PROPHET 1998 meteorological overview and air-mass classification

The Program for Research on Oxidants: Photochemistry, Emissions, and Transp ort (PROPHET) summer 1998 measurements intensive was conducted from a tower above a forested site in northern Michigan. This paper provides a brief ov erview of the meteorological conditions and establishes that the study peri od was moderately drier and warmer than the climatological mean. The paper also identifies and chemically characterizes the major air mass types influ encing the site. Meteorological analyses and back trajectories establish th at air mass origin oscillated between relatively clean Canadian regions (ex cept for periods influenced by Canadian forest fires) and regions of greate r anthropogenic emissions in the contiguous United States. Higher mixing ra tios of ozone, CO, NOx and NOz were generally associated with southerly tra nsport, which occurred 24% of the time. Lower mixing ratios were observed u nder northerly transport, which occurred 44% of the time. The dominance of northerly transport was due to a stronger than normal Hudson Bay low. The r emaining 32% of the time was occupied by,transitional periods between disti nct air masses. A positive slope exists between ozone and CO when photochem ically aged air masses are selected. However, no meaningful relationship be tween ozone and CO was observed when northerly and southerly transport peri ods were considered separately. Comparison to other summertime rural locati ons suggests that rapid and frequent transitions between air masses of cont rasting source regions play an important role in maintaining the 0.3 O-3/CO slope commonly observed in eastern North America.