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National Ambient Air Quality Standards

The Federal Clean Air Act

The U.S. Environmental Protection Agency's (EPA) Clean Air Act establishes national ambient air quality standards (NAAQS) for six "criteria" air pollutants:

  1. Carbon Monoxide (CO)
  2. Lead (Pb)
  3. Nitrogen Oxides (NOx)
  4. Ozone (O3)
  5. Particulate Matter (PM)
  6. Sulfur Dioxide (SO2)

Units of measure for the standards are parts per billion (ppb) by volume, milligrams per cubic meter of air (mg/m3), and micrograms per cubic meter of air (µg/m3). These standards are designed to protect public health and the environment.  All counties in the United States are classified by the United States EPA based on whether or not they meet the NAAQS for a particular pollutant. Counties where pollutant concentrations regularly exceed the NAAQS are in violation of the standards and are classified as "nonattainment." States are required to develop and implement plans that will bring nonattainment areas into compliance with the NAAQS by a specified date. The Clean Air Act also contains additional specific requirements for emission reduction and planning activities related to emission sources (point sources, on-road mobile sources, non-road mobile sources, and non-point sources) and for nonattainment areas that fall into certain categories.


Ground-level ozone is the criteria pollutant of concern in the Austin-Round Rock (A-RR) region.  On March 12, 2008 the EPA lowered the NAAQS for ground-level ozone, from the 1997 standard of 84 parts per billion (ppb), to 75 ppb. The 8-hour ozone standard is based on averaging ozone concentrations over a rolling 8-hour block of time.  Attainment of this more stringent ozone standard is determined by the fourth highest eight-hour daily maximum at any regulatory monitor in an area, averaged over a three-year period; this average is called the design value. 

The 1997 standard-and the implementation rules for that standard-will remain in place for implementation purposes as EPA undertakes rulemaking to address the transition from the 1997 ozone standard to the 2008 ozone standard. The TCEQ must make recommendations to EPA no later than March 2009 based on either 2006-2008 or 2007-2009 monitoring data for areas to be designated attainment, nonattainment, or unclassifiable for the new ozone standard. EPA will issue final designations of attainment, nonattainment and unclassifiable areas no later than March 2010 unless there is insufficient information to make these designation decisions. In that case, EPA will issue designations no later than March 2011.

Ozone Standards and Guidelines
Jurisdiction 1 Hour 8 Hours Annual/Seasonal
US EPA NAAQS 124 ppb* 75 ppb none
US EPA (proposed NAAQS) 124 ppb* 60-70 ppb 7-15 ppm-hours
Canadian Standards 82 ppb 65 ppb 15 ppb
European Union Standards none 60 ppb none
World Health Organization Guidelines none 50 ppb none
Austin Area Design Value 87 ppb 74 ppb 29 ppb average, 9 ppm-hours
*EPA revoked the one-hour standard in 2004, although some regulatory requirements still remain

The following table shows how many days an ozone monitor in the Austin area exceeded 8-hour concentrations of 60, 65, 70, 75, and 84 ppb since 2008:

Number of Days of Ozone Exceedances in Austin Area
8-Hour Average Ozone Concentration 2011 (as of April 15) 2010 2009 2008
84 ppb (1997 ozone standard) 0 0 0 0
75 ppb (2008 ozone standard) 0 4 5 2
70 ppb (upper end of proposed standard) 1 12 7 17
65 ppb (midpoint of proposed standard) 3 19 20 26
60 ppb (lower end of proposed standard) 6 32 39 47

[Sources: EPA - 1997 Standard Fact Sheet, 2008 Standard Fact Sheet(pdf)]

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