Relationships between direction of wind flow and ozone inflow concentrations at rural locations outside of St. Louis,MO

The relationships between the monthly and seasonal averages of the daily 1200–1500 h O₃ inflow concentrations and wind flow direction have been evaluated. The O₃ measurements used are those during inflow of air parcels from upwind to rural monitoring stations outside of St. Louis, MO. The results obtained are consistent with the O₃ measurements reported from other monitoring studies at rural locations both to the west and east of St. Louis. Although there is a stratospheric component to these ground level rural O₃ concentrations, it is likely that most of the O₃ measured during the warmer months of the year is associated with photochemical O₃ formation in the planetary boundary layer and in the free troposphere. A substantial part of the increments in rural O₃ concentrations which occur from west to east of St. Louis during the warmer months of the year appear best to be accounted for as a result of photochemical formation O₃ precursors from anthropogenic emission sources to the east of St. Louis. Differences in the values of meteorological parameters with wind flow direction account for only a small part of the differences in O₃ concentration observed.