## 4 Discussion
### Comparative Analysis with Local Sensors  
In the following, we propose a comparative analysis between the aerosol number concentration retrieved from the Colibri and Fidas 200 during the fog-oil event. The objective is to validate the proposed methodology for short-range micro-lidar. Prior to the comparison, we should recall the different nature of the two instruments. Elastic backscatter lidars are remote-sensing instruments that provide range-dependant aerosol products whereas optical particle counters are local sensors that measure aerosols properties (e.g., particle size distribution, particle concentration) in a single location. The direct products of the Fidas 200 are the particle matter concentrations (PM1, PM2.5, PM10) at relatively high temporal resolution (1s), nevertheless, elaborated products such as particle and mass concentration are time-integrated products. Considering that, both instruments will measure the fog-oil event, but Fidas 200 is retrieving the particle number concentration as a function of the diameter for the air column where Fidas 200 is located (4 m away to the fog-oil generator and about 25 m from the micro-lidar). Meanwhile, the aerosol number concentration retrieved from the Colibri micro-lidar measurements are provided with a native spatio-temporal resolution for a scattering-volume along the tunnel.  
To use the Fidas 200 product as a reference during the event, comparable products should be computed for both instruments in order to preserve their principle of the measurement. The temporal mean of the \(\mathrm{N}_{p}\) is calculated here and is directly provided by the Fidas 200. For the Colibri micro-lidar, a range and  
Figure 8: Calibration error propagation of \(K_{\mathrm{0}}^{*}\) and \(\mathrm{O}(\mathrm{r})\) on the Colibri micro-lidar products: attenuated backscatter profiles (top panel), backscatter profiles (middle panel), and aerosol number concentration (bottom panel).  
temporal mean was performed around the fog-oil plume from 0 to 40 m and from 0 to 60 s. This makes it possible to carry out a comparison in Table 3 between the mean values retrieved from both instruments.  
Table 3 presents the time and range-averaged aerosol number concentration from the Colibri and Fidas 200 instruments \(<\) N\({}_{p}>=\) 4350 part cm\({}^{-3}\) and \(<\) N\({}_{p}>=\) 3990 part cm\({}^{-3}\) respectively. This comparison yields a good agreement between the Colibri micro-lidar and the Fidas 200. The differences might be associated on the one hand to the different nature of the instrument: while the Colibri micro-lidar is a remote-sensing instrument that measures range-resolved number concentration of particles, Fidas 200 is an in-situ instrument that performs measurements from a single location. On the other hand, the differences in temporal resolution between instruments might yield erroneous results, considering that Fidas 200 reported less than 15 measurements during the fog-oil event, which is far from being directly comparable with the millisecond temporal resolution of the Colibri micro-lidar. Finally, this analysis assesses the performances of the Colibri micro-lidar for providing quantitive number concentration profiles for complex aerosol events in the short-range, close to their sources, and with high range-resolution and high temporal-resolution.