Effects of orography on air quality In deep and narrow valleys

Mountainous terrain modifies the atmospheric boundary layer (ABL) structure and its evolution in comparison with flat homogeneous terrain and consequently affects the transport and mixing of pollutants. The assessment of the airflow is more challenging as detail of the terrain arises. It is inappropriate to apply concepts originally developed over flat terrain to such an environment.

In this study, the physical processes, which drive the evolution of the atmosphere structure (particularly with the ABL) in deep and narrow valleys, have been characterised using observations as well as high-resolution numerical simulations. Data sets from the POVA (POllution des Vallées Alpines) programme, devoted to air quality in the Chamonix and Maurienne valleys (French Alps), were analyzed. A large-eddy simulation code, the Advanced Regional Prediction System (ARPS), was adapted to reproduce intensive observation periods using a horizontal resolution down to 300 m. Model results were carefully compared with the measurement data from the field campaigns. Specific features as well as seasonal variability of flow over highly complex topography were thus characterized and quantified.

Flow fields simulated by the model have then been passed to an air quality model suitable to investigate photochemistry over such complex terrain. A detailed emission inventory was necessary for chemical-transport simulation using the same horizontal resolution as the ARPS model. The model used afterwards was found to provide a reasonable estimate of transport and mixing of pollutants over both valleys. The modelling system was also used to assess 2010 air quality within both valleys using weather type and emission scenarios.

In particular, in urban areas, terrain-induced exchange processes interfere with the urban-scale circulations, which are driven by the land-use heterogeneities. It is worth to note that little is known about the enhancement or suppression of the Urban Heat Island (UHI) within a valley. In stable conditions, the UHI is balanced by the cold air pool that is likely to form within the valley. This balancing effect needs to be quantified since it drives the mixing ability of the air mass within the valley.

Acknowledgements & collaborations:

J.-P. Chollet (Laboratory of Geophysical and Industrial Fluid Flows, Joseph Fourier University, Grenoble, France) and his former Ph.D. students

Selected references:

Chemel, C. and J.-P. Chollet, 2008. Seasonal variability of flow characteristics in two deep and narrow alpine valleys from large-eddy simulation. To be submitted pretty soon.

Brulfert, G., C. Chemel, E. Chaxel, J.-P. Chollet, B. Jouve, and H. Villard, 2006. Assessment of 2010 air quality in two Alpine valleys from modelling: Weather type and emission scenarios. Atm. Environ. 40, 7893-7907.

Chemel, C. and J.-P. Chollet, 2006. Observations of the daytime boundary layer in deep alpine valleys. Boundary-Layer Meteorol. 119, 239-262.

Brulfert, G., C. Chemel, E. Chaxel, and J.-P. Chollet, 2005. Modelling photochemistry in alpine valleys. Atmos. Chem. Phys. 5, 2341-2355.