# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 6 Conclusions  
In this paper, the local wind structure is retrieved in the steep-sided Passy basin that is part of the Arve river valley, close to Mont-Blanc (French Alps). Our aim was to identify the features of the local winds that may contribute to (i) the high levels of PM10 monitored in winter in the Passy basin and (ii) the spatial PM10 heterogeneities observed locally over a few tens of kilometres within this basin. This work relies on data from the Passy-2015 field experiment that took place during winter 2014/2015. More precisely, this work focuses on a persistent temperature inversion episode documented during an Intensive Observation Period of the field experiment. Data from radiosoundings, ground stations of the METEO-FRANCE operational network and a scanning DWL were used. The scanning DWL database was first validated through a comparison with a profiler DWL, allowing a good level of confidence regarding the velocity fields retrieved. The scanning DWL proved to be a really useful instrument, providing access to 3D measurements and thus revealing the local wind structures that could not be seen by profiler instruments. Moreover, additional relevant information can be derived from the data given by this instrument, such as the aerosol layer height or a qualitative overview of the aerosol content derived from the CNR signal. Within the Arve river valley, PM10 concentrations recorded in the Passy basin are significantly higher than concentrations recorded in the upstream Chamonix valley under a persistent temperature  
Figure 15: Horizontal map of ventilation index obtained from PPI scan, 40 m AVL for (**a**) the full 11 February, (**b**) the early morning stage [0:00 a.m.:10:59 a.m.] UTC; (**c**) the daytime stage [11:00 a.m.:4:59 p.m.] UTC; and (**d**) the evening stage [5:00 p.m.:11:59 p.m.] UTC. Blue represents ventilation in the toward-lidar LOS direction and red represents ventilation away-from-lidar, which is located at the centre (0,0). The black star indicates the location of Site 1 within the Passy basin and the black dot shows the location of the TEOM-FDMS station, which measured \([PM10]\) at Passy. The Sallanches station (SLNCH) is out of the plot, 6.2 km away the center in the west–northwest direction, close to the southern sidewalls of the basin (blue star in Figure 1). inversion episode. Beyond the variability in emissions rates that may exist, this work reveals that the accumulation of pollutants within the Passy basin is consistent with the combined effects of:  
* a night-time stagnation of pollutants in the lowest layers, induced by the back-and-forth transport of particles by wind reversal on an hourly timescale. * a reduction of the dilution efficiency because of the discharge of the urbanized tributary valley emissions into the basin at night. Such particles may be integrated into the low-layers in the basin, during daytime convection,
* a lack of daytime ventilation because of the combined effects of thermal stratification and topography, including the Servoz passageway between the Passy basin and the Chamonix valley, which prevent particles contained in the up-valley flow from being vented out. Under less stable conditions, the communication between the Passy basin and the upstream Chamonix valley is restored, leading to similar PM10 between the two sections of the Arve river valley. Within the Passy basin, significant differences in PM10 levels are also observed between Passy and Sallanches, although they are only 5 km apart.