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Lidar ratios and Angstrom exponents of continental, maritime, and desert aerosols were calculated to evaluate the effects of aerosol composition on these parameters. Their correlation was assessed using correlation analysis and curve fitting. The Pearson correlation coefficient between the lidar ratio and the Angstrom exponent was larger than 0.95 in all cases. We verified the reliability of the Pearson correlation coefficient using the significance test. The relationship between the lidar ratio and the Angstrom exponent of continental aerosol can be described by a cubic polynomial model; thus, the function relation between the change in lidar ratios at different laser wavelengths depends on the fitting coefficients and the Angstrom exponent. The relationship between the lidar ratio and the Angstrom exponent of both maritime and desert aerosols can be described by a linear model. In these aerosols, the linear change in lidar ratios at different laser wavelengths remains unaffected by the Angstrom exponent. The changes in the lidar ratio in maritime aerosol at 355 nm and 532 nm are −0.7 times and −0.18 times that at 1064 nm, respectively. For desert aerosol, the changes in the lidar ratio at 355 nm and 532 nm are 0.37 times and 1.88 times that at 1064 nm, respectively.

From August to October 2010 lidar measurements of aerosols in the troposphere were performed at Otlica observatory, Slovenia, using a vertical scanning elastic lidar. The lidar data sample, which contains 38 nighttime vertical profiles of the mean aerosol extinction, was combined with continuous ozone concentration (O3), particulate matter concentrations (PM) and daily radiosonde data. The obtained radiosonde- and lidar-derived heights of the atmospheric boundary layer (ABL), which varied considerably from day to day, were found to be in good agreement. The mean values of the aerosol optical depth (AOD) at 355 nm, were calculated separately for the ABL and for the free troposphere (FT). A ten-fold increase of the FT AOD was observed during the days with predicted presence of Saharan dust above the lidar site. To correlate AOD values with the type and origin of aerosols, backward trajectories of air-masses above Otlica were modeled using the HYSPLIT model and clustered. High ABL AOD values were found to be correlated with local circulations and slowly approaching air masses from the Balkans and low values with northwestern flows. The highest values correlated with southwestern flows originating in northern Africa.

Multi-longitudinal-mode (MLM) high-spectral-resolution lidar (HSRL) selects an optical interferometer with the periodic transmittance function as the spectral discriminator to separate aerosol Mie scattering signals and molecular Rayleigh scattering signals excited by the MLM pulsed lasers, such as Mach-Zehnder interferometer, Michelson interferometer, Fabry-Perot etalon and so on. In this paper, we deduce a novel algorithm for retrieving aerosol backscattering coefficient, optical depth, and aerosol extinction coefficient in the application of the MLM HSRL, in which the lidar returns should be considered as a quasi-monochromatic light. Under such a condition, the effective transmittance of Mach-Zehnder interferometer is a function of the complex degree of coherence, which plays an important role in the retrieval algorithms of aerosol optical properties. The retrieval algorithms of aerosol backscattering coefficient, optical depth and aerosol extinction coefficient based on the complex degree of coherence are presented in detail and their correctness is verified by the system simulation using an experimental Mie-Rayleigh lidar returns. Meanwhile, the absolute errors of the measurements of aerosol backscattering coefficient and optical depth are analyzed from the systematic errors and random errors. The simulation results show that the presence of aerosol layers or cloud layer deteriorates considerably the accuracy of the retrieved aerosol backscattering coefficient and optical depth, and the total absolute error of aerosol backscattering coefficient is ignored, as well as the total absolute error of optical depth can be controlled within 0.4 in the majority of the lidar data when the relative error of δ is within 20%. Both measurement examples and continuous observations from the constructed MLM HSRL have proven that the MLM HSRL has the capability in the application of fine detection of aerosol optical properties.

The real aerosol size distributions were obtained by aerodynamic particle sizer spectrometer (APS) in China YinChuan. The lidar ratios at wavelengths of 355 nm, 532 nm and 1064 nm were calculated using Mie theory. The effective radius of aerosol particles r eff and volume C / F ratio (coarse/fine) V c/f were retrieved from the real aerosol size distributions. The relationship between multiwavelength lidar ratios and particle r eff and V c/f were investigated. The results indicate that the lidar ratio is positive correlated to the particle r eff and V c/f . The lidar ratio is more sensitive to the coarse particles. The short wavelength lidar ratio is more sensitive to the particle V c/f and the long wavelength lidar ratio is more sensitive to the particle r eff . The wavelength dependency indicated that the lidar ratios decrease with increasing the wavelength. The lidar ratios are almost irrelevant to the shape and total particles of aerosol size distributions.

In this study, a multi-wavelength polarization lidar was developed at the Lidar Center for Atmosphere Remote Sensing, in Xi’an, China to study the vertical distribution of the optical and microphysical properties of smog aerosols. To better understand smog, two events with different haze conditions observed in January 2015 were analyzed in detail. Using these data, we performed a vertical characterization of smog evolution using the lidar range-squared-corrected signal and the aerosol depolarization ratio. Using inversion with regularization, we retrieved the vertical distribution of aerosol microphysical properties, including volume size distribution, volume concentration, number concentration and effective radius. We also used the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model to analyze aerosol sources during the two episodes. Our results show that the most polluted area in the lower troposphere during smog episodes is located below a height of 1 km above the ground level; under more severe smog conditions, it can be below 0.5 km. In the case of severe smog, we found a large number of spherical and fine particles concentrated in the very low troposphere, even below 0.5 km. Surprisingly, a dust layer with a slight depolarization ratio was observed above the smog layer.

The use of Raman and high-spectral lidars enables measurements of a stratospheric aerosol extinction profile independent of backscatter, and a multi-wavelength (MW) lidar can obtain additional information that can aid in retrieving the microphysical characteristics of the sampled aerosol. The inversion method for retrieving aerosol particle size distributions and microphysical particle parameters from MW lidar data was studied. An inversion algorithm for retrieving aerosol particle size distributions based on the regularization method was established. Based on the inversion of regularization, the inversion method was optimized by choosing the base function closest to the aerosol distribution. The logarithmic normal distribution function was selected over the triangle function as the base function for the inversion. The averaging procedure was carried out for three main types of aerosol. The 1% averaging result near the minimum of the discrepancy gave the best estimate of the particle parameters. The accuracy and stabilization of the optimized algorithm for microphysical parameters were tested by scores of aerosol size distributions. The systematic effects and random errors impacting the inversion were also considered, and the algorithm was tested by the data, showing 10% systematic error and 15% random error. At the same time, the reliability of the proposed algorithm was also verified by using the aerosol particle size distribution data of the aircraft. The inversion results showed that the algorithm was reliable in retrieving the aerosol particle size distributions at vertical heights using lidar data.

It is difficult to independently obtain aerosol optical parameters at the near-infrared wavelength using lidar. A new high-spectral-resolution technology is proposed to obtain the aerosol extinction coefficient profile at 1064 nm. The high-spectral-resolution detection method by extracting Mie scattering and suppressing Cabannes–Brillouin scattering is applied in the lidar at 1064 nm to improve the detection signal-to-noise ratio. The system parameters of near-infrared high-spectral-resolution LiDAR (N HSRL) must be calibrated in this method; then, the optical parameters are accurately deduced. The effect of the system parameters (transmittances of the spectral filter for molecule and aerosol) on the detection results in this method is discussed and analysed. The transmittance of the spectral filter for aerosol scattering is a key parameter in the retrieval of the extinction coefficient, and the sensitivity of the inversion results to the transmittance is analysed and discussed. The method to distinguish the large deviation data of the extinction coefficient is discussed and provided, and the third derivative of the optical thickness profile is adopted to determine the data. According to the characteristics of the inversion signal, the method to correct the extinction coefficient profile is also presented. The correction algorithm can obviously increase the inversion accuracy of the extinction coefficient. The 1064-nm N HSRL Lidar was developed at Xi'an University of Technology. A dual-channel Fabry-Perot Etalon was designed and used to extract the aerosol Mie scattering signal and suppress Cabannes–Brillouin scattering in this N HSRL. The N HSRL lidar observations were performed, and extinction coefficient profiles at 1064 nm were obtained. The results demonstrate that the method can obtain an accurate profile of the extinction coefficients.

In order to investigate the aerosol variability over the southwest region of Slovenia, an ultraviolet scanning elastic backscatter LiDAR was utilized to make the vertical scan for atmospheric probing. With the assumption of horizontal atmospheric homogeneity, aerosol optical variables were retrieved from the horizontal pixel data points of two-dimensional range-height-indicator (RHI) diagrams by using a multiangle retrieval method, in which optical depth is defined as the slope of the resulting linear function when height is kept constant. To make the data retrieval feasible and precise, a series of key procedures complemented the data processing, including construction of the RHI diagram, correction of Rayleigh scattering, assessment of horizontal atmospheric homogeneity and retrieval of aerosol optical variables. The measurement example demonstrated the feasibility of the ultraviolet scanning elastic backscatter LiDAR in the applications of the retrieval of aerosol extinction and determination of the atmospheric boundary layer height. Three months’ data combined with the modeling of air flow trajectories using Hybrid Single Particle Lagrangian Integrated Trajectory Model were analyzed to investigate aerosol variability. The average value of aerosol extinction with the presence of land-based air masses from the European continent was found to be two-times larger than that influenced by marine aerosols from the Mediterranean or Adriatic Sea.

A complex optical system used in polarization lidars often modifies the input polarization of the return signal so that it may significantly impact depolarization estimates and introduce errors to polarization lidar measurements. In most cases, retardation, depolarization, and misalignment of the system exist at the same time and interact with each other. Polarization effects of the system cannot be represented by a simple correction coefficient, so they cannot be removed using a traditional calibration method. Detailed analysis and correction technologies were provided to remove systematic biases in estimating depolarization values from a polarization lidar owing to multiple optical components. The Mueller matrices from an emitter to a receiver were calculated, and the expression for an aerosol depolarization parameter including system polarization effects was derived and obtained. In addition, the correction algorithm based on the Mueller matrix was introduced and provided. A polarization lidar was established, and the polarization characteristics of its optical components were measured with a laboratory ellipsometer; then, the Mueller matrix of the receiver was calculated and obtained. Lidar observations were performed, and our correction algorithm was applied to lidar field data. The results show that the correction method can significantly remove systematic polarization effects.

Aiming at the detection of water vapor mixing ratio, particle linear depolarization ratio, extinction coefficient and cloud information, the Water vapor, Cloud and Aerosol Lidar (WVCAL) was developed by the lidar group at Ocean University of China. The Lidar consists of transmitting subsystem, receiving subsystem, data acquisition and controlling subsystem and auxiliary subsystem. These parts were presented and described in this paper. For the measurement of various physical properties, three channels including Raman channel, polarization channel and infrared channel are integrated in this Lidar system. In this paper, the integration and working principle of these channels is introduced in details. Finally, a measurement example which was operated in coastal area-Qingdao, Shandong province, during 2014 is provided.

In order to improve accuracy of aerosol measurements, a novel method using twin scanning lidars is presented; this method is able to overcome the incomplete overlap range of vertical lidar as well as provide 2D spatial distributions. The scanning lidar setups in the opposite directions are employed as remote sensing tools. Aerosol measurements are performed with cross scanning from the ground to the height of interest. Aerosol optical properties are retrieved using numerical approximation, in which differences between the measured values and the constructed values of the logarithmic range-square-corrected lidar data in the cross-scanning region are minimized. In the data retrieval, we utilize a matrix formulation, in which a Cartesian 2D range-height-indicator diagram is constructed. To verify this method, scanning measurements by ultraviolet Mie scanning lidar performed at different time intervals were taken as the cross-scanning measurements from the twin scanning lidars. With the retrieved spatial distributions of aerosol optical properties, such as aerosol backscatter, aerosol extinction, and lidar ratio, the regional aerosol studies showed that aerosol loading was relatively small and in the presence of multiple layers, which may be influenced by airflow from long-range transportation and cause a large impact on the local environment. To conclude, the presented method using twin scanning lidars is feasible for aerosol measurement in the application of horizontally atmospheric inhomogeneity.

Atmospheric aerosol particles influence the Earth’s radiation balance both directly and indirectly. The aerosol size distribution (ASD) is one of the most important microphysical properties. In this paper, the generalized cross-validation (GCV) regularization method is used for the retrieval of ASD from three-wavelength lidar optical data. The numerical simulations are carried out using synthetic backscatter and extinction coefficients. Simulations results demonstrate that the ASD depends on particle refractive index. Choosing the suitable refractive index is crucial to retrieve aerosol size distribution accurately. Moreover, the numerical results show that, for the same refractive index, it is more suitable to retrieve broad ASD, which has larger mode width σ. The GCV regularization method has been tested for a set of experimental data from three-wavelength lidar, which provides backscatter coefficient at 355, 532 and 1064 nm and extinction coefficient at 355 and 532 nm. Experimental result shows that the retrieved size distribution belongs to the urban industrial type and fine mode. The result shows good agreement with the actural atmospheric aerosol size distribution of local area. Both the simulation and the expriment demonstrate that the GCV regularization method is feasible to retrieve the aerosol size distribution.

A portable Micro-pulsed Mie scattering lidar at the laser wavelength of 532 nm has been developed for routine observation of atmospheric optical properties of the lower troposphere, including boundary layer structure, cloud, the distribution of aerosol and horizontal visibility and so on. The configuration of lidar and its design methods including the hardware and software were described in details. The lidar system was controlled by compact computer, including self adjustment for coaxial lidar, three-dimensional scanning, real-time data processing of visualization and inversion online. The experimental results illustrate that the system can measure the atmospheric aerosols up to the range of near 5 km at daytime and up to 15 km at nighttime under the measurement conditions of laser energy of 50 μJ, signal averaging time of 40s, a receiving aperture 254 mm, range resolution of 7.5 m and analog detection model, which can provide scientific measurement data for studying the atmospheric environment change, particularly for resolving the particulate pollutant generation, transmission and diffusion characteristics.

Long-term observations of atmosphere aerosol optical properties over Xi’an area have been carried out by a Raman-Mie lidar and a Micro-pulsed 3D Scanning Mie lidar, which were built at Xi’an University of technology with the laser wavelength of 355nm and 532nm, respectively. The Raman-Mie lidar is used for observation of the atmospheric temperature, water vapor and aerosol profiles simultaneously. In order to deeply discuss the temporal-spatial evolution of the mixed-layer within the urban boundary layer (UBL), the method of combining the absolute minimum of first derivative and second derivative of the range-squared-corrected signal (RSCS) of lidar was used to retrieve the mixed-layer depth (MLD). By using continuous observations of 24-hour (THI display), the MLD in temporal and spatial variation are clearly revealed. Also, the results of continuous observations from July 2006 to October 2011 have been analyzed for revealing the seasonal cycle and the annual cycle of the MLD. By analyzing the average MLD, it is obviously shown that the MLD of seasonal cycle is higher in summer than in winter over Xi’an area. Otherwise, by investigating the relationship of atmospheric boundary layer height, relative humidity and temperature, and the dependence characteristics and a general disciplinarian between them are then obtained. The achievement is of great importance for studying the proliferation of urban pollution and obtaining a complete meteorological status of the urban atmosphere.

Aiming at the detection of atmospheric water vapor mixing ratio, depolarization ratio, backscatter coefficient, extinction coefficient and cloud information, the Water vapor, Cloud and Aerosol Lidar (WACAL) is developed by the lidar group at Ocean University of China. The lidar consists of transmitter, receiver, data acquisition and auxiliary system. For the measurement of various atmospheric physical properties, three channels including Raman channel, polarization channel and infrared channel are integrated in WACAL. The integration and working principle of these channels are introduced in details. The optical setup, the housekeeping of the system and the data retrieval routines are also presented. After the completion of the construction of the lidar, the WACAL system was installed in Ocean University of China (36.165°N, 120.5°E), Qingdao for the measurement of atmosphere during 2013 and 2014. The measurement principles and some case studies corresponding to various atmospheric physical properties are provided. Finally, the result of one continuous measurement example operated on 13 June 2014 is presented. The WACAL can measure the aerosol and cloud optical properties as well as the water vapor mixing ratio. It is useful for studying the direct and indirect effects of the aerosol on the climate change.

A Micro-pulsed lidar at 532nm has been developed and used for routine observations of optical property of aerosol and the temporal-spatial evolution of the mixed-layer depth (MLD) of atmosphere. The mixed-layer depth of atmosphere is retrieved by seeking the absolute minimum of first derivative or second derivative of the range-squared- corrected lidar signal. Through 24 hours of continuous observation over Xi'an, the result shows that the lidar can clearly observe the diurnal variation of MLD of the Xi'an, which is of great importance for studying the proliferation of urban pollution and obtaining a complete meteorological status of the urban atmosphere.

The Raman lidar technique has the capability of accurately profiling the extinction coefficient of aerosol. A Mie-Raman lidar was developed for measuring the optical properties of atmospheric aerosol, such as extinction coefficient, volume backscatter coefficient and lidar ratio. The vibrational Raman signal of nitrogen and Mie-Rayleigh scattering signal are separated spatially by use of the high-resolution grating and narrow bandpass filters, and then detected by two photomultiplier tubes with a pre-amplifier. Preliminary experiments show that the Mie-Raman lidar system operated in analog detection mode has the capability of measuring aerosol profiles up to a height of 3km with 250mJ of laser energy and 8 minute integration time.

Dust aerosol or sand storm has become the popular attention topic of the world currently. In order to understand and study the aerosol optical properties, particularly for dust aerosol produced in the spring weather condition, and to investigate their effects on atmospheric pollution status, a Mie scattering lidar was developed to detect the time and spatial distribution of the aerosol and the atmospheric visibility at Xi'an, China. The lidar system employs a Nd:YAG pulsed laser at a eye-safe wavelength of 355nm as a transmitter, and a Schmidt-Cassegrain telescope as a receiver. A spectroscope filter combined with a high-resolution grating was used to separate the main lidar returns and to block the solar background simultaneously for daytime measurement. The observation experiments with lidar have been carried out from the spring of 2007. The data of the extinction coefficients of aerosol and atmospheric visibility taken under the different atmospheric conditions are demonstrated. The comparison results of visibility measurement using lidar and other tool show that the lidar system is feasible, and the aerosol observation results show that the main aerosol pollution of Xi'an is from the floating dust aerosol, which is usually suspended at a height of near 1km.

Telescopes in polarization lidar often modify the input polarization of the return signal, such that the telescope may significantly impact the depolarization estimates of aerosol and introduce error to the polarization lidar measurements. The error cannot be corrected by a traditional calibration constant. We present a method to correct the polarization effect of the telescope. We analyze the polarization effect of a telescope on the basis of the Mueller formalism, and introduce an algorithm for correcting the depolarization parameter of aerosol. A Newton telescope and a Cassegrain telescope are often chosen as the receiver in lidar. Their polarization models are established, and the Mueller matrices are calculated. The components of these matrices are dependent on wavelength, incident angle of the incoming light, and surface properties. The polarization impact of the telescope in lidar can be calibrated by a parameter, and the effects of different telescopes are discussed. The polarization crosstalk induced by the Newton telescope is obvious. The depolarization parameters change greatly with coating and wavelength, and they are calculated and presented. Whereas the crosstalk of a Cassegrain telescope is much smaller, the error can reach the level of 10−3 and can be negligible. The method presented in this paper could also be upgraded by taking into account all of the optical devices instead of only the telescope.

A multifunctional UV Rayleigh-Mie Raman lidar at a wavelength of 355nm has been developed for profiling the temperature, humidity and aerosol optical properties in the troposphere. The temperature is determined from the linewidth of molecular Rayleigh scattering and the humidity is determined from the intensity of water vapor vibration-Raman line centered at 407.5nm. The lidar system is relatively compact, which uses laser energy of 200mJ at 20Hz and 25-cm-diameter telescope. The measurement shows that the temperature error of less than 1K and relative humidity error of less than 5% were obtainable up to a height of 3km and 1.5km, respectively, with 5 min observation time. The measurement results were compared with radiosonde measurements and good agreements were obtained.

A compact, high-sensitivity, eye-safe lidar system has been developed using an efficient KTP optical parametric oscillator (OPO) at the wavelength of 1.57 μm. Mie scattering intensities and depolarization ratios were detected for two-dimensional mapping of distribution of aerosol concentrations by rapid laser beam scanning. The system is useful and practical for sensing urban and industrial atmosphere.

A UV vibrational Raman lidar has been built and used to make quantitative measurements of water vapor and aerosol optical properties over Xi'an, China. Vertical profiles of the water vapor mixing ratio and aerosol extinction coefficient are retrieved. The water vapor mixing ratio is calibrated with radiosonde data. The diurnal variations of the water vapor mixing ratio, aerosol extinction coefficient, and aerosol optical depth are obtained. The results obtained in the form of a time-height indicator (THI) display clearly showed the relationship between water vapor and aerosols, in which the gradual enhancement of water vapor density results in aerosol accumulation in the early morning, in particular in the lower troposphere. The seasonal variations of the water vapor mixing ratio and aerosol optical depth over Xi'an were observed and analyzed using the average monthly distribution obtained by lidar for the first time, which will provide useful scientific data and real-time monitoring methods for studying local climate change.