Meeting NAAQS for O3 and PM2.5 and Reducing Regional Haze

In 1997, the U.S. Environmental Protection Agency (EPA) promulgated new National Ambient Air Quality Standards (NAAQS) for two criteria pollutants: O3 and PM. These standards were developed because of new scientific data that indicated deleterious health effects from exposure to concentrations of the two pollutants that were below the current NAAQS. Meeting the new standards will require additional reductions in pollutant emissions. Moreover, because O3 and PM are secondary pollutants (produced in the atmosphere from reactions involving primary pollutants), it will be necessary to determine which pollutant emissions to reduce and to devise appropriate monitoring systems to assess progress toward meeting the new standards. All of these tasks will be major challenges for the AQM system in the United States over the next decade.

The standard for O3 has been changed from a maximum 1-hr peak concentration of 120 parts per billion by volume (ppbv) to an 8-hr average concentration of 80 ppbv.1 It has proved to be extremely difficult to attain the previous (1-hr) O3 NAAQS. In the United States, approximately 56 areas composed of 233 counties have yet to attain it after decades of trying to do so (EPA 2003p). It will probably be even more difficult to meet the new O3 8-hr standard (NARSTO 2000). Retrospective analysis of air quality data indicates that there will be many more exceedances of the new 8-hr standard than the previous 1-hr standard. More frequent exceedances will occur in areas already in nonattainment of the 1-hr standard, and new exceedances will occur in areas currently in attainment of the 1-hr standard. Many of these new nonattainment areas will be in rural areas that do not have the major sources of the various air pollutants that produce O3. The decline of the 8-hr averaged O3 concentrations (11%) in the United States has been slower than that of the 1-hr averaged O3 concentrations (18%) over the past two decades (EPA 2002a). Therefore, additional pollution control strategies are likely to be needed to meet the new O3 NAAQS—for example, a further departure from the local emission-control approach demanded in the current state implementation plan (SIP) process and the enhanced development of multistate airshed2 management approaches, such as those embodied by the Ozone Transport Assessment Group (OTAG) and the resulting requirement to submit a revised NOx SIP.


EPA is phasing out the 1-hr 0.12-ppm standards (primary and secondary) and putting in place the 8-hr 0.08-ppm standards. However, the 0.12-ppm standards will not be revoked in a given area until that area has achieved 3 consecutive years of air quality data meeting the 1-hr standard (EPA 2001a).


The geographic extent of a pollutant or its precursor emissions in air is often referred to as an airshed.

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