Objective Air pollution control has higher requirements for environmental monitoring equipment. As an active remote sensing instrument, lidar is currently a powerful tool for monitoring the three-dimensional distribution characteristics of atmospheric aerosols in the troposphere. Lidar can compensate for the insufficient spatial distribution rate of existing ground monitoring data, mainly using various monitoring modes such as horizontal, vertical, navigation, and networking. Among them, horizontal scanning can effectively monitor horizontal visibility, dynamically extract the location and transmission path of pollution sources. Moreover, vertical monitoring can analyze the spatiotemporal changes in aerosol vertical diffusion, sedimentation transport, optical characteristics, boundary layer changes and their cloud information, as well as sand and dust monitoring. This article introduces a self-developed infrared lidar that can detect aerosol distribution, visibility, and boundary layer height in real-time under environmental pollution and multiple sudden weather conditions. It has unique advantages in analyzing the diffusion and sedimentation trends of polluted air masses. In the context of increasing observation needs, it can provide more scientific and effective data support for environmental management decisions and meteorological services.
Methods The detection principle is based on the Mie scattering lidar equation. The schematic diagram and appearance of the aerosol lidar product structure are shown (Fig.2). The system mainly consists of three parts of laser emission unit, optical receiving unit, and data acquisition and processing unit. The main technical specifications are shown (Tab.1). The fundamental frequency 1 064 nm linearly polarized laser of Nd: YAG laser is used as the detection light source, and the telescope uses an aspherical lens as the main mirror. The aspherical lens has the advantages of small aberration and short focal length, which can reduce the volume of the receiving module. The subsequent optical and detection units are composed of fiber coupling devices and small core diameter multimode fibers, which can effectively control the telescope's receiving field of view. The optical channel measures the scattering signal generated by the interaction between 1 064 nm outgoing laser and atmospheric particles. By using the measurement data of the channel and combining with the above inversion method, the distribution characteristics of optical parameters of tropospheric aerosols and clouds can be obtained.
Results and Discussions This radar can be effectively applied in fields such as obtaining the spatiotemporal distribution characteristics of atmospheric aerosol pollution, monitoring horizontal visibility, and monitoring vertical boundary layers. The specific application conclusions are as follows. 1) Infrared detection lidar horizontal scanning monitoring can timely detect pollution sources in the area. During the monitoring period, a total of 10 main pollution source areas were discovered (Fig.3), distributed in the southeast and northeast sides of the radar points. Based on the comprehensive analysis of particulate matter concentration data from national urban environmental air quality monitoring stations within the scanning area, the pollution source spreads southwestward under the influence of northeast winds, resulting in a significant increase in particulate matter and carbon monoxide concentrations at downwind stations simultaneously. 2) Two infrared detection lidars were observed at the same location as the visibility meter, and the results showed that the correlation coefficient of visibility between the two lidars was 0.98, with a relative error of 7.76% (Fig.10-11). The trend of change was consistent, and the instrument performance was relatively stable. The relative error in comparison and analysis with the visibility meter data is less than 20%, which is consistent with the standard data. The daily variation of visibility shows a unimodal distribution, with higher daytime visibility compared to nighttime. From the 26th to the 27th, visibility decreases. Combined with environmental air quality analysis, an increase in particulate matter concentration is the main factor affecting visibility. 3) The infrared lidar and sounding balloon simultaneously inverted the boundary layer height, and the results showed that the absolute deviation between the lidar boundary layer height and the sounding balloon was 200 m, which can accurately invert the boundary layer height. During the monitoring period, the height of the boundary layer inverted by the lidar showed a diurnal trend, with an average height of 1.2 km (Fig.12). However, during the nighttime period, the height of the boundary layer was significantly higher than that of the sounding inversion, which was affected by residual pollution at night and was not a stable boundary layer height at night.
Conclusions Infrared detection aerosol lidar has the characteristics of strong atmospheric penetration, little influence by sky background light, and sensitivity to large particles, and has achieved good results in the spatial and temporal distribution characteristics of atmospheric aerosol pollution, horizontal visibility monitoring, and vertical boundary layer monitoring. Horizontal scanning can obtain a map of particulate matter distribution in a large area, find pollution sources in time, and evaluate the impact of pollution sources by combining data such as wind speed, wind direction, particulate matter and carbon monoxide concentration near the ground. The system can accurately capture the distribution and transmission of atmospheric aerosols in real time, and accurately invert information such as horizontal visibility and boundary layer height, and has a wide range of application scenarios in the field of atmospheric parameter monitoring.
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