Environmental Services Program

National Ambient Air Quality Standards

In the past, measured ambient values were compared directly to the standard to determine if a violation of the standard had occurred. The new regulations require that statistical smoothing and averaging procedures be applied to the measured values before they are compared to the standards.


Ozone (O3)
Inhalable Particulates (PM10)
Fine Inhalable Particulates (PM2.5)


Ozone Standards
Averaging Time Primary Standard Secondary Standard
Eight-hour 0.075 ppm 0.075 ppm
  1. Method of Comparison for the Eight-Hour Standard.
    1. Each hour of the day marks the beginning of an eight-hour averaging period. The eight-hour average value for each time period is recorded at the beginning hour. For example, the average of the eight hourly ozone measurements for the time period of 0000 to 0800 is recorded in the 0000 hour time slot. Similarly, the average of the eight hourly ozone measurements for the time period 0100 to 0900 is recorded in the 0100 time slot.
    2. This procedure produces 24 eight-hour average values for each day. From these 24 eight-hour rolling averages the daily maximum average is noted. Even though some of the hours used to produce the averages may belong to the next calender day.
    3. Each daily maximum eight-hour average is compared to all other daily maximum eight-hour averages obtained for the ozone season of the calender year.
    4. The fourth highest daily maximum average for each site for the calender year is then noted.
    5. This value is averaged with two similarly obtained fourth highest daily maximum averages from the last two consecutive calender years in order to obtain a three-year average of the fourth highest daily maximum average.
    6. All measurements and averages calculated up to this point are reported in parts per million (ppm) to the third decimal place with the remaining places to the right being truncated.
    7. Next the three-year average is compared to the standard.
    8. At this point, for comparison to the standard, the three-year average is reported to three decimal places. The numbers to the right are truncated. 
    9. It follows from this that a minimum value for a violation of the standard would be a three-year average of the fourth high daily maximum average that equals .075 ppm.

Link to EPA:  http://www.epa.gov/ttn/naaqs/standards/ozone/s_o3_index.html

Inhalable Particulates (PM10) Standards
Averaging Time Primary Standard Secondary Standard
24-hour 150 ug/m3 150 ug/m3
  1. EPA has revoked the annual PM10 standards, because there is insufficient evidence linking health problems to long-term exposure to inhalable coarse particle pollution.
  2. Method of Comparison for the 24-hour Standard.
    1. An area will meet the 24-hour PM10 standard when the 150μg/m3 level is not exceeded
      more than once per year on average over a three year period. This is the same form as the current 24-hour standard.

Link to EPA: http://www.epa.gov/oar/particlepollution/pdfs/20060921_factsheet.pdf

Fine Inhalable Particulates (PM2.5) Standards
Averaging Time Primary Standard Secondary Standard
One-year 15.0 ug/m3 15.0 ug/m3
24-hour 35 ug/m3 35 ug/m3

The methods for comparing the new PM2.5 standards to ambient measurements is as follows:

    1. Method of Comparison for the Annual Standard.
      1. Average all twenty-four hour daily measurements to obtain a quarterly average and repeat for each quarter.
      2. Take the four quarterly averages and average them to obtain a yearly average.
      3. If a state has elected to designate more than one monitoring site to represent an area the yearly averages from all designated sites are to be averaged together.
      4. Average three consecutive yearly averages to to obtain a three-year average.
      5. To compare the three-year average to the standard, the average must be rounded to the nearest 0.1 microgam per cubic meter (ug/m3). Use the convention that if the number 5 is found in the hundredths place, the number rounds up.
      6. If the three-year mean is greater than 15.0 ug/m3, a violation of the standard has occurred.
    2. Method of Comparison for the 24-hour Standard.
      1. Determine the 98th percentile value by the following method:
        1. Arrange from highest value to lowest value all 24-hour measurments for a calendar year.
        2. Assign the number one to the lowest value, the number two to the second value and so on untill each of the values is assigned a number, with the largest number (n) being assigned to the highest value .
        3. Multiply the largest number(n) times 0.98.
        4. The whole number part of the above product, plus one, is the number of the 98th percentile value.
        5. Using the value number obtained in step "D", determine the 98th percentile value by matching the value number to the list in step "B".
      2. Average the 98th percentile value with the 98th percentile values of two other consecutive years.
      3. Round the above three-year average to the nearest one microgram per cubic meter (ug/m)3. Use the convention that if the number 5 is found in the tenths place, the number rounds up.
      4. If the rounded three-year average of the 98th percentile values is greater than 35 ug/m3, a violation of the standard has occurred.

 Link to EPA: http://www.epa.gov/oar/particlepollution/pdfs/20060921_factsheet.pdf

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