# Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
## 6 Conclusions
In addition to the effects mentioned above, the following wind pattern specificities may contribute to these variations:  
* a reduction of the night-time dilution at Passy because of the down-valley flow, which remains above the lowest layers in the eastern part of the basin but is likely to get through the lower layers in the other part of the basin, leading to a west-east gradient in dilution,
* a northeast-southwest ventilation gradient observed when looking at the 24-h average with a westerly transport in the southern part of the basin and an easterly transport in the northern part of the basin, probably with more polluted air. This northeast-southwest gradient is consistent with the observed heterogeneity of the backscattered signal maps derived from the scanning DWL CNR signal. To sum up, the complexity and very particular characteristics of the basin topography are mirrored in the local winds that are heterogeneous. Under stable wintertime conditions, the observed wind systems lead to an important variability in ventilation within the Passy basin. Instead of distributing the emissions among the valley, the wind system favors their accumulation in places where the highest PM10 concentrations are measured. This study finally highlights the importance of local dynamincs on PM10 distibution. Several questions regarding the mechanisms leading to the local wind structures observed arise from this work. A high resolution numerical modelling study is currently in progress in order to investigate these questions. The model has shown a good ability to represent the persistent temperature inversion dynamics observed during the first IOP [37]. Focus will now move to the role of the western part of the Passy basin, not covered by instruments during the field experiment, the Servoz passageway and its impact on the valley wind structures, and the possible role of the northern and southern slopes on the north-south local wind gradient. Particular attention will be paid to the elevated down-valley wind in the eastern part of the Passy basin as it appears to be a key element for the impact of local winds on air quality within the basin. The Passy-2015 field experiment was supported by ADEME through the French national programme LEFE/INSU and by METEO-FRANCE. We thank the cities of Passy and Sallanches for their kind support. The field experiment was led by CNRM while LEGI is the principal investigator of the LEFE/INSU project. Data are managed by SEDOO at Observatoire Midi-Pyrenes ([[http://passy.sedoo.fr](http://passy.sedoo.fr)]([http://passy.sedoo.fr](http://passy.sedoo.fr))). We thank all the teams involved in the field experiment: GMEI/LISA, GMEI/4M, GMEI/MNPCA and GMEI/TRAMM at CNRM; LEGI, IGE, Atmo Auvergne-Rhone-Alpes and NCAS (UK). A special thanks to the people involved in the rawinsondes preparation and data acquisition under difficult meteorological conditions at night-time. [PERSON] wrote the paper and conducted the data analysis in the framework of her Ph.D. program. [PERSON] designed the scanning strategy of the lidar during the field experiment and helped in the interpretation of the results and manuscript revisions along with [PERSON] and [PERSON]; [PERSON] brought its expertise on stable flows; [PERSON] brought its expertise on boundary layer dynamics; [PERSON] largely contributed to the analysis of the dynamics of the temperature inversion episode; [PERSON] contributed to data-processing and supported the determination of the measurement accuracy. [PERSON]-M.D. and T.D. ensured the scanning Doppler wind lidar data acquisition during the field-experiment and helped on technical details and data-processing. **Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.