in 472 nm, 513 nm and 560 nm channels have comparable value. Thus, from the analysis of the ratios of fluorescence backscattering in available channels, we show that it is possible to identify smoke layers. The particle classification based on single channel fluorescence capacity (ratio of the fluorescence backscattering to elastic one), has limitations at high relative humidity (RH). Fluorescence capacity is indeed decreasing when water uptake of particles enhances the elastic scattering. However, the spectral variation of fluorescence backscattering does not evidence any dependence on RH and can be therefore considered for aerosol identification. [ABSTRACT FROM AUTHOR]

Lidar sounding is used for the analysis of possible contribution of the corner reflection (CR) effect to the total backscattering in case of ice crystals. Our study is based on observations of mixed phase clouds performed during the SHADOW campaign in Senegal. Mie-Raman lidar allows measurements at 355 nm and 532 nm at 43 dg. off-zenith angle, so the extinction and backscattering Ångström exponents can be evaluated. In some measurements we observed the positive values of backscattering Ångström exponent, which can be attributed to the corner reflection by horizontally oriented ice plates.