激光雷达探测建筑基坑开挖阶段气溶胶水平分布

Aerosol Horizontal Distribution Detected by Lidar in Excavation Stage of Construction Site Foundation Pit

建筑工地扬尘等气溶胶是大气污染的重要来源,激光雷达是探测建筑工地基坑开挖阶段气溶胶水平时空分布的有力手段。一台532 nm波长的便携式微脉冲激光雷达发射激光并接收气溶胶颗粒与激光相互作用的后向散射回波信号,结合颗粒物浓度监测仪,利用Klett法和分段斜率法反演气溶胶水平消光系数和PM_(10)质量浓度。结果表明,用激光雷达探测的消光系数反演工地PM_(10)质量浓度是可行的。当基坑开挖、渣土外运等施工作业开始时,工地内的气溶胶颗粒浓度明显高于周边区域,且会向周边扩散;停止施工后,在无风情况下,气溶胶颗粒会聚集在工地区域内,PM_(10)质量浓度会维持在一个高水平阶段。研究结果为掌握并控制工地气溶胶颗粒排放情况提供了技术支撑。

Objective The dust and other particulate matter generated during the excavation of construction site foundation pits are major sources of urban atmospheric aerosols and an important factor in the frequent occurrence of severe weather,such as haze.Aerosol particles on construction sites are derived from slag transportation and general construction dust,which are unorganized and irregular emissions.However,monitoring data in limited locations cannot fully and accurately reflect the spatial and temporal distribution of aerosol levels in the construction area.Lidar with high temporal and spatial resolution,large range,and real-time remote sensing monitoring capabilities has gradually grown into an indispensable observation device in the field of urban fine particle monitoring and atmospheric science research.Therefore,lidar is an effective method for detecting the spatial and temporal distribution of aerosols during the excavation of foundation pits at construction sites.Methods In this study,aportable micropulse lidar with a 532-nm wavelength emits the laser and receives the backscattered echo signal generated by the interaction between aerosol particles and lasers.Combined with the particle concentration monitor,the Klett and segmented-slope methods are used to invert the aerosol.The horizontal extinction coefficient and particulate matter(PM_(10))mass concentration are used to ensure that emission of pollutant particles in the entire construction site is always in a known and controllable state.First,a curve with distance as the abscissa and logarithm of the signal as the ordinate is obtained according to the lidar equation.Then,the least squares method is used to plot each small distance in the curve to obtain the linear equation of each distance,and its 1/2slope is the value of the horizontal extinction coefficient.The aerosol extinction coefficient of a certain distance with the highest correlation among the aerosol extinction coefficients at each distance is selected as the horizontal extinction coefficient at the boundary position.Finally,the distribution of the horizontal extinction coefficient on the detection path is obtained by substituting the boundary value of the extinction coefficient into the Klett equation.Although the atmosphere is assumed to be uniform,it is based on the long-distance detection data,and the average extinction coefficient value of the whole process is taken.Results and Discussion Lidar obtains the backscattered signal of aerosol particles on the construction site to the laser,revealing the horizontal distribution of aerosol during the excavation stage of the construction site.The detection results show that the change in the aerosol particle-level distribution curve at different moments in the excavation area of the site foundation pit first rose and then fell,and an extreme point appeared at 0.18km.However,the position of the extreme point was not fixed(Fig.4).The correlation between the extinction coefficient measured by the lidar and PM_(10)mass concentration measured by the particulate matter quality monitor reached 0.81,indicating that using the extinction coefficient to retrieve the PM_(10)mass concentration is feasible.The relative error between the measured value of PM_(10)mass concentration at 0.21km from October 19to 22and inversion hourly average value was between 10%and 30%.Continuous change in the concentration profile of pollutants are essential for determining the location of pollution sources and emission rules and can better reflect the temporal and spatial distribution and change of aerosol levels.The excavation of foundation pits and the export of muck are mainly concentrated in the period of 30-48hand 54-72h.The range of 0.09-0.24km is the area where the PM_(10)mass concentration changes continuously.Simultaneously,owing to the influence of the southeast wind direction,particulate matter,such as aerosols generated in the construction site,will drift with the wind and spread to the construction site within 1-2km.Conclusions The results show that the dust and other aerosols generated during the excavation of the construction site’s foundation pit are concentrated in the area of 150 mfrom 0.09-0.24km.The PM_(10)mass concentration in the construction site area is significantly higher than that in the surrounding area;however,it will diffuse to the surrounding area,resulting in certain periods.The PM_(10)mass concentration in the surrounding area is higher than that in the construction site.With the normalization of construction,the change in PM_(10)mass concentration-level profile and the time of peak appearance are inseparable from the construction schedule of the site.After stopping the construction,aerosol particles will gather in the site area without obvious wind speed.The PM_(10)mass concentration will be maintained at a high level.Thus,the discharge of pollutant particles on the construction site can be grasped and controlled in time so that the discharge of pollutant particles on the entire construction site is always in a standard and controllable state.