Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial distribution and temporal evolution of pollutant concentrations. They can be very complex and difficult to represent in numerical weather prediction models, particularly under stable conditions. Better knowledge of these local winds from observations is also a prerequisite to improving air quality prediction capability. This paper analyses local winds during the Passy-2015 field experiment that took place in a section of the Arve river valley, near Chamonix-Mont-Blanc. This location is one of the worst places in France regarding air quality. The wind analysis, which is mainly based on scanning Doppler lidar data sampling a persistent temperature inversion episode, reveals features consistent with the higher pollutant concentrations observed in this section of the valley as well as their spatial heterogeneities. In particular, an elevated down-valley jet is observed at night in the northern half of the valley, which, combined with a weak daytime up-valley wind, leads to very poor ventilation of the lowest layers. A northeast-southwest gradient in ventilation is observed on a daily-average, and is consistent with the PM10 heterogeneities observed within the valley.

Les conditions anticycloniques hivernales (ciel clair, nuits froides) conduisent à la formation de couches stables persistantes qui favorisent les épisodes de pollution, particulièrement en terrain montagneux. La vallée de l'Arve est très sensible à ce phénomène, en particulier près de la ville de Passy (Haute-Savoie), située à 20 kilomètres en aval de Chamonix-Mont-Blanc, où la qualité de l'air est l'une des moins bonnes de France.Au-delà du suivi de la qualité de l'air, tel que réalisé par Atmo Auvergne-Rhône-Alpes ou par le projet DECOMBIO piloté par l'Institut des Géosciences et de l'Environnement (IGE), il est primordial d'améliorer la connaissance de la dynamique atmosphérique à l'échelle de la vallée en conditions stables pour mieux comprendre comment, couplée au cycle et à la géographie des émissions, elle pilote la dispersion des polluants. C'est la motivation du projet Passy, initié en 2014. Ce projet s'appuie sur les observations de la campagne Passy-2015, présentées dans cet article avec quelques premiers résultats. L'objectif général de cette campagne est de documenter la dynamique atmosphérique au sein de la vallée de l'Arve lors des épisodes de pollution hivernale.Les travaux menés dans le cadre du projet et de l'analyse des données de la campagne s'inscrivent au sein d'une collaboration sur plusieurs années entre les différents partenaires. Ils contribueront à affiner la prévision du temps et de la qualité de l'air dans ce type de vallée, et plus généralement en conditions stables. Il s'agit en particulier d'améliorer la capacité à prévoir des phénomènes critiques, comme les températures minimales, le verglas, le brouillard, les évènements de pollution ou encore les zones de pollution intense.

Mountainous terrains are known for driving their own dynamics which respond to the local morphological arrangement of the area. Thermally driven flows in particular develop at slope and valley scales and ensure a certain degree of pollutant dispersion under quiescent wintertime synoptic conditions. The present study focuses on a section of the Arve River valley situated close to Mont Blanc which frequently suffers from severe pollution episodes under stable wintertime conditions whilst surrounding valleys appear to be less affected. The particular shape of this basin‐like section and its location at the confluence of several tributary valleys raises the question of the extent to which local circulations participate in pollutant trapping over restricted sectors. A set of high‐resolution numerical simulations are designed in order to improve our understanding of the local flow structure and their sensitivity to thermal stratification, radiative forcing and snow cover. The tributary valleys play a major role in both daytime and night‐time dynamics by deflecting the entering daytime flux and constraining the night‐time flow trajectories. In addition, the basin morphology greatly influences the circulations. During daytime a two‐layer wind structure is developed and driven by spatial variations in sun exposure which is particularly heterogeneous under wintertime forcing. Early spring radiative forcing or the presence of snow allow both of them to develop more homogeneous circulations through the reduction of sun exposure variations, except when snow cover is restricted to shaded basin sidewalls. At night, a three‐layer wind structure is developed. It favours air mass ventilation in the western linear branch of the basin whilst stagnation and recirculation prevail in the curved part of the valley which is also the most polluted. This study therefore highlights spatial variations in circulation patterns consistent with the accumulation of observed pollutants and their heterogeneous distribution.

Derivatization techniques based on α-effect amines and H catalysis are commonly used for the measurement of carbonyl compounds (CCs), whether in environmental, food, or biological samples. Here, we investigated the potential of aniline-based catalysts to improve derivatization rates of selected carbonyls by using dansylacetamidooxyamine (DNSAOA) as a reagent. Kinetic experiments were performed in aqueous solutions by varying catalyst and CC concentrations and delivered insights into the reaction mechanism. Using anilinium acetate (AnAc), rate constants varied linearly with the catalyst concentration with rate enhancements toward H-catalyzed reactions as high as ca. 90 and 200 for acetone and benzaldehyde, respectively. Owing to contamination problems when using AnAc, anilinium chloride (AnCl) was chosen for the optimized analysis of real samples at low concentration. Rate enhancements for derivatization reaction of 4.4 (methylglyoxal), 6.0 (glyoxal), 12 (acetone), 20 (formaldehyde), and 47 (hydroxyacetaldehyde) were obtained using 0.1 M AnCl. The optimized method was successfully applied to the determination of the above compounds in natural snow and meltwater samples. Limits of detection (LODs) and limits of quantification (LOQs) were in the 2-14 and 7-41 nM range, respectively, i.e., low enough to allow for the analysis of most natural samples. Satisfactory relative recoveries (92.8 ± 3.8-118.3 ± 4.4%) and intra-day precision (2.7-11.3%) were achieved. Finally, we think that this approach could be applied not only to every α-effect nitrogen reagent-with the most evident profit of lowering derivatization times and particularly those required for low-reactive ketones-but also to the derivatization of CCs onto coated solid sorbents.