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Elastic Lidar Theory Practice And Analysis Methods

Three gravity-wave-induced clouds and a glaciating altocumulus layer were continuously monitored with lidar at Leipzig, Germany, on 21 November 2003. The midtropospheric clouds formed in Saharan dust at heights from 3.5 km (-9°C) to 6.5 km (-27°C). Distinct ice formation in the altocumulus was triggered by the gravity wave. For the first time, an aerosol/cloud study presents height profiles of temperature, water vapor mixing ratio, relative humidity, dust, and cloud optical properties (volume extinction and backscatter coefficients, lidar ratio, depolarization ratio) within the same air column, solely derived from lidar data. The three gravity-wave-induced clouds did not show any sign of ice formation. The aged dust particles below 4.5-km height were probably partly coated and mixed with hygroscopic material and thus deactivated concerning ice nucleation. Ice crystals were generated in isolated air parcels at the cloud edges of a young, optically thin altocumulus layer between 5- and 6.5-km height. An aged altocumulus deck composed of a geometrically thin liquid water layer at cloud top and an extended ice crystal layer (ice virga) was observed 2 hours later in the same height range. Strong ice formation occurred in the altocumulus during the downdraft induced by the gravity wave. Contact freezing was probably the main reason for the observed ice formation on dust particles. Ice depolarization ratios were relatively low with values from 10 to 15% in the altocumulus and indicate plate-like crystals.

Ice formation in Saharan dust over central Europe observed with temperature/humidity/aerosol Raman lidar : Quantifying the radiative and biogeochemical impacts of mineral dust

The Ron Brown cruise during ACE-Asia (March-April 2001) encountered complex aerosol that at times was dominated by marine, polluted, volcanic, and dust aerosols. Average total light scattering coefficients (σ sp for D p < 10 μm, relative humidity (RH) = 19%, and X = 550 nm) ranged from 23 (marine) to 181 Mm -1 (dust). Aerosol hygroscopicity ranged from deliquescent with hysteresis (marine frequently and polluted variably) to hygroscopic without hysteresis (volcanic) to nearly hygrophobic (dust-dominated). Average deliquescence and crystallization RH were 77 ± 2% and 42 ± 3%, respectively. The ambient aerosol was typically on the upper branch of the hysteresis loop for marine and polluted air masses and the lower branch for dust-dominated aerosols. Averagef(RH = ambient), defined as σ sp (RH = ambient)/σ sp (RH = 19%), ranged from 1.25 (dust) to 2.88 (volcanic). Average h(RH ∼60%), defined as f(RH) upper branch/ f(RH) lower branch, were 1.6, 1.3, 1, and 1.25 for marine, polluted, volcanic, and dust, demonstrating an importance of hysteresis to optical properties. Hemispheric backscatter fraction (b) at ambient RH ranged from 0.077 (marine) to 0.111 (dust), while single scattering albedo (w) at ambient RH ranged from 0.94 (dust and polluted) to 0.99 (marine).

Mixtures of pollution, dust, sea salt, and volcanic aerosol during ACE-Asia: Radiative properties as a function of relative humidity : Characterization of Asian aerosols and their radiative impacts on climate

A portable unmanned Mie-scattering Scheimpflug lidar system has been designed and implemented for atmospheric remote sensing. The Scheimpflug lidar system employs a continuous-wave high-power 808 nm laser diode as the light source and the emitted laser beam is collimated by an F6 lens with a 100 mm aperture. Atmospheric backscattering light is collected by a F5 lens with a 150 mm aperture and then detected by a 45° tilted image sensor. The separation between the transmitting and the receiving optics is about 756 mm to satisfy the Scheimpflug principle. Unmanned outdoor atmospheric measurements were performed in an urban area to investigate system performance. Localized emissions can be identified by performing horizontal scanning measurements over the urban atmosphere for 107° approximately every 17 min. The temporal variation of the vertical aerosol structure in the boundary layer has also been studied through zenith scanning measurements. The promising result shows great potential of the present portable lidar system for unmanned atmospheric pollution monitoring in urban areas.

https://www.mdpi.com/2072-4292/11/7/837/pdf

Preliminary Studies on Atmospheric Monitoring by Employing a Portable Unmanned Mie-Scattering Scheimpflug Lidar System

Previous studies have shown that dust aerosols may accelerate the melting of snow and glaciers over the Tibetan Plateau. To investigate the vertical structure of dust aerosols, we conducted a ground-based observation by using multi-wavelength polarization lidar which is designed for continuous network measurements. In this study, we used the lidar observation from September to October 2020 at the Ruoqiang site (39.0°N, 88.2°E; 894 m ASL), located at the junction of the Taklimakan Desert–Tibetan Plateau. Our results showed that dust aerosols can be lifted up to 5 km from the ground, which is comparable with the elevation of the Tibetan Plateau in autumn with a mass concentration of 400–900 μg m−3. Moreover, the particle depolarization ratio (PDR) of the lifted dust aerosols at 532 nm and 355 nm are 0.34 ± 0.03 and 0.25 ± 0.04, respectively, indicating the high degree of non-sphericity in shape. In addition, extinction-related Ångström exponents are very small (0.11 ± 0.24), implying the large values in size. Based on ground-based lidar observation, this study proved that coarse non-spherical Taklimakan dust with high concentration can be transported to the Tibetan Plateau, suggesting its possible impacts on the regional climate and ecosystem. 

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Polarization Lidar Measurements of Dust Optical Properties at the Junction of the Taklimakan Desert–Tibetan Plateau

A dual-wavelength Scheimpflug lidar system, utilizing a 4-W 808-nm and 1-W 407-nm multimode laser diodes as light sources and two CMOS sensors as detectors, is developed for the studies of the aerosol extinction coefficient and the Angstrom exponent. The system performance has been successfully validated by a two-week continuous measurement campaign on a near horizontal path in May 2018 at Dalian, which is a coastal city in Northern China. The aerosol extinction coefficients retrieved by the Fernald method show good correlations with particle concentrations and relative humidities (RHs). It has been found that the enhancement factor of the backscattering coefficient at the short wavelength due to hygroscopic growth is larger than that at the long wavelength for the aerosol particles off the coast of the Yellow Sea. The Angstrom exponent obtains from the aerosol extinction coefficients at the two wavelengths, varies between 0 and 2, and is found to relate with the mass concentration fraction of fine mode particles, specifically PM2.5 particles. Moreover, the Angstrom exponent has a positive correlation with the RH, implying a bimodal or multimodal size distribution of aerosol particles in the measurement season.

Dual-wavelength Mie-scattering Scheimpflug lidar system developed for the studies of the aerosol extinction coefficient and the Ångström exponent.

Multiple-wavelength polarization lidar techniques have been of great interest for the studies of aerosol backscattering color ratio, Angstrom exponent, particle size distribution, hygroscopic growth, etc. Conventional lidar techniques are mainly based on the time-of-flight principle. In this paper, a three-wavelength polarization Scheimpflug lidar (SLidar) system, based on the Scheimpflug imaging principle, has been developed for studying optical properties of atmospheric aerosols. The SLidar system utilizes low-cost, compact, multimode laser diodes as light sources and two complementary metal oxide semiconductor (CMOS) sensors as detectors. The depolarization ratio was measured at the 808 nm band by successively detecting atmospheric backscattering signals from two orthogonally polarized laser beams with a polarization CMOS camera, while the 520 nm and the 405 nm backscattering signals were recorded by a second CMOS camera based on the time-multiplexing scheme. Atmospheric remote measurements were carried out in May and July 2019 on a near-horizontal path. The aerosol extinction coefficient, linear volume depolarization ratio, and the Angstrom exponent have been retrieved and evaluated to study aerosol properties during different atmospheric conditions, which were in good agreement with optical properties reported by previous studies.

Three-wavelength polarization Scheimpflug lidar system developed for remote sensing of atmospheric aerosols

The Scheimpflug lidar (SLidar) technique has been recently developed for various remote sensing applications, where the lidar signal is detected by an image sensor according to the Scheimpflug principle instead of the time-of-flight principle. Comparison studies between the SLidar technique and the conventional pulsed lidar technique are crucial for understanding the principle as well as the measurement results of the SLidar technique. In this work, a 520-nm Scheimpflug lidar system and a 532-nm pulsed lidar system have been developed for comparison studies. Atmospheric remote measurements as well as statistical analysis have been carried out on a near-horizontal path and on a slant direction with an elevation angle of 30$^\circ $∘. The temporal-spatial variations of the atmospheric backscattering maps measured by the 520-nm SLidar system and the 532-nm pulsed lidar system generally agreed well. The median extinction coefficient measured by the SLidar and the pulsed techniques has shown similar temporal evolution during the near-horizontal comparison study, and a correlation coefficient of 0.99 has been achieved through statistical analysis on all lidar measurements. Moreover, the root-mean-square error (RMSE) ratio for each extinction coefficient profile has also been evaluated, and the mean value of the RMSE ratio for all lidar measurements was about 11% in homogeneous atmospheric conditions. During slant comparison studies, the RMSE ratio between the SLidar curve and the pulsed lidar curve was less than 5% in the region of 0.5–2 km, and it generally increased with the increase of measurement distance, primarily due to the decreased range resolution of the SLidar technique. The promising results suggested that the SLidar technique, featuring a short blind range, could be suitable for aerosol sensing, particularly in the planetary boundary layer.

Comparison studies of the Scheimpflug lidar technique and the pulsed lidar technique for atmospheric aerosol sensing.

A polarization imaging lidar, employing a linearly polarized continuous-wave 450-nm laser diode and a polarization-sensitive image sensor has been proposed and demonstrated for atmospheric polarization studies The polarization image sensor, fabricated with four-directional on-chip polarizers at 0°, 45°, 90° and 135°, is able to capture a four-directional polarized image of the laser beam that has been transmitted into the atmosphere, from which four polarized atmospheric backscattering signals with 0°, 45°, 90°, and 135° polarization angles can be simultaneously measured A theoretical model has been established to retrieve the linear volume depolarization ratio (LVDR) as well as the misalignment (offset) angle between the polarization plane of the transmitted laser beam and the polarization plane of the 0°-directional on-chip polarizer of the image sensor, without employing additional optical components and sophisticated system adjustments Atmospheric validation measurements have been carried out during a haze event The promising results have shown a great potential of utilizing the proposed polarization imaging lidar technique for various atmospheric polarization studies

A polarization-sensitive imaging lidar for atmospheric remote sensing

Lidar techniques have been widely employed for atmospheric remote sensing during past decades. However, an important drawback of the traditional atmospheric pulsed lidar technique is the large blind range, typically hundreds of meters, due to incomplete overlap between the transmitter and the receiver, etc. The large blind range prevents the successful retrieval of the near-ground aerosol profile, which is of great significance for both meteorological studies and environmental monitoring. In this work, we have demonstrated a new experimental approach to calibrate the overlap factor of the Mie-scattering pulsed lidar system by employing a collocated Scheimpflug lidar (SLidar) system. A calibration method of the overlap factor has been proposed and evaluated with lidar data measured in different ranges. The overlap factor, experimentally determined by the collocated SLidar system, has also been validated through horizontal comparison measurements. It has been found out that the median overlap factor evaluated by the proposed method agreed very well with the overlap factor obtained by the linear fitting approach with the assumption of homogeneous atmospheric conditions, and the discrepancy was generally less than 10%. Meanwhile, simultaneous measurements employing the SLidar system and the pulsed lidar system have been carried out to extend the measurement range of lidar techniques by gluing the lidar curves measured by the two systems. The profile of the aerosol extinction coefficient from the near surface at around 90 m up to 28 km can be well resolved in a slant measurement geometry during nighttime. This work has demonstrated a great potential of employing the SLidar technique for the calibration of the overlap factor and the extension of the measurement range for pulsed lidar techniques.

https://www.mdpi.com/2072-4292/12/7/1227/pdf

Teng Ma

Papers

Dual-wavelength Mie-scattering Scheimpflug lidar system developed for the studies of the aerosol extinction coefficient and the Ångström exponent.

Three-wavelength polarization Scheimpflug lidar system developed for remote sensing of atmospheric aerosols

Comparison studies of the Scheimpflug lidar technique and the pulsed lidar technique for atmospheric aerosol sensing.

A polarization-sensitive imaging lidar for atmospheric remote sensing

Experimental Calibration of the Overlap Factor for the Pulsed Atmospheric Lidar by Employing a Collocated Scheimpflug Lidar