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Short-range elastic backscatter lidar (SR-EBL) systems are remote sensing instruments for studying low atmospheric boundary layer processes. This work presents a field campaign oriented to filling the gap between the near-surface aerosol processes regarding aerosol radiative properties and connecting them with the atmospheric boundary layer (ABL), centering attention on the residual layer and the ABL transition periods. A Colibri Aerosol Lidar (CAL) instrument, based on the short-range lidar with high spatio-temporal resolution, was used for the first time in the ACTRIS AGORA facility (Andalusian Global Observatory of the Atmosphere) in Granada (Spain). This study showed the possibility of combining lidar and in situ measurements in the lowermost 150 m. The results address, on the one hand, the characterization of the short-range lidar for developing a method to find the calibration constant of the system and to correct the incomplete overlap to further data exploitation. On the other hand, relevant radiative properties such as the temporal series of the aerosol lidar ratio and extinction coefficient were quantified. The campaign was divided in three different periods based on the vehicular emission peak in the early mornings, namely, before, during, and after the emission peak. For before and after the emission peak data classification, aerosol properties presented closer values; however, large variability was obtained after the emission peak reaching the maximum values of extinction and a lidar ratio up to 51.5 ± 11.9 (Mm) − 1 and 36.0 ± 10.5 sr, respectively. During the emission peaks, the values reached for extinction and lidar ratio were up to 136.8 ± 26.5 (Mm) − 1 and 119.0 ± 22.7 sr, respectively. [ABSTRACT FROM AUTHOR]

• A novel supercontinuum laser-based apparatus designed for measuring light-scattering of particles while imaging via digital in-line holography to provide size and shape information. • Measurements of two-dimensional (2D) light-scattering patterns, including the exact backscattering direction, of atmospheric particles across a broad spectrum, from 450 to 850 nm in wavelength, and for different polarization states. • Lidar-relevant quantities lidar are measured, including the backscattering phase function and the linear depolarization ratio for glass particles, volcanic ash (Eyjafjallajökull and La Palma), and dust particles (Arizona Test, Gobi Desert, Saharan Desert). This study presents a new experiment to measure the polarization-resolved two-dimensional light-scattering patterns, near and including the backscattering direction, of fixed particles representative of some atmospheric-aerosol types. The measurements are conducted across a broad spectrum, from 450 to 850 nm, and for different polarization states. A supercontinuum laser, achromatic optics, and polarizers are used to measure the scattering patterns. Important quantities for applications such as lidar are also measured including the backscattering phase-function and the linear depolarization ratio in the exact backscattering direction. Microscopic images of the particles are provided by digital in-line holography as well. Thus, in some cases, it is possible to interpret the spectro-polarimetric measurements with knowledge of the size and shape of the particles involved. Spherical particles of known properties are used to calibrate the method. Then, Arizona test dust, Gobi and Sahara Desert dust, and ash from the Eyjafjallajökull and La Palma volcanic eruptions are studied to illustrate the method's potential for lidar applications. [ABSTRACT FROM AUTHOR]

This study presents a synergistic approach to the study of the aerosol optical and microphysical properties measured in La Palma, Spain, during the 2021 eruption of the Cumbre Vieja volcano (from 19 September to 13 December 2021). This study aims to characterize the different phases of the volcanic eruption using the spatio-temporal evolution of the event together with the mass concentration quantification of four different atmospheric layers. The impact of the plume's pathway that reached the South of France is analyzed. Here, passive and active remote sensors were used, namely CL51 and CL61 ceilometers and AERONET sunphotometers. The attenuated backscattering ranged from 0.8 to 9.1 × 10 − 6 (msr) − 1 and the volume depolarization ratio measured nearby the volcano was up to 0.3. The ash plume remained within the first 4 km agl, with intense episodes that reached mean aerosol optical depth values of up to 0.4. Thirteen study cases were selected where coarse mode was dominant over fine mode. For the data selection, the fine and coarse lidar ratios found were 3.9 ± 0.8 and 21.0 ± 3.8 sr in the north and 6.9 ± 1.8 and 30.1 ± 10.3 sr in the south. The ash mass concentration reached moderate levels with maximum values of up to 313.7 μ gm − 3 . [ABSTRACT FROM AUTHOR]

Black carbon aerosol emissions are recognized as contributors to global warming and air pollution. There remains, however, a lack of techniques to remotely measure black carbon aerosol particles with high range and time resolution. This article presents a direct and contact-free remote technique to estimate the black carbon aerosol number and mass concentration at a few meters from the emission source. This is done using the Colibri instrument based on a novel technique, referred to here as Picosecond Short-Range Elastic Backscatter Lidar (PSR-EBL). To address the complexity of retrieving lidar products at short measurement ranges, we apply a forward inversion method featuring radiometric lidar calibration. Our method is based on an extension of a well-established light-scattering model, the Rayleigh–Debye–Gans for Fractal-Aggregates (RDG-FA) theory, which computes an analytical expression of lidar parameters. These parameters are the backscattering cross-sections and the lidar ratio for black carbon fractal aggregates. Using a small-scale Jet A-1 kerosene pool fire, we demonstrate the ability of the technique to quantify the aerosol number and mass concentration with centimetre range-resolution and millisecond time-resolution. [ABSTRACT FROM AUTHOR]

This work assesses the ability of equivalent-sphere models to reproduce the optical properties of soot aggregates relevant for lidar remote sensing, i.e. the backscattering and extinction cross sections. Lidar cross-sections are computed with a spectral discrete dipole approximation model over the visible-to-infrared (400–5000 nm) spectrum and compared with equivalent-sphere approximations. It is shown that the equivalent-sphere approximation, applied to fractal aggregates, has a limited ability to calculate such cross-sections well. The approximation should thus be used with caution for the computation of broadband lidar cross-sections, especially backscattering, at small and intermediate wavelengths (e.g. UV to visible). [ABSTRACT FROM AUTHOR]

• Aerosol light extinction and backscattering are reviewed with a lidar perspective. • The elastic lidar equation is derived from the radiative transfer equation. • Extinction and backscattering cross-sections of aerosol particles are interpreted from wave-optics and phasor analysis. • Digital holography and small-angle light-scattering methods are proposed for measuring the extinction properties of single aerosols. • A new elastic backscatter short-range micro-lidar system, Colibri, is presented for measuring the backscattering properties of aerosol plumes. Elastic backscatter lidar is an established method useful to characterize the particles forming an atmospheric aerosol. Fundamental to lidar measurements are the extinction and backscattering properties of the aerosol particles in the lidar beam. This review aims to give the reader an understanding for how lidar measurements are made, modeled, and applied. Specific emphasis is placed on extinction, which manifests attenuation of the lidar beam by the aerosol, and the backscattering that ultimately constitutes the measured signal. The so-called lidar equation, which quantifies this signal in terms of aerosol-particle properties, is derived from radiative transfer theory. Extinction is examined in terms of the redistribution of energy from the lidar beam by the particles and is shown to be inherently linked to interference between the incident and scattered light. Ways to measure aerosol-particle extinction are reviewed, including a recent method where the wave interference constitutes a digital hologram that may be processed to yield both the cross section and an image of the particle. The semi-graphical method of phasor analysis is also presented and used to reveal connections between the scattering characteristics of a particle and the distribution of electric field within it. Finally, a recently developed elastic backscatter lidar system, Colibri, is described and an example of its measurement capabilities are presented. [ABSTRACT FROM AUTHOR]

• A laboratory experiment is performed to measure soot depolarization at 180.0°. • Fresh soot lidar depolarization ratios are provided in laboratory at 355, 532 nm. • The soot aggregates are freshly emitted from a pool fire in laboratory ambient air. • The soot LDR is numerically computed by applying the Superposition T-Matrix method. • The soot parameters agreeing with the laboratory LDR are hence discussed. In this paper, a controlled-laboratory experiment is carried out to evaluate the lidar depolarization ratio of freshly emitted soot aggregates in the exact backward scattering direction at 180.0°. The experiment is performed at two wavelengths simultaneously, namely 355 and 532 nm, often used in polarimetric lidar remote sensing. The soot aggregates are generated from a kerosene JET A-1 pool fire in laboratory ambient air and microscopic images confirm the fractal morphology of generated soot aggregates. Then, the Superposition T-Matrix (STM) method is applied to numerically simulate the soot aggregates backscattering properties for different soot particles refractive indices, monomer radii and monomer numbers. The range of these parameters which ensures the lowest discrepancy between the laboratory measurement and the STM-computations is discussed within experimental and numerical error bars. We find that the polydisperse monomers model gives an overall better evaluation of the ratio F 22 (π)/F 11 (π). In the polydisperse case, our numerical and laboratory experimental findings agree at both wavelengths for a refractive index m = 2.65 + i1.32 and monomer number N m > 40 at a mean monomer radius of r p = 30 nm (N m > 160 at r p = 27.5 nm). We believe this work may be useful for the light scattering and remote sensing communities and may also help future studies aimed at better understanding the impact of soot particle aggregates on the Earth's climate, which still needs to be precisely quantified. [ABSTRACT FROM AUTHOR]

• The superposition T-matrix method (STMM) is used to simulate light scattering from soot. • The simulations cover a wide range of wavelength and several scattering observables. • Comparisons between the STMM and Rayleigh-Debye-Gans approximation are made. • The linear depolarization ratio is roughly 10 times larger for superaggregates than chain-like. The modeling of radiative properties of soot particles can be addressed by a number of methods. Among them, the canonical Rayleigh-Debye-Gans (RDG) theory and the Superposition T-Matrix Method (STMM) are particularly well adapted to the task. Here, both RDG and STMM are used to model the radiative properties of two types of fractal aggregates indicative of soot relevant to lidar sensing. One type, a canonical aggregate with a chain-like morphology, corresponds to low-sooting flames and is well characterized by a single fractal dimension. The other type, a superaggregate, exhibits multiple fractal dimensions and is seen in heavily sooting flames. Radiative properties such as the differential scattering cross-section, total scattering cross-sections, backscattering cross-sections, and linear depolarization-ratio (LDR) are calculated for a range of wavelengths from 300 − 1100 nm while including the wavelength dependence of the soot refractive index. Of particular interest is that depolarization by a superaggregate is found to be approximately ten times larger than the depolarization by a canonical chain-like aggregate and both exhibit a power-law with wavelength. Results for specific wavelengths of interest to lidar (355, 532, and 1064 nm) are tabulated for lidar applications. Additionally, RDG theory yields fairly accurate results for modeling the radiative properties, including lidar relevant parameters such as backscattering cross-sections, only for canonical soot fractal aggregates. [ABSTRACT FROM AUTHOR]

• Lidar-relevant parameters (backscattering, extinction, lidar ratio, and depolarization ratio) are computed using MSTM for statistically representative ensembles of soot fractal aggregates. • The monomer radius strongly influences all of the studied lidar relevant radiative properties • The number of monomers per aggregate has also a strong effect on the lidar cross-sections, but has a limited impact on the lidar parameters such as the Lidar Ratio and the linear depolarization ratio. • The aggregate fractal dimension has a weak impact on all lidar relevant radiative properties. This study presents the averaged lidar-relevant radiative properties of numerically generated soot fractal aggregate ensembles. The radiative properties of these aggregates have been computed using the Superposition T-Matrix Method, with emphasis put on those that are most relevant to lidar application; the backscattering and extinction cross-sections, the Lidar Ratio and the Linear Depolarization Ratio. These lidar-relevant radiative properties have been computed over a broad spectrum, going from the ultraviolet to the near infrared, in order to address lidar instrument need for a priori knowledge in signal inversion procedures and measurement analysis. By averaging the computed radiative properties according to each set of morphological parameters, we obtain statistically representative results and we study the impacts of morphological changes on these lidar-relevant radiative properties. Our results show a strong impact of the primary particle radius on all considered radiative properties, while the number of primary particles induce significant variations on the cross-sections only. The fractal dimension, although being an essential morphological parameter, has a weak influences on the lidar-relevant radiative properties. [ABSTRACT FROM AUTHOR]