## 4 Experiments and Results
### CO\({}_{2}\) Concentration Outcomes  
Before the field measurement, a zero offset calibration process was first carried out using off-off line testing. Both wavelengths of the improved system were adjusted to the \(\lambda_{\text{off\_1}}\) values in Table 1 during the process of the zero offset calibration. In this case, the atmospheric extinction coefficient and target reflectance of the two wavelengths are the same. The effect of speckle noise was very small, as the speckle field between the two wavelengths was highly correlated. With this arrangement, and according to Equation (3), the measured differential optical depth is expected to be zero. An open-path measurement experiment was done in Hongkou District, Shanghai, China, with the dual \(\lambda_{\text{off\_1}}\) wavelengths and a concrete exterior wall surface 580 m away as a reflective hard-target. The integral path is shown by an orange double-arrow line in Figure 14. As shown in Figure 15, the correlation coefficient of the normalized intensities of two channels was greater than 0.9997, as the wavelength separation was close to zero. The CO\({}_{2}\) concentration results in Figure 16 show that a 5 ppm offset from 0 ppm remained, which could be eliminated by correction. The drift of the concentration baseline was less than 1 ppm. On this path, we adjusted the wavelength of the on laser to the \(\lambda_{\text{off\_1}}\) wavelength in Table 1, and an in situ CO\({}_{2}\) sensor (Instrumentation for Biological Sciences Ltd., Li-7500A CO\({}_{2}\)/H\({}_{2}\)O Analyzer) was placed on the path. After correcting the fixed offset (5 ppm), the measurement results of the CO\({}_{2}\) concentration on this path were obtained for 21 h in summer (with a 10 s average measurement interval). The measurement results of the in situ sensor (shown in the rectangular frame in Figure 17) changed drastically due to rain during this period, where the raindrops fell on the window mirror of the open optical path of the Li-7500 to form interference. The measurement of the IPDA lidar was not affected by the rainfall process. In other time periods, the diurnal variations measured by the IPDA lidar were consistent with those of the in situ sensor. The standard deviation of the comparison error between the IPDA lidar and in situ sensor was about 5.566 ppm, as shown in Figure 18. Figure 19 shows the measurement results of the IPDA lidar and the in situ sensor in summer shower weather. The in situ sensor could not work normally for most of the time. There were no obvious peak and valley for the diurnal variations of the CO\({}_{2}\) concentration, according to the IPDA lidar measurement results in the shower weather. Figure 13: Intensity time-series for different wavelength separations; (**a**) intensity of on and off wavelengths separated by 16 GHz; (**b**) intensity of on and off wavelengths separated by 36.4 GHz; (**c**) intensity ratio of time-series in (**a**) after 1 s of smoothing time; (**d**) intensity ratio of time-series in (**b**) after 1 s of smoothing time. Figure 16: IPDA lidar CO\({}_{2}\) mixing ratio offset measurement using off-off line testing. Figure 14: Integration paths for IPDA lidar testing. The orange double arrow line is the 580 m horizontal path. The red double arrow line is the 1.5 km horizontal path. The base map comes from the China National Platform for Common Geospatial Information Services ([[http://www.tianditu.gov.cn/](http://www.tianditu.gov.cn/)]([http://www.tianditu.gov.cn/](http://www.tianditu.gov.cn/))). Figure 15: Normalized intensities with dual off wavelengths. Figure 19: Simultaneous measurements of the diurnal variations of atmospheric CO\({}_{2}\) with the improved IPDA lidar system and an in situ sensor. The integral path length is 580 m and the weather was rainy. Figure 17: Simultaneous measurements of the diurnal variations of atmospheric CO\({}_{2}\) with the improved IPDA lidar system and an in situ sensor. Black boxes indicate rainy weather.