Range-resolved bistatic imaging lidar for the measurement of the lower atmosphere

Environmental CW range-resolved S-lidars with Si/InGaAs arrays: limitations and capabilities under sky background

In this paper, we discuss some features of open-path remote sensing inherent to CW range-resolved S-lidars (S comes from Scheimpflug) as a new, to the best of our knowledge, and promising class of laser instruments for environmental monitoring. In many remote-sensing applications, the accompanying skylight can degrade the sensitivity and overload the photodetectors, which is also very relevant for S-lidars with Si and InGaAs arrays. We paid special attention to the topical problem of predicting the limitations and potential of S-lidars in the VIS and SWIR spectral bands, where the sky background is particularly strongly affected. For this purpose, the index of immunity against external backgrounds as a quantitative indicator of S-lidars' potential insensitivity to the current skylight is introduced. Its evaluation is carried out by comparing the potentially achievable signal-to-noise ratios at the detector output in the presence and absence of external illumination. The detector response to the skylight in the photon-counting mode is normalized to appropriate parameters of the array in order to use dimensionless estimates in describing the variability of conditions. Characteristic spectral and dark-current-related features distinguishing the response of Si and InGaAs array detectors in the presence of background illumination are taken into account. It is then shown how to determine the minimum required full well capacity of the array in order to neglect the skylight contribution and ensure stable operation of S-lidars. The proposed methodology is aimed at providing a rationale for design solutions to expand the applicability of this promising type of remote sensors.

Initial experimental multi-wavelength EEM (Excitation Emission Matrix) fluorescence lidar detection and classification of atmospheric pollen with potential applications toward real-time bioaerosols monitoring

Fluorescence has the potential to identify the types of substances associated with aerosols. To demonstrate its usefulness in environmental studies, we investigated the use of Excitation-Emission-Matrix (EEM) fluorescence in lidar bioaerosol monitoring. First, the EEM fluorescence of cedar, ragweed, and apple pollens as typical bioaerosols found around our surroundings were measured using a commercial fluorescence spectrometer. We found that the patterns of fluorescence changed depending on the pollen type and excitation wavelength and it meant that studying these EEM fluorescence patterns was a good parameter for identifying pollen types. Then, we setup a simple EEM fluorescence lidar to confirm the usefulness in lidar bioaerosol monitoring. The lidar consisted of three laser diodes and one light emitting diode with output at 520 nm, 445 nm, 405 nm and 325 nm, respectively, an ultra violet camera lens as a receiver, and a fluorescence spectrum detection unit. Comparing the lidar simulation results with the EEM fluorescence dataset supported the possibility of performing bioaerosol monitoring using the EEM fluorescence lidar. Based on the results and the current technology, a feasible design of a bioaerosol detection EEM fluorescence lidar is proposed for future rel-time remote sensing and mapping of atmospheric bioaerosols.

Triple charge-coupled device cameras combined backscatter lidar for retrieving PM2.5 from aerosol extinction coefficient

The vertical aerosol extinction coefficient profile is the key to evaluating the aerosol radiation effect. In addition, it is also the premise of indirect inversion of PM_2.5 concentration from the lidar signal. A novel lidar system combining tripe charge-coupled device (CCD) cameras and backscatter lidar is proposed to retrieve accurately the aerosol extinction coefficient. The relative humidity segmentation method is used to convert the aerosol extinction coefficient into PM_2.5 concentrations. The results show that the combination of triple CCDs and backscatter lidar can obtain a wider-range extinction coefficient profile. The variation trend of fitted PM_2.5 concentrations and PM_2.5 concentrations measured by the in situ instrument show a high correlation. The correlation coefficient reaches 0.850.

Adaptive digital filter for the processing of atmospheric lidar signals measured by imaging lidar techniques

The lidar signal measured by the atmospheric imaging lidar technique is subject to sunlight background noise, dark current noise, and fixed pattern noise (FPN) of the image sensor, etc., which presents quite different characteristics compared to the lidar signal measured by the pulsed lidar technique based on the time-of-flight principle. Enhancing the signal-to-noise ratio (SNR) of the measured lidar signal is of great importance for improving the performance of imaging lidar techniques. By carefully investigating the signal and noise characteristics of the lidar signal measured by a Scheimpflug lidar (SLidar) based on the Scheimpflug imaging principle, we have demonstrated an adaptive digital filter based on the Savitzky-Golay (S-G) filter and the Fourier analysis. The window length of the polynomial of the S-G filter is automatically optimized by iteratively examining the Fourier domain frequency characteristics of the residual signal between the filtered lidar signal and the raw lidar signal. The performance of the adaptive digital filter has been carefully investigated for lidar signals measured by a SLidar system under various atmospheric conditions. It has been found that the optimal window length for near horizontal measurements is concentrated in the region of 90-150, while it varies mainly in the region of 40-100 for slant measurements due to the frequent presence of the peak echoes from clouds, aerosol layers, etc. The promising result has demonstrated great potential for utilizing the proposed adaptive digital filter for the lidar signal processing of imaging lidar techniques in the future.

Mini-Scheimpflug lidar system for all-day atmospheric remote sensing in the boundary layer

Development of a lightweight, low-cost, easy-to-use and low-maintenance lidar technique has been of great interest for atmospheric aerosol remote sensing in recent years and remains a great challenge. In this work, an 808 nm mini-Scheimpflug lidar (SLidar) system with about 450 mm separation between the transmitter and the receiver has been developed by employing a 114 mm aperture Newtonian telescope (F4). System performances, such as the beam characteristic, the range resolution, and the signal-to-noise ratio of the lidar signal, have been carefully investigated. Despite employing a small receiving aperture, all-day measurements were still feasible with about a one-minute signal averaging for both the horizontal urban area monitoring and the slant atmospheric sounding in the boundary layer. The lidar signal in the region of 29-50 m with a scattering angle less than 179.5° could be slightly underestimated due to the variation of the phase function. The extinction coefficient evaluated in the region between 29 and 2000 m according to the Klett method agreed well with the concentrations of particulate matters for both horizontal and slant measurements. The promising result demonstrated in this work has shown great potential to employ the robust mini-SLidar system for atmospheric monitoring in the boundary layer.

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

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.

Method to retrieve aerosol extinction profiles and aerosol scattering phase functions with a modified CCD laser atmospheric detection system

Vertical distributions of ambient aerosols and their corresponding optical properties are crucial to the assessment of aerosol radiative effects. Traditionally, ambient aerosol phase function is assumed as a constant of input parameter in the retrieval of the vertical distribution of aerosol optical characteristics from remote sensing measurements (e.g. lidar or camera-laser based instruments). In this work, sensitivity studies revealed that using constant aerosol phase function assumptions in the algorithm would cause large uncertainties. Therefore, an improved retrieval method was established to simultaneously measure ambient aerosol scattering phase functions and aerosol scattering function profiles with a modified charge-coupled device-laser aerosol detection system (CLADS), which are then combined to yield vertical profiles of aerosol extinction coefficients. This method was applied and evaluated in a comprehensive field campaign in the North China Plain during January 2016. The algorithm showed robust performance and was able to capture temporal variations in ambient aerosol scattering phase functions and aerosol scattering function profiles. Aerosol extinction coefficients derived with simultaneously measured aerosol phase functions agreed well with in-situ measurements, indicating that uncertainties in the retrieval of aerosol extinction vertical profiles have been significantly reduced by using the proposed method with the modified CLADS. The advantage of this modified CLADS is that it can accomplish these aerosol measurements independent of other supplementary instruments. Benefiting from its low cost and high spatial resolution (∼1 m on average) in the boundary layer, this measurement system can play an important role in the research of aerosol vertical distributions and its impacts on environmental and climatic studies.

The Determination of Aerosol Distribution by a No-Blind-Zone Scanning Lidar

A homemade portable no-blind zone laser detection and ranging (lidar) system was designed to map the three-dimensional (3D) distribution of aerosols based on a dual-field-of-view (FOV) receiver system. This innovative lidar prototype has a space resolution of 7.5 m and a time resolution of 30 s. A blind zone of zero meters, and a transition zone of approximately 60 m were realized with careful optical alignments, and were rather meaningful to the lower atmosphere observation. With a scanning platform, the lidar system was used to locate the industrial pollution sources at ground level. The primary parameters of the transmitter, receivers, and detectors are described in this paper. Acquiring a whole return signal of this lidar system represents the key step to the retrieval of aerosol distribution with applying a linear joining method to the two FOV signals. The vertical profiles of aerosols were retrieved by the traditional Fernald method and verified by real-time observations. To effectively and reliably retrieve the horizontal distributions of aerosols, a composition of the Fernald method and the slope method were applied. In this way, a priori assumptions of even atmospheric conditions and the already-known reference point in the lidar equation were avoided. No-blind-zone vertical in-situ observation of aerosol illustrated a detailed evolution from almost 0 m to higher altitudes. No-blind-zone detection provided tiny structures of pollution distribution in lower atmosphere, which is closely related to human health. Horizontal field scanning experiments were also conducted in the Shandong Province. The results showed a high accuracy of aerosol mass movement by this lidar system. An effective quantitative way to locate pollution sources distribution was paved with the portable lidar system after validation by the mass concentration of suspended particulate matter from a ground air quality station.

Twin scanning lidars for accurate measurement of lower tropospheric aerosols by numerical approximation

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.

Small-scale Scheimpflug lidar for aerosol extinction coefficient and vertical atmospheric transmittance detection

In this paper, a new prototypical Scheimpflug lidar capable of detecting the aerosol extinction coefficient and vertical atmospheric transmittance at 1 km above the ground is described. The lidar system operates at 532 nm and can be used to detect aerosol extinction coefficients throughout an entire day. Then, the vertical atmospheric transmittance can be determined from the extinction coefficients with the equation of numerical integration in this area. CCD flat fielding of the image data is used to mitigate the effects of pixel sensitivity variation. An efficient method of two-dimensional wavelet transform according to a local threshold value has been proposed to reduce the Gaussian white noise in the lidar signal. Furthermore, a new iteration method of backscattering ratio based on genetic algorithm is presented to calculate the aerosol extinction coefficient and vertical atmospheric transmittance. Some simulations are performed to reduce the different levels of noise in the simulated signal in order to test the precision of the de-noising method and inversion algorithm. The simulation result shows that the root-mean-square errors of extinction coefficients are all less than 0.02 km^-1, and that the relative errors of the atmospheric transmittance between the model and inversion data are below 0.56% for all cases. The feasibility of the instrument and the inversion algorithm have also been verified by an optical experiment. The average relative errors of aerosol extinction coefficients between the Scheimpflug lidar and the conventional backscattering elastic lidar are 3.54% and 2.79% in the full overlap heights of two time points, respectively. This work opens up new possibilities of using a small-scale Scheimpflug lidar system for the remote sensing of atmospheric aerosols.

Atmospheric aerosol monitoring by an elastic Scheimpflug lidar system

This work demonstrates a new approach - Scheimpflug lidar - for atmospheric aerosol monitoring. The atmospheric backscattering echo of a high-power continuous-wave laser diode is received by a Newtonian telescope and recorded by a tilted imaging sensor satisfying the Scheimpflug condition. The principles as well as the lidar equation are discussed in details. A Scheimpflug lidar system operating at around 808 nm is developed and employed for continuous atmospheric aerosol monitoring at daytime. Localized emission, atmospheric variation, as well as the changes of cloud height are observed from the recorded lidar signals. The extinction coefficient is retrieved according to the slope method for a homogeneous atmosphere. This work opens up new possibilities of using a compact and robust Scheimpflug lidar system for atmospheric aerosol remote sensing.

An inexpensive active optical remote sensing instrument for assessing aerosol distributions

Air quality studies on a broad variety of topics from health impacts to source/sink analyses, require information on the distributions of atmospheric aerosols over both altitude and time. An inexpensive, simple to implement, ground-based optical remote sensing technique has been developed to assess aerosol distributions. The technique, called CLidar (Charge Coupled Device Camera Light Detection and Ranging), provides aerosol altitude profiles over time. In the CLidar technique a relatively low-power laser transmits light vertically into the atmosphere. The transmitted laser light scatters off of air molecules, clouds, and aerosols. The entire beam from ground to zenith is imaged using a CCD camera and wide-angle (100 degree) optics which are a few hundred meters from the laser. The CLidar technique is optimized for low altitude (boundary layer and lower troposphere) measurements where most aerosols are found and where many other profiling techniques face difficulties. Currently the technique is limited to nighttime measurements. Using the CLidar technique aerosols may be mapped over both altitude and time. The instrumentation required is portable and can easily be moved to locations of interest (e.g. downwind from factories or power plants, near highways). This paper describes the CLidar technique, implementation and data analysis and offers specifics for users wishing to apply the technique for aerosol profiles.

Atmospheric aerosol profiling with a bistatic imaging lidar system

Atmospheric aerosols have been profiled using a simple, imaging, bistatic lidar system. A vertical laser beam is imaged onto a charge-coupled-device camera from the ground to the zenith with a wide-angle lens (CLidar). The altitudes are derived geometrically from the position of the camera and laser with submeter resolution near the ground. The system requires no overlap correction needed in monostatic lidar systems and needs a much smaller dynamic range. Nighttime measurements of both molecular and aerosol scattering were made at Mauna Loa Observatory. The CLidar aerosol total scatter compares very well with a nephelometer measuring at 10 m above the ground. The results build on earlier work that compared purely molecular scattered light to theory, and detail instrument improvements.

Boundary layer scattering measurements with a charge-coupled device camera lidar

A CCD-based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system uses a CCD camera, wide-angle optics, and a laser. Imaging a vertical laser beam from the side allows high-altitude resolution in the boundary layer all the way to the ground. The dynamic range needed for the molecular signal is several orders of magnitude in the standard monostatic method, but only approximately 1 order of magnitude with the CLidar method. Other advantages of the Clidar method include low cost and simplicity. Observations at Mauna Loa Observatory, Hawaii, show excellent agreement with the modeled molecular-scattering signal. The scattering depends on angle (altitude) and the polarization plane of the laser.