Lidar has been widely used in the field of atmospheric detection with the advantages of high spatial-temporal resolution and high detection sensitivity. The overlap factor of a lidar system arises from the incomplete overlap between the laser beam and the field of view of the receiver, which results in the distortion of the received backscattered signal in the near-field range. The accurate observation of aerosol optical parameters near the ground is important for the monitoring of the atmospheric environment, air quality, and atmospheric visibility. The overlap factor at a certain distance is defined as the ratio of the beam energy entering the receiving field of view to the actual backscattering energy. In general, the overlap factor is estimated by either theoretical methods or experimental methods. The theoretical methods are to calculate the overlap factor according to the structural parameters of the lidar system. However, some parameters are often rather difficult to accurately obtain in practice or theory, such as the performance of the optical elements, the beam divergence angle, and the beam direction. The experimental methods are to calculate the overlap factor with the experimental observation data. Some use the deviation between the Raman solution and the Fernald solution of a backscattering coefficient to calculate the overlap factor. The main limitation is that the Fernald method of aerosol backscattering coefficient requires the assumption of lidar ratio or boundary conditions, which will introduce great errors. Besides, the experimental methods strongly depend on the accurate estimation of atmospheric conditions. Thus, it is necessary to propose a stable algorithm for overlap factors to correct signals and aerosol optical parameters in the near-field range.

An experimental method for the overlap factor of Raman-Mie scattering lidar is proposed in this paper, which is applicable to lidar systems equipped with a Raman scattering channel. The method is based on the Raman inversion method for aerosol optical parameters. By analyzing the inversion characteristics of aerosol optical parameters, it is found that in a transition area, the aerosol backscattering coefficient is not influenced by the overlap factor, while the aerosol extinction coefficient is influenced greatly. In the Raman inversion method, the aerosol extinction coefficient and backscattering coefficient are independently inversed without the assumption of lidar ratios. Thus, the lidar ratio profile can be obtained in a complete overlap area. According to the inversed optical parameters, the overlap area height is determined preliminarily, and then the lidar ratio in the transition area is assumed to be equal to that at the overlap area height. The product of the aerosol backscattering coefficient and the lidar ratio is used to preliminarily correct the missing signal of the aerosol extinction coefficient in the transition area. The Raman scattering signal is derived from the inverse equation of the aerosol extinction coefficient, and then the preliminarily corrected Raman scattering signal is forward modeled. The overlap factor is obtained by dividing the experimental observed Raman signal by the forward modeled Raman signal. The blind area, transition area, and overlap area are distinguished according to the overlap factor profile. The Raman scattering and Mie scattering echo signals and the aerosol optical parameters in the near-field range are corrected, respectively. For the transition area, the definition of the overlap factor is used for signal correction. For the blind area, the slope consistency method is used to supplement the signal, that is, the slope of the standard atmospheric model is used to linearly estimate the signal.

The atmospheric observation experiment is carried out with an independently developed Raman-Mie scattering lidar system. For a single set of experimental observation data, the aerosol extinction coefficient and backscattering coefficient are retrieved, respectively. The overlap factor profile is obtained, and then the echo signals and aerosol optical parameters are corrected, respectively. The estimated extinction coefficient on the ground by lidar is compared with that observed via a visibility meter to verify the correctness of the algorithm. For the observation data from an experiment lasting for 4 hours, the time-height-intensity (THI) diagrams of the aerosol extinction coefficient before and after the correction by an overlap factor are given (Fig. 5). The corrected aerosol extinction coefficient below about 0.6 km can be obtained, which can obviously reflect the stratification structure and spatial-temporal variations of atmospheric aerosols near the ground. The estimated aerosol extinction coefficients of long-term observations on the ground are compared with those simultaneously observed via a visibility meter, and they show good consistency with a regression coefficient R up to 0.993 (Fig. 7). The measured overlap factors are compared with those calculated with the theoretical method, and the relative biases are analyzed separately. The error of overlap factors can be controlled within ±8%.

The error of the proposed method is calculated and analyzed. The results show that the proposed method can accurately calculate the overlap factor profile of the Raman lidar system. After correction by an overlap factor, the signal profile in the transition area and the estimated linear signal in the blind area can be obtained. The improved method is of great significance for the correction and supplement of near-field signals with lidar.