# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 5 Discussion
### Consequences of the Observed Wind Structures on Air Quality
#### 5.2.2 PM10 Differences between Passy and Sallanches  
The PM10 dynamics of Passy and Sallanches, both lying within the basin, are similar as observed in Figure 2, but PM10 concentrations are significantly higher at Passy. A ventilation index was derived to visualize the average ability of the wind to disperse pollutants at every location of the basin. This was computed by integrating the \(V_{los}\) field over a certain period using horizontal PPI scans, at 40 m AVL. The index is given in metres-per-hour.  
The ventilation index is displayed for 11 February in Figure 15a. A decomposition in three stages of the 11 February is proposed with (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. Ventilation in the toward- and away-from-lidar directions are represented in blue and red, respectively. Panel a shows that the daily-averaged ventilation index intensity does not exceed 600 metres-per-hour reflecting a globally weak ventilation of the basin low-level layer. In comparison, it reaches 4000 metres-per-hour the 14 February, which was out of the inversion episode (not shown).  
In the east of the basin, above Passy, the daily-averaged ventilation is westerly. The decomposition per stage shows that this place is mainly affected by the daytime up-valley wind (panel c) with intensity above 1500 meters-per-hour while the night-time index does not exceed 300 metres-per-hour (panels b and d). This can be related to the night-time down-valley jet, which is only observed at a higher altitude in this part of the basin (Figure 8). As a result, the east of the basin is mainly ventilated during the day and remains stagnant at night.  
In the west of the basin, on a daily-average, the northern part is exposed to an easterly ventilation, while the southern part is ventilated in the opposite westerly direction (panel a). Panel c shows a rather homogeneous structure during the daytime stage while panel d highlights a north-south sheared zone in the evening stage. This is again likely to be related to the night-time down-valley jet, which is at altitude in the eastern part of the basin but falls to a lower level in the middle of the basin, remaining at this lower altitude through the western part. The daily-averaged index in the west therefore results in a predominance of (i) the day-time up-valley wind in the south and (ii) the night-time down-valley wind in the north.  
Finally, on a daily-average, Sallanches and Passy are both exposed to a westerly ventilation. This results in an advection of the Sallanches PM10 towards Passy. The main difference between the two parts of the basin occurs at night. During the evening stage, the atmosphere above Passy is almost motionless, while Sallanches is affected by an easterly ventilation. This difference may participate in the higher \([PM10]\) measured at Passy. Similar patterns with north-south and west-east gradients are observed on the daily ventilation index and \(\beta T^{2}\) maps (Figures 15a and 13, respectively). The trajectory of the down-valley wind therefore appears to be of major importance in the PM10 distribution within the Passy basin.