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Calculating the 8-Hour Ozone Standard

How is the 8-hour ozone concentration-based standard calculated? Unlike the form of the previous 0.075 ppm 8-hour average ozone standard that was established in 2008, the daily maximum 8-hour average concentration for a given day is derived from the highest of the 17 consecutive 8-hour averages beginning with the 8-hour period from 7:00 a.m. to 3:00 p.m. and ending with the 8-hour period from 11:00 p.m. to 7:00 a.m. the following day (i.e., the continuous 8-h averages running from 7:00 a.m. to 11:00 p.m.). Once this calculation is made, identify the concentration at the 4th highest 8-h daily maximum during the year. Second, average the 4th highest value for each of the last 3 years. The 8-hour standard is based on the concept that if the 3-year average of the 4th highest 8-hour daily maximum is greater than or equal to 0.071 ppm, the area will be in violation of the current standard. For the revised 8-hour ozone standard, attainment occurs when the 3-year average of the 4th highest 8-hour daily maximum concentration is equal to or less than 0.070 ppm.

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