During the summer 2003 a POVA intensive observation period (IOP) aimed at determining the sources of airborne pollutants and monitoring their concentrations in two French Alpine valleys: the Chamonix and the Maurienne valleys (see figure 1 for geographic location). The Pollution of Alpine Valleys (POVA) program was launched in 2000 after the traffic interruption under the Mont-Blanc that followed the tragic accident in the tunnel. The Mont-Blanc tunnel were reopened at the end of 2002 what caused the high duty vehicle traffic to be back in the Chamonix and the Courmayeur (Italy) valleys. The summer 2003 IOP was the last of a series of four IOPs (two in wintertime and two in summertime) and took place from 5 to 12 July in the Chamonix valley.