## 2 Material and Methods
### Calibration  
A critical problem for performing short-range measurements with elastic backscatter lidar arises from the incomplete overlap between the lidar transmitter and receiver. This incomplete overlap problem is a major source of uncertainty for profiling aerosol radiative and microphysical properties with short-range lidars. From an instrumental perspective, it can be seen as a lidar collection efficiency that varies with range. The origin of this variation originates from the changes of the image-plane position with the range of the scattering volume, which affects the efficiency of the lidar collection. As a result, the light backscattered by aerosols in the short-range may not completely reach the optical sensor. This zone is often referred to as the lidar incomplete overlap or blind zone. Nevertheless, a wide range of techniques has been proposed to determine the overlap function of lidar systems. Theoretical methods [38; 39] require complete knowledge of all the lidar specifications with sufficient accuracy. Because such accurate information is generally not available for most lidar systems, experimental approaches have been undertaken to estimate the overlap function [40; 41; 42; 43; 44; 45; 46]. The Colibri micro-lidar was specially designed with a bi-static multi-axial configuration for profiling aerosols in the short-range with a reduction of the incomplete overlap zone. An evaluation of the range-dependent overlap function is proposed using a simple two-step method. First, a series of lidar measurements on a low reflectance Lambertian surface (Spectralon, Labsphere) are carried out at different ranges. Second, an analytical function reported by [PERSON] et al. [47] is used to fit the measurements using the bi-static lidar parameters. The advantage of our method based on low reflectance surfaces is that it does not rely on the usual assumption of homogeneous atmospheres used for estimating the overlap function. To prevent any damages of the photodetector due to intense reflection on the surface, a neutral density filter may be inserted without affecting the geometric calibration. Figure 2 reports the experimental evaluation of the overlap function for a series of ranges in the short-range. The results are fit by the analytical function using the lidar systems parameters from Table 1. Our results indicate a high correspondence between analytical and experimental overlap functions for the Colibri micro-lidar. A non-uniform Gaussian energy distribution within the laser beam may explain the small disagreement between the analytical and experimental overlap functions. However, the laser characteristics measured in the laboratory allow us to expect stable laser power (<1.5%), pulse to pulse stability, and divergence over time. Our method suggests that the micro-lidar architecture and configuration are suitable for short-range aerosols profiling. The lidar radiometric calibration is performed at the full-overlap distance using as well a low reflectance surface (Spectralon, Labsphere). These surfaces are highly diffusive due to the porosity of their PTFE-based surface and their reflectance properties are well characterized and close to a Lambertianreflector at normal incidence [48]. Elastic lidar instruments commonly do not have direct access to the backscattered power \(P(\mathrm{r})\) but provide range-corrected signals RCS such as:  
\[\mathrm{RCS(r)}=\mathrm{K_{o}^{*}\,O(r)\,U(r)}\qquad[\mathrm{V\ m^{2}}] \tag{12}\]  
where \(\mathrm{K_{o}^{*}}\) is the instrument lidar constant directly derived from the previously defined radiometric lidar constant \(\mathrm{K_{o}}\) as:  
\[\mathrm{K_{o}^{*}=S\,G\,K_{o}=S\,G\,E_{o}\frac{c}{2}A\qquad[\mathrm{V\ m^{3}\ sr}]} \tag{13}\]  
where S is the sensor responsivity \([\mathrm{A\ W^{-1}}]\) and G is the gain \([\mathrm{V\ A^{-1}}]\) of the low-noise trans-impedance amplifier.