Highly time-resolved measurements of PM2.5, its major constituents, particle size distributions (9 nm to 20 μ m), CO, NO/NO2, and O3, and meteorological parameters were made from February through November 2002, at the Baltimore Supersite at Ponca St. using commercial and prototype semi-continuous instruments. The average PM2.5 mass concentration during the study period was 16.9 μ g/m3 and a total of 29 PM2.5 pollution episodes, each in which 24-h averaged PM2.5 mass concentrations exceeded 30.0 μ g/m3 for one or more days, were observed. Herein, 6 of the worst episodes are discussed. During these events, PM2.5 excursions were often largely due to elevations in the concentration of one or two of the major species. In addition, numerous short-term excursions were observed and were generally attributable to local sources. Those in OC, EC, nitrate, CO, and NOx levels were often observed in the morning traffic hours, particularly before breakdown of nocturnal inversions. Moreover, fresh accumulation aerosols from local stationary combustion sources were observed on several occasions, as evidenced by elevations in elemental markers when winds were aligned with sources resulting in PM2.5 increments of ∼ 17 μ g/m3. Overall, the results described herein show that concentrations of PM2.5 and its major constituents vary enormously on time scales ranging from < 1 hr to several days, thus imposing a more highly complex pattern of pollutant exposure than can be captured by 24-hr integrated methods, alone. The data suggest that control of a limited number of local sources might achieve compliance with daily and annual PM2.5 standards.