Geometric Factor Correction Method for Raman Lidar Under Partial Occlusion Conditions
Objective As a new type of active remote sensing equipment,lidar is increasingly widely used in the measurement of atmospheric components such as aerosols,water vapor,and ozone.Raman lidar used for aerosol and water vapor detection has outstanding advantages such as high detection accuracy,high spatiotemporal resolution,and real-time measurement capabilities.It is suitable for various mobile platforms such as vehicle mounted and airborne systems and has become one of the main technical means for accurately detecting the distribution of atmospheric aerosols and water vapor.The receiving field of view of a lidar cannot completely coincide with the laser beam at close range.The laser beam gradually enters the receiving field of view,so the echo signal received by the lidar at close range is only a partial echo signal of the laser beam.To describe this effect,a geometric overlap factor,abbreviated as the geometric factor,is defined.Due to the influence of geometric factors,the measurement results of lidar in the close range geometric factor area are inaccurate.The closer the distance,the more significant this effect becomes.Since atmospheric water vapor is mainly distributed below the troposphere,if we want to use lidar to obtain accurate water vapor distribution profiles,it is necessary to calibrate and calculate the geometric factors.This article focuses on the situation where the receiving telescope's field of view is partially obstructed by obstacles during horizontal experimental measurement of geometric factors in the practical application of gas-soluble glue water vapor Raman lidar.We have made some improvements to the correction method of geometric factors.Methods We propose an improved geometric factor correction method to solve the problem of partial occlusion of the telescope's receiving field of view in horizontal experimental measurements of gas-soluble glue water vapor Raman lidar.This method is based on the experimental method commonly used for geometric factor correction,which involves measuring the lidar along the horizontal direction under horizontally uniform atmospheric conditions.Improvements have been made to the experimental scheme and data processing methods.Firstly,a shading device is used to completely block the lower half of the telescope's field of view(Fig.2).The position of the shading device can be flexibly adjusted according to the occlusion of the object,ensuring that the unobstructed part can be fully received on the path.The improved experimental plan is as follows:① a shading device is used to cover half of the received field of view of the telescope,followed by horizontal measurement with a lidar in the azimuth direction under good visibility conditions;② the pitch angle is adjusted to 90°,ensuring the shading device in an unobstructed state for a set of vertical measurements with the same parameters;③ the shading device is quickly removed to perform another set of vertical measurements under the same conditions.To obtain the geometric factor of the lidar before the method improvement,distance squared correction is applied to the echo signal measured horizontally by the lidar in the first step.An appropriate linear range is then selected for fitting[Fig.4(a)],and the ratio of the two measurements is processed to determine the geometric factor[Fig.4(b)].Since the occlusion state of the telescope remains unchanged during horizontal measurement in step ① and vertical measurement in step ②,the geometric factors processed in step ① can be used for correcting the vertical occlusion measurement echo signal in step ②(Fig.5).In step ③,the shading device is quickly removed to perform a set of vertical measurements with the same parameters as in step ②.The interval between the two vertical measurements is very short,allowing the assumption that the atmospheric state remains unchanged.The echo signal of the vertical unobstructed measurement without geometric factor correction in step ③ and the echo signal of the vertical unobstructed measurement with geometric factor correction in step ② are plotted together(Fig.6).Normalizing the ratio of the two signals provides the true and accurate geometric factor of the aerosol water vapor Raman lidar(Fig.7).Results and Discussions Continuous atmospheric observation experiments are conducted using a self-developed aerosol water vapor Raman lidar system,and the extinction coefficient profiles of aerosols before and after geometric factor correction using the improved method are inverted(Fig.10).We compare and analyze the calculation results of the 532 nm wavelength optical thickness of the lidar before and after the improvement of the geometric factor correction method with the continuous measurement results of the 550 nm wavelength optical thickness of the solar radiometer at the same time and space(Fig.11).The correlation analysis results show that the correlation coefficient between the improved geometric factor correction method and the measurement results of the lidar and solar radiometer is as high as 0.9779(Fig.12),indicating good consistency between the two.From the calculation results of relative error,the relative error between the improved lidar optical thickness(532 nm)calculation results and the solar radiometer(550 nm)measurement results is within 10％,with an average relative error of 3.81％and a maximum relative error of 8.23％.The average relative error before the method improvement is 8.34％,and the maximum relative error is 18.26％.The accuracy of the improved method is 2.19 times that of the original method.The reliability of the lidar measurement results and the rationality and accuracy of the improved geometric factor correction method have been fully verified.Conclusions We calculate and analyze the correction effect of the proposed improved geometric factor correction method.The results indicate that this method can accurately calculate the geometric factors of Raman lidar systems under partial occlusion conditions.After the geometric factor correction obtained by the improved method,the accuracy and reliability of the lidar measurement results are good.The improved geometric factor correction method has a certain reference value for the practical application of aerosol water vapor Raman lidar systems.
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