Annual and Multi-Year Variability in Ozone Expected Exceedances

Ozone data from 184 metropolitan statistical areas (MSAs) for the time period 1979-1989 were analyzed to: (1) put the large number of O₃ exceedances observed in 1988 into perspective, and (2) determine if increasing the averaging time used to calculate the O₃ NAAdS violation rate would significantly decrease the number of MSAs that experienced a change in their annual attainment status. The analyses indicate that 1988 had more exceedances in more MSAs than any other year, but 1979-1981 had more O₃ NAAQS violations. The analyses also indicate that perceived attainment status flip-flops are significantly reduced by increasing the averaging period from the current three-year period to a longer time period.