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Shortly after 12 noon on August 15, 1995, the second highest 1-hour averaged O3 mixing ratio on record in Atlanta was observed at a monitoring site located to the west and generally upwind of the downtown core. Analysis of relevant data suggest that the peak mixing ratio at the site was caused by a highly localized and transient event: the intersection of a plume emanating from a nearby power plant with the monitoring site. In addition to representing an exceedence of the 1-hour-120-ppbv U.S. National Ambient Air Quality Standard (NAAQS), the O3 mixing ratios observed at the site would have also been in excess of the new 8-hour-80-ppbv NAAQS. Without more extensive observational data it is not possible to assess spatial and temporal frequency of this type of event, although SO2 data from downtown Atlanta, and modeling studies suggest that there may be significant influence by these plumes on the measurement of O3 at Atlanta monitoring sites.