The Federal Clean Air Act in the US sets standards for six pollutants. These pollutants are called "criteria pollutants," and ozone is one of these. The Clean Air Act sets National Ambient Air Quality Standards ("NAAQS;" concentrations that are not to be exceeded in outdoor air) for these six pollutants. The criteria pollutants in the US at present are SO2 (sulfur dioxide); NO2; Pb (lead); CO (carbon monoxide); O3, and fine particulates ("PM-10" and "PM-25" -- the numbers refer to microns of diameter for the particles).
Criteria pollutants that are regulated in this fashion were chosen for regulation by the EPA because they are:
That is, they aren't pollutants that are of concern only in small areas or that result from unique or unusual sources.
The NAAQS for ozone until 1997 was 0.12 ppm for a maximum hourly concentration, not to be exceeded more than one day per year averaged over three years. (Re-stated, the expected number of days with a maximum hourly concentration > 0.12 ppm was not to exceed one per year averaged over 3 yrs.)
As we'll see, even this standard was exceeded in many places in the US, and hourly highs can be much greater than 0.12 ppm -- for example, in the LA basin, concentrations can exceed 0.5 ppm!!
In 1996, EPA proposed a new NAAQS for ozone and also proposed regulating PM-2.5 as well as PM-10. It These standards are now approved, even though challenges in court delayed implementation of the new ozone standard; we'll look more at the new standard later.)
Click tropospheric ozone to return to the index of topics discussed relative to this pollutant. To return to the master Table of Contents for these BI 301 pages, click "contents." For reminders on how to move about within and among these pages, click "navigate."
Page maintained by Patricia Muir at Oregon State University. Last updated November 19, 2006