The Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive was conducted from a tower above a forested site in northern Michigan. This paper provides a brief overview of the meteorological conditions and establishes that the study period was moderately drier and warmer than the climatological mean. The paper also identifies and chemically characterizes the major air mass types influencing the site. Meteorological analyses and back trajectories establish that air mass origin oscillated between relatively clean Canadian regions (except for periods influenced by Canadian forest fires) and regions of greater anthropogenic emissions in the contiguous United States. Higher mixing ratios of ozone, CO, NOx, and NOz were generally associated with southerly transport, which occurred 24% of the time. Lower mixing ratios were observed under northerly transport, which occurred 44% of the time. The dominance of northerly transport was due to a stronger than normal Hudson Bay low. The remaining 32% of the time was occupied by transitional periods between distinct air masses. A positive slope exists between ozone and CO when photochemically aged air masses are selected. However, no meaningful relationship between ozone and CO was observed when northerly and southerly transport periods were considered separately. Comparison to other summertime rural locations suggests that rapid and frequent transitions between air masses of contrasting source regions play an important role in maintaining the 0.3 O3/CO slope commonly observed in eastern North America. ¿ 2001 American Geophysical Union