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Technology Transfer Network / NAAQS
Ozone Implementation

Ozone and Carbon Monoxide 1996-98 Air Quality Data Update

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

The following is a brief summary of EPA's 1998 air quality update for ozone and carbon monoxide nonattainment areas.

Ozone (O3): Today's list updates ozone air quality monitoring data for the three year period, 1996-98. During this current three year period,

  • 36 of the original 98 areas designated nonattainment for the 1-hour O3 National Ambient Air Quality Standard (NAAQS) in 1991 failed to meet the NAAQS in 1996-98 (Table 1).


  • 11 additional counties failed to meet the 1-hour O3 NAAQS in 1996-98 (Table 2).


  • 300 counties have average annual 4th maximum 8-hour daily maximum O3 concentrations greater than the level of the 8-hour O3 NAAQS (Table 3).


EPA set the 1-hour O3 standard at 0.12 parts per million (ppm) daily maximum 1-hour average concentration not to be exceeded more than once per year on average. Compliance with the 1-hour ozone standard is judged on the basis of the most recent three years of ambient air quality monitoring data. The 1-hour ozone standard is not met at a monitoring site if the average number of estimated exceedances of the ozone standard is greater than 1.0 (1.05 rounds up). The level of the 8-hour O3 NAAQS is 0.08 ppm. The 8-hour O3 standard is not met if the 3-year average of the annual 4th highest daily maximum 8-hour O3 concentration is greater than 0.08 ppm (0.085 rounds up).

Carbon Monoxide (CO): Today's list updates CO air quality data for the two year period, 1997-98. During this two year period,

  • 3 of the original 42 areas designated nonattainment for the 8-hour CO NAAQS in 1991 failed to meet the CO NAAQS in 1997-98 (Table 4).


  • 3 additional areas failed to meet the CO NAAQS in 1997-98 (Table 4).


EPA's National Ambient Air Quality Standard for carbon monoxide is 9 parts per million (ppm) nonoverlapping 8-hour average concentration not to be exceeded more than once per year. The CO standard is not met at a monitoring site if there are two or more exceedances of the level of the CO NAAQS in either of the two most recent calendar years of monitoring data.

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