Daily PM2.5 and PM10 measurements at sites in and around Tucson, Arizona, are examined for the period 1990-1995. The data are spatially averaged to produce values that are deemed representative of the PM conditions for the given days. The data are examined using the principal component analysis and clustering. Antecedent and contemporaneous surface and upper-air observations are then investigated for the top twenty extreme events to determine the atmospheric conditions that result in higher PM concentrations. It is determined that SSC2 type 3 classification (i.e., dry tropical) is most responsible for high PM10 values in Tucson. Further analysis is being performed to determine the SSC2 classification type(s) that lead to high concentrations of PM2.5.