Spatial variations of PM_2.5 during the Pittsburgh air quality study

Measurements of 24 h PM_2.5 total mass, sulfate, ammonium, and organic carbon show similar concentrations within experimental error at six sampling locations separated by more than 300 km. The measurements were obtained during summer 2001 in the center of Pittsburgh as well as in less-populated areas upwind and down-wind of the city. Measurable differences among the six sites were observed for nitrate and elemental carbon during the same time period. In contrast, measurable differences were observed for total mass and all five chemical species at the same sites during winter 2002. The results suggest that concentrations may be remarkably uniform over large areas due to secondary aerosol production from gases emitted in upwind areas. Meteorological back-trajectories show that concentrations can steadily increase along an airmass trajectory, and that regions downwind of a city such as Pittsburgh may be affected by city emissions; however, PM_2.5 levels measured within the city may not be significantly affected by local emissions if background levels are sufficiently high.