## 3 Results and Analysis
Figure 2: Scatter diagram of the extinction coefficient and PM\({}_{2.5}\) above 600 m.  
Demarcated by the time of transmitter adjustment, the data in Figure 1 are divided into two groups. The first set is five groups of data measured on May 12-13 (LT), and the second set is three groups of data measured on May 20 (LT). Intermittent thin clouds appeared at low altitudes in the first group, and the weather in the second group was fine and clean. Figure 4a shows the overlap factor calculated based on the first set of data. The gray area represents the variation range of the five overlap factor profiles, the boundary is the black dashed lines; the black solid line is the averaged overlap factor, and the red dashed line is the result of the Raman method. For comparison, Figure 4b shows the relative deviation of the results of the two methods. The deviation between the two methods is very small above 0.45 km, and the relative deviation between 0.14 and 0.45 km is less than 6.25%. However, the deviation increases rapidly as the altitude decreases and exceeds 10% below 0.12 km. This is because, at low altitudes close to the height of the detection blind zone, the overlap factor value is too small. Figure 4c,d show the calculation results of the second set of data, and the legend is consistent with Figure 4a,b. As the atmosphere is relatively clean and stable, the fluctuation range of the overlap factor is small. The relative deviation is less than 3.55% above 0.26 km and reaches the maximum value of 10.05% at 0.15 km. Below 50 m close to the blind zone, the absolute value of the relative deviation exceeds 10%. To verify the correction effect of the calculated overlap factor, the overlap factor in Figure 4c is used for data correction from 15:00 on May 22 to 06:00 on May 23 (LT). The time series of 355 nm range corrected signals before and after overlap factor correction are shown in Figure 5a,c, and the corresponding aerosol extinction coefficients are shown in  
Figure 4: The overlap factors are calculated by the two methods and their relative deviations in different weather conditions: (**a**,**b**) thin cloud weather; (**c**,**d**) clean weather. Figure 5b,d, respectively. After 19:30 on 22 July (LT), the near-surface aerosol extinction coefficients gradually increased after overlap factor correction. To facilitate analysis, the time curves of 75 m, 150 m, 300 m, and 750 m near the height range affected by the overlap factor were selected and compared with the corrected extinction coefficient with the ground observation curve. As shown in Figure 6, before 20:00 on May 22 (LT), due to the relatively uniform mixing of aerosols in the boundary layer, the aerosol extinction coefficients of all heights have the same changing trend, and the values are relatively close. After that, as the top of the boundary layer descends, aerosols converge toward the near-ground area, the changing trend of extinction coefficients at different heights is no longer consistent, and the numerical gap increases. As the height decreases, the trend of the curve gradually coincides with that of the surface ground. To sum up, the ability of Lidar to effectively detect low-altitude aerosols is greatly improved by the correction of the overlap factor. Figure 5: Range corrected signal (**a**,**c**) and aerosol extinction coefficient (**b**,**d**) from 15:00 on May 22 to 06:00 on May 23 (LT) before and after overlap factor correction. Figure 6: The curve of the aerosol extinction coefficient with time for different heights.