Abstract : The framework of this PhD work is the development of the PAES network (Atmospheric Pollution at Synoptic Scale), devoted to ozone and CO measurements at high-altitude sites in France. The spatial and temporal representativity of high surface station has been evaluated, in comparisons with airborne (MOZAIC) and satellite (MOPITT) data. Monitoring the atmospheric changes turns out to be possible from surface data. High altitude stations also appear to be to some extent representative of the variability of the free troposphere despite the influence of the surface. The air quality over Western Europe and its evolution since the 1980s have been portrayed through CO and ozone data from PAES and other French and European networks. Finally, associating observations during the PIC 2005 field campaign (June 13th - July 7th 2005 around the Pic du Midi) and modeling (Méso-NH) allows the understanding of the reversed diurnal cycle observed in climatologies from some high altitude stations. Specific study about the June 23th 2005 underlines the role of the transport at synoptic scale and of the boundary layer - free troposphere exchanges on the variability of air composition at high-altitude sites.