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, a portable 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 (PM₁₀) 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 1/2 slope 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 0.18 km. However, the position of the extreme point was not fixed (Fig. 4). The correlation between the extinction coefficient measured by the lidar and PM₁₀ mass concentration measured by the particulate matter quality monitor reached 0.81, indicating that using the extinction coefficient to retrieve the PM₁₀ mass concentration is feasible. The relative error between the measured value of PM₁₀ mass concentration at 0.21 km from October 19 to 22 and 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--48 h and 54--72 h. The range of 0.09--0.24 km is the area where the PM₁₀ 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--2 km.

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 m from 0.09--0.24 km. The PM₁₀ 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 PM₁₀ mass concentration in the surrounding area is higher than that in the construction site.

With the normalization of construction, the change in PM₁₀ 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 PM₁₀ 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.