From 1997 to 2003, airborne measurements of O3, CO, SO2, and aerosol properties were made during summertime air pollution episodes over the mid-Atlantic United States (34.7–44.6°N, 68.4–81.6°W) as part of the Regional Atmospheric Measurement, Modeling, and Prediction Program (RAMMPP). Little diurnal variation was identified in the CO, SO2, and Ångström exponent profiles, although the Ångström exponent profiles decreased with altitude. Boundary layer O3 was greater in the afternoon, while lower free tropospheric O3 was invariant at ∼55 ppbv. The single scattering albedo increased from morning to afternoon (0.93 ± 0.01–0.94 ± 0.01); however, both profiles decreased with altitude. A cluster analysis of back trajectories in conjunction with the vertical profile data was used to identify source regions and characteristic transport patterns during summertime pollution episodes. When the greatest trajectory density lay over the northern Ohio River Valley, the result was large O3 values, large SO2/CO ratios, highly scattering particles, and large aerosol optical depths. Maximum trajectory density over the southern Ohio River Valley resulted in little pollution. The greatest afternoon O3 values occurred during periods of stagnation. North-northwesterly and northerly flow brought the least pollution overall. The contribution of regional transport to afternoon boundary layer O3 was quantified. When the greatest cluster trajectory density lay over the Ohio River Valley (∼59% of the profiles), transport accounted for 69–82% of the afternoon boundary layer O3. Under stagnant conditions (∼27% of the profiles), transport only accounted for 58% of the afternoon boundary layer O3. The results from this study provide a description of regional chemical and transport processes that will be valuable to investigators from the Baltimore, New York, and Pittsburgh EPA Supersites.