# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 2 Passy-2015 Field Experiment  
### Context: A Steep Sided Polluted Alpine Valley  
The Arve River Valley is located in the French Alps close to the border between France and Switzerland. It starts in the vicinity of Mont-Blanc and extends over about 100 km to Lake Geneva. This study focuses on a section of the Arve River Valley close to the town of Passy, hereafter referred to as the Passy basin. The topography of the Passy basin and its surroundings is presented in Figure 1. This section of the valley is approximately 23 km long, has a maximum width of 2 km and an average valley bottom slope of 1%. The basin is confined by the Giffre range (about 2000 m Above Sea Level (ASL)) to the north, the Aravis range (about 2000 m ASL) to the west and the Mont-Blanc range (rising to 4808 m ASL) to the southeast. The floor in the centre of the Passy basin is around 560 m ASL, giving a valley depth of more than 1500 m. The Passy basin is connected with the upstream part of the Arve river valley (Chamonix, 1030 m ASL) and the downstream part (Marnaz, 480 m ASL) through two constricted passageways, at Servoz and Cluzes, respectively. At Servoz, the valley is less than 50 m wide and its floor rises by 200 m over a distance of 4 km. This forms a natural barrier between the upper part of the valley and the Passy basin. Two smaller tributary valleys are connected to the south of the basin, orientated in a northeasterly direction (\"Megeve\" valley) and northwesterly direction (\"Saint-Gervais\" valley). The Arve river valley regularly experiences severe pollution episodes in winter. The valley is urbanized, with 27,000 residents within the Passy basin (Passy, Sallanches), 10,000 residents in the upstream town of Chamonix and about 6000 residents in each of the southern tributary valleys (Megeve and Saint-Gervais). An Atmosphere Protection Plan (2015) has been in force for the whole Arve River Valley since February 2012 for winter PM10 pollution episodes among other things. PM10 can have anthropogenic or natural origins, the former being the main source of pollution within the valley. Anthropogenic PM10 results from residential heating (wood burning), transport (the valley has been one of the major access roads to Italy through the Mont-Blanc tunnel since it was opened in 1965) and industrial emissions. In 2015, the local air quality agency, Atmo Auvergne-Rhone-Alpes, recorded 44 and 20 days when PM10 concentration exceeded the air quality standard at Passy and Sallanches, respectively [33]. In contrast, the neighbouring cities outside the Passy basin were significantly less affected, with 10 days at Chamonix (20 km up the valley) and only 3 days at Marnaz (20 km down the valley). Figure 2a displays the time evolution of PM10 for the four towns through January and February 2015. Grey areas represents the pollution episodes based on PM10 levels measured at Passy. It can be seen that PM10 time series at Passy and Sallanches display similar trends. However, concentrations at Sallanches remain systematically lower during pollution events, whereas the two towns are only 5 km apart. At Chamonix, the PM10 time series appears significantly distinct from those at Passy and Sallanches during pollution peaks but remains similar the rest of the time. Table 1 lists the pollution episodes represented in grey and gives the day of the episode during which the maximum PM10 concentration was reached for the three stations. While the maximum was reached on the same day for the two stations within the Passy basin, it was shifted towards the beginning or the end of the episodes for the Chamonix up-valley station. Figure 2b displays the time evolution of the sea level pressure (Psea) measured in the middle of the Passy basin. In addition, the Psea time series recorded by the METEO-FRANCE automatic weather station at Annecy, 45 km west of the Passy basin, is represented. The two series are very close, with an average difference of 0.65 hPa and a maximum difference of 3.5 hPa.