Pic du Midi (2877 m asl, 42.93642N, 0.14260E) is a high-altitude observatory for astronomy and atmospheric sciences (http://www.obs-mip.fr/pic-du-midi) on top of an isolated summit in the Pyrenees. The summit is situated well to the north of the Pyrenean main divide and thus forms a promontory over the French plain. This site is 150 km to the east of the Atlantic coast and thus most often exposed to oceanic westerlies or north-westerlies. However, advection of continental air from the Iberian Peninsula flowing over the Pyrenees is not rare.
In the context of the GEOmon programme (www.geomon.eu), a study was conducted to characterize and compare the areas of influence ("footprints") of various atmospheric background measurement sites in Europe (Henne et al., Atmospheric Chemistry and Physics, 2010). Pic du Midi (PDM) was categorized as ``mostly remote'' (as Sonnlick and Jungfraujoch in the Alps). Pic du Midi is located away from major urban or industrial areas. The population in radii of 10 and 50 km amount to about 13,000 and 412,000 inhabitants, respectively (see http://geomon.empa.ch/index.php?id=PDM), concentrated in two main cities: Pau (217,000 inhabitants) and Tarbes (110,000 inhabitants, Fig. 2), situated 55 km and 30 km away to the northwest, respectively. Outside of these two main cities, countryside and mountain areas are sparsely inhabited, with exception of the small town of Bagnères-de-Bigorre (9000 inhabitants) located 14 km north of PDM.
Beyond ICOS measurements (carbon dioxide, methane, carbon monoxide, water vapor), permanent monitoring of other gaseous species (ozone, carbon monoxide, mercury), aerosol particles and meteorology is also being performed at PDM (http://www.aero.obs-mip.fr/observation/p2oa).
