Dynamical Control on Air Pollution in the Arve River Valley during a Persistent Cold-Air-Pool Event

The Arve River Valley experiences severe air pollution episodes during wintertime, usually associated with persistent cold-air pools (PCAPs). A field campaign was held during two months of the winter of 2014-2015 in the Arve River Valley, around the city of Passy, as part of the Passy project (Paci et al. 2015, Staquet et al. 2015). Recent work (Chemel et al. 2016), showed that pollutant concentration can be highly variable at the hourly time-scale in this section of the valley. It was proposed that advection and mixing processes induced by the nighttime valley-wind system, may have an essential role in the sub-diurnal temporal and spatial variation of pollutant concentrations. Moreover, numerical simulations in idealized valleys performed by our group pointed out that the particular geometry of this section of valley can lead to a peculiar behaviour of the valley winds.

These elements motivate the present work, which aims to characterize the locally-induced dynamical processes in a PCAP, that in turn control the spatial and temporal variability of the pollutant concentrations within the valley.

The Weather Research and Forecasting (WRF) model is used for this purpose. Five nested domains are used, with a maximum horizontal resolution of hundred meters and the first mass point located 10m above the ground.

The numerical model results were evaluated with near-surface data (2-m air temperature, 10-m wind and surface fluxes) and vertical profiles of wind and temperature collected during the field campaign. The comparison highlighted the key role of the snow representation (cover, height, density and albedo) in the simulation of the cold-air pool near-surface structure, as already indicated by previous works (Tomasi et al. 2014). Hence, satellite products were used to initialize the snow cover and albedo, while the snow height was initialized on the basis of in situ estimates and the snow density on estimates of the snow age. These modifications led to a better agreement between the model results and the measurements.

The impact of the tributaries and the constriction at the valley end, on the evolution of the PCAP in its persistent stage, was clarified by the analysis of the simulated circulation within the valley. The latter is affected by spatial inhomogeneities of the thermal structure of the PCAP. These inhomogeneities are driven by the competing heating processes taking place within the valley. A heat budget analysis was performed at different locations along the valley axis in order to study the spatial variability of the cooling processes. Finally, the features observed will be compared with those in other valleys.

References:

Chemel, C., Arduini, G., Staquet, C., Largeron, Y., Legain, D., Tzanos, D., et al. (2016), Valley heat deficit as a bulk measure of wintertime particulate air pollution in the Arve River Valley, Atmos. Environ. 128:208-215.

Paci, A., Staquet, C., and 43 co-authors (2015), The Passy-2015 field experiment: an overview of the campaign and preliminary results, Proc. of the 33rd International Conference on Alpine Meteorology, Innsbruck, Austria.

Staquet, C., Paci, A., and 24 co-authors (2015), The Passy project: objectives, underlying scientific questions and preliminary numerical modeling of the Passy Alpine valley, Proc. of the 33rd International Conference on Alpine Meteorology, Innsbruck, Austria.

Tomasi, E., Giovannini, L., Zardi, D. and De Franceschi, M. (2014), High-resolution numerical simulations of wintertime atmospheric boundary layer processes in the Adige Valley during an ALPNAP project field campaign, Proc. of the 21st Symposium on Boundary Layers and Turbulence - American Meteorology Society, Leeds, United Kingdom.