Along a transect ranging from the Pacific to the summit of Cerro Tololo at 2200 m above sea level (a.s.l.) in Chile at 30 øS, coarse particle concentration was measured at three sites in a one-year study from December 2002 to 2003. The climate of this area is characterized by semi-arid conditions. Intensive thermally induced valley winds occurring particularly in summer, possibly transporting polluted air masses form the coastal area into the mountain region. This may question the background characteristics of Cerro Tololo as suitable site in Chile for a monitoring station of the Global Atmospheric Watch (GAW) program. Particle sampling by a passive sampling technique was employed to analyze composition of particles, find out effective sources and trace possible transport of suspended particles in this area. Temporal resolution of sampling was one week. Composition and size distribution of weekly deposition samples were determined by microscopic single particle analysis and bulk ionchromatographic analysis. Coarse particle concentration decreases differently along the transect as a function of occurring sources and depending on reactivity of components. The percentage of a black particle component remains at the same level of about 4% at all the three sites, confirming biomass burning as a wide-spread pollution. The percentage of sea salt, by contrast, amounts on average about 50% of total coarse particle concentration at the coastal area, quickly decreasing to about 10% along the transect due to losses by particle deposition and chemical reaction during transport. Ammonium shows a different behaviour, characterized by enhanced values at the central part of the transect, due to emissions from agricultural activities in this area. A shift in size distribution between the different sites is explained not only by gravitational fractionation but also by chemical conversion during transport. As a result, it is concluded that an impact of contaminated air at Cerro Tololo cannot be ruled out at a low level particularly in summer. However, the amount, composition and size distribution of the coarse fraction rather confirms the pristine area characteristics of Cerro Tololo and its background conditions