Drivers of Severe Air Pollution Events in a Deep Valley During Wintertime.

During wintertime, urbanized mountainous areas are frequently affected by severe particulate air pollution episodes due to the direct emission of particulate matter (PM) particles and precursor gases leading to secondary aerosols. In addition to the increase of emission of PM during winter in complex terrain, such as in the French Alpes (Jaffrezo et al. 2005), other dynamical factors may lead to the accumulation of pollution in such confined terrain. In particular, atmospheric stability, which may suppress turbulent mixing and block ventilation (e.g. Zoras et al. 2006). These stable boundary layers in valleys are often related to persistent cold-air pools (CAPs), which can last for days and lead to high pollutant concentrations exceeding air-quality standards (Silcox et al. 2012).

In the present work we focus our attention in a section of the Arve river Valley, located in the French Alps. In recent years the relatively small city of Passy, located in this valley, has regularly experienced air-quality issues during wintertime when the atmosphere is strongly stratified. Passy has become one of the most polluted cities in the region, recording pollution levels as high as Lyon, which is the third large city in France and has ten times more inhabitants than Passy. The aim of this work is to determine the contribution of the dynamical processes to the life cycle of air pollution episodes around Passy.

We use numerical simulations based on the Weather Research and Forecasting Model (WRF) in its version 3.5.1. PM is modelled as a passive scalar in the present work. Five nested domains have been used with a horizontal resolution as fine as hundred meters and the first mass point located 10 m above the ground. The emission inventory has been developed in close collaboration with the regional air-quality agency (Atmo Aura) and is representative of the year 2016. Results from the model were evaluated using data from the network of air-quality stations in the valley (operated by Atmo Aura), and data from the Passy field campaign (Paci et al. 2015, Staquet et al. 2015).

The contribution of air pollution advected from the tributaries to the air pollution within this section of the valley have been studied. Results suggest that it is very small compared to that from local sources, suggesting that in-situ emissions are primarily responsible for the high concentration of PM in the valley. The ventilation characteristics of the valley and their influence on the PM concentration have been investigated. The analysis indicates that due to the limited ventilation in some particular sites inside the valley, the hourly variability of PM concentration at these sites is driven by that of the emissions.

References:

Jaffrezo, J. L., Aymoz, G., Delaval, C., & Cozic, J. (2005). Seasonal variations of the water soluble organic carbon mass fraction of aerosol in two valleys of the French Alps. Atmospheric Chemistry and Physics, 5(10), 2809-2821.

Paci, A., Staquet, C., & Allard, J. (2015). al. The passy-2015 field experiment: an overview of the campaign and preliminary results. In Proc. of the 33rd International Conference on Alpine Meteorology, Innsbruck, Austria.

Silcox, G. D., Kelly, K. E., Crosman, E. T., Whiteman, C. D., & Allen, B. L. (2012). Wintertime PM2. 5 concentrations during persistent, multi-day cold-air pools in a mountain valley. Atmospheric environment, 46, 17-24.

Staquet, C., Paci, A., & Allard, J. (2015). The Passy project: objectives, underlying scientific questions and preliminary numerical modeling of the Passy Alpine valley. Proc. of the 33rd International on Alpine Meteorology, Innsbruck, Austria.

Zoras, S., Triantafyllou, A. G., & Deligiorgi, D. (2006). Atmospheric stability and PM10 concentrations at far distance from elevated point sources in complex terrain: Worst-case episode study. Journal of Environmental management, 80(4), 295-302.