Atmospheric Pollution Monitoring in Urban Area by Employing a 450-nm Lidar System

Broadband continuous-wave differential absorption lidar for atmospheric remote sensing of water vapor

What we believe to be a novel low-cost broadband continuous-wave water vapor differential absorption lidar (CW-DIAL) technique has been proposed and implemented by combing the Scheimpflug principle and the differential absorption method. The broadband CW-DIAL technique utilizes an 830-nm high-power multimode laser diode with 3-W output power as a tunable light source and a CMOS image sensor tilted at 45° as the detector. A retrieval algorithm dedicated for the broadband CW-DIAL technique has been developed to obtain range-resolved water vapor concentration from the DIAL signal. Atmospheric remote sensing of water vapor has been carried out on a near-horizontal water vapor path to validate the performance of the broadband CW-DIAL system. The retrieved water vapor concentration showed a good consistency with those measured by an air quality monitoring station, with a correlation coefficient of 0.9669. The fitting error of the water vapor concentration is found to be less than 10%. Numerical simulation studies have revealed that the aerosol-induced error on the water vapor concentration is below 5% with a background water vapor concentration of 5 g/m3 for most atmospheric conditions. The experimental results have successfully demonstrated the feasibility of the present broadband CW-DIAL technique for range-resolved water vapor remote sensing.

Theoretical and experimental investigation of the molecular depolarization ratio for broadband polarization lidar techniques

The molecular depolarization ratio (MDR) is of great significance for polarization lidar techniques in terms of validating the measurement accuracy, etc. However, previous studies mainly focused on cases with narrowband laser linewidths, and the transmittance of the Cabannes line in the receiver has been assumed constant. In this work, the narrowband theoretical model of MDR has been re-examined by taking the transmittance of the Cabannes line into account. A large relative deviation of beyond 200% has been found if the wavelength-shift reaches up to 0.5 nm for a receiving bandwidth of 0.5 nm at 532 nm, which is much larger than the case without considering the transmittance of the Cabannes line, i.e., only 15%, reported in previous studies. Besides, a broadband theoretical model has been proposed to evaluate the MDR for polarization lidar using high-power multimode laser diodes as light sources. Simulation studies have revealed that the MDR is highly related to the laser linewidth, the receiving bandwidth, as well as the wavelength-shift between the laser wavelength and the center wavelength of the receiver. The MDR at 520 nm calculated by the broadband theoretical model is about 21% larger than the value evaluated without considering the laser linewidth, when the receiving bandwidth is equivalent to the laser linewidth (e.g., 2 nm). Validation measurements, employing a 520-nm imaging-based polarization lidar with a 3.4-nm laser linewidth and a 10-nm receiving bandwidth, illustrated that the volume depolarization ratio in a clean atmospheric region (0.129±0.0025) was highly consistent with the theoretical MDR (0.132). The good agreement between theoretical and experimental results demonstrated a high measurement accuracy of the imaging-based polarization lidar and excellent feasibility of the broadband theoretical model.

Real-Time Synchronous 3-D Detection of Air Pollution and Wind Using a Solo Coherent Doppler Wind Lidar

The monitoring and tracking of urban air pollution is a challenging environmental issue. The approach of synchronous 3-D detection of wind and pollution using a solo coherent Doppler wind lidar (CDWL) is developed and demonstrated. The 3-D distribution of pollutant is depicted by the backscatter coefficient based on signal intensity of CDWL. Then, a high-resolution wind field is derived to track the local air pollution source with its diffusion and to analyze transboundary air pollution episodes. The approach is experimentally implemented in a chemical industry park. Smoke plumes caused by point source pollutions are captured well using plan position indicator (PPI) scanning with low elevation. A typical source of pollution is located, combining the trajectory of the smoke plume and the horizontal wind vector. In addition, transboundary air pollution caused by the transport of dust storms is detected in a vertical profile scanning pattern, which is consistent with the results of national monitoring stations and backward trajectory models. Our present work provides a significant 3-D detection approach to air pollution monitoring with its sources, paths, and heights by using a solo-CDWL system.

Integration of Aerobiological Information for Construction Engineering Based on LiDAR and BIM

In green urban areas, the allergenic factor is important when selecting trees to improve the quality of life of the population. An application of laser imaging detection and ranging (LiDAR) in building information modelling (BIM) is the capture of geo-referenced geometric information of the environment. This study presents the process of digitalisation of a green infrastructure inventory based on the geolocation and bioparameters of the cypress species. The aerobiological index (IUGZA) was estimated by developing green infrastructure BIM models at different detail levels and with a new BIM dimension (6D) for the urban environment. The novelty of the study is the modelling of urban information for evaluating the potential environmental impact related to the allergenicity of the urban green infrastructure using LiDAR through BIM. The measurements of cypress trees based on bioparameters and distances were applied to the IUGZA. This innovation for describing the current 3D environments and designing new scenarios in 6D may prevent future problems in urban areas during construction projects.

Evaluation of systematic errors for the continuous-wave NO2 differential absorption lidar employing a multimode laser diode

The NO₂-differential absorption lidar (NO₂-DIAL) technique has been of great interest for atmospheric NO₂ profiling. Comprehensive studies on measurement errors in the NO₂-DIAL technique are vital for the accurate retrieval of the NO₂ concentration. This work investigates the systematic errors of the recently developed continuous-wave (CW) NO₂-DIAL technique based on the Scheimpflug principle and a high-power CW multimode laser diode. Systematic errors introduced by various factors, e.g., uncertainty of the NO₂ differential absorption cross-section, differential absorption due to other gases, spectral drifting of the λ_on and λ_off wavelengths, wavelength-dependent extinction and backscattering effect, have been theoretically and experimentally studied for the CW-DIAL technique. By performing real-time spectral monitoring on the emission spectrum of the laser diode, the effect of spectral drifting on the NO₂ differential absorption cross-section is negligible. The temperature-dependent NO₂ absorption cross-section in the region of 220-294 K can be interpolated by employing a linear fitting method based on high-precision absorption spectra at 220, 240, and 294 K. The relative error for the retrieval of the NO₂ concentration is estimated to be less than 0.34% when employing the interpolated spectrum. The primary interference molecule is found to be the glyoxal (CHOCHO), which should be carefully evaluated according to its relative concentration in respect to NO₂. The systematic error introduced by the backscattering effect is subjected to the spatial variation of the aerosol load, while the extinction-induced systematic error is primarily determined by the difference between the aerosol extinction coefficients at λ_on and λ_off wavelengths. A case study has been carried out to demonstrate the evaluation of systematic errors for practical NO₂ monitoring. The comprehensive investigation on systematic errors in this work can be of great value for future NO₂ monitoring using the DIAL technique.

Urban air pollution monitoring using scanning Lidar

We have developed a scanning Lidar system in this work to detect urban air pollution changes in real time and locate the sources of urban air pollution. We first proposed an algorithm to retrieve atmospheric extinction coefficients, which we used to create Lidar maps. Using Lidar map of the average extinction coefficients, we identified the locations of the local maximum values, and hence, the positions of the urban air pollution sources. Experimental results indicate that this method is effective for urban air pollution monitoring.

Recent Advances in Aerosol and Trace Gas Monitoring by Employing the Scheimpflug Lidar Techniques

Lidar techniques, based on the time-of-flight principle, have been widely employed in atmospheric remote sensing since decades. The Scheimpflug lidar (SLidar) technique, which employing robust high-power laser diodes as light sources and highly integrated CCD/CMOS image sensor as detectors, has been recently developed for various atmospheric applications. Range-resolved atmospheric backscattering signal is obtained by capturing the backscattering imaging of transmitted continuous-wave laser beam based on the Scheimpflug principle. This paper reported recent advances in aerosol and trace gas monitoring by employing the SLidar techniques.

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

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.

LED Mini Lidar for Atmospheric Application

The creation of a compact and easy-to-use atmospheric lidar has been the aim of researchers for a long time. Micro Pulse Lidars (MPL) and commercialized ceilometers were designed for such purposes. Laser Diodes (LD) and Diode-Pumped Solid State (DPSS) Laser technology has evolved, making lidar system more compact; however, their vulnerability to static electricity and fluctuation of electrical power prevented the growth of atmospheric lidar technology as a system suited to all kinds of users. In this study, a mini lidar with a Light Emitting Diode (LED)-based light source was designed and developed. As LED lamp modules do not need a heat sink or fan, they are resilient and can emit light for long periods with constant intensity. They also offer ease of handling for non-professionals. On the other hand, a LED lamp module has a large divergence, when compared to laser beams. A prototype LED mini lidar was thus developed, with focus on transmitting power optimization and optical design. This low-cost lidar system is not only compact, but also offers near-range measurement applications. It visualizes rapid activities of small air cells in a close range (surface atmosphere), and can verify and predict the condition of the surface atmosphere. This paper summarizes the principle, design, practical use and applications of the LED mini-lidar.