The results of the first long-period campaign on particulate matter performed in the city of Buenos Aires are reported. Twenty four hour averages of atmospheric concentrations of PM10, PM2.5 and total suspended particles (TSP) were measured at a site near downtown Buenos Aires from December 1998 to September 1999. The values of PM2.5 concentrations correlate well to the concentrations of carbon monoxide (CO) during the winter period, indicating that direct traffic emissions have an important contribution to PM2.5. The data are less correlated for the case of PM10, indicating that the coarse fraction has a different source, probably resuspended dust. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) images of atmospheric particles on nucleopore filters are shown. Information about the ion and metal content of the particles is reported.