40 CFR Part 50, Appendix P to Part 50 -

Interpretation of the Primary and 

Secondary National Ambient Air Quality 

Standards for Ozone

Appendix P to Part 50—Interpretation of the Primary and Secondary National Ambient Air 

Quality Standards for Ozone

1. General

(a) This appendix explains the data handling conventions and computations necessary for 

determining whether the national 8-hour primary and secondary ambient air quality standards for 

ozone (O3) specified in § 50.15 are met at an ambient O3 air quality monitoring site. Ozone is 

measured in the ambient air by a reference method based on appendix D of this part, as applicable, 

and designated in accordance with part 53 of this chapter, or by an equivalent method designated in 

accordance with part 53 of this chapter. Data reporting, data handling, and computation procedures 

to be used in making comparisons between reported O3 concentrations and the levels of the O3

standards are specified in the following sections. Whether to exclude, retain, or make adjustments to 

the data affected by exceptional events, including stratospheric O3 intrusion and other natural 

events, is determined by the requirements under §§ 50.1, 50.14 and 51.930.

(b) The terms used in this appendix are defined as follows:

8-hour average is the rolling average of eight hourly O3 concentrations as explained in section 2 of 

this appendix.

Annual fourth-highest daily maximum refers to the fourth highest value measured at a monitoring site 

during a particular year.

Daily maximum 8-hour average concentration refers to the maximum calculated 8-hour average for a 

particular day as explained in section 2 of this appendix.

Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to determine 

compliance, calculated as shown in section 3 of this appendix.

O3 monitoring season refers to the span of time within a calendar year when individual States are 

required to measure ambient O3 concentrations as listed in part 58 appendix D to this chapter.

Year refers to calendar year.

2. Primary and Secondary Ambient Air Quality Standards for Ozone

2.1Data Reporting and Handling Conventions

Computing 8-hour averages. Hourly average concentrations shall be reported in parts per million 

(ppm) to the third decimal place, with additional digits to the right of the third decimal place 

truncated. Running 8-hour averages shall be computed from the hourly O3 concentration data for 

each hour of the year and shall be stored in the first, or start, hour of the 8-hour period. An 8-hour 
average shall be considered valid if at least 75% of the hourly averages for the 8-hour period are 

available. In the event that only 6 or 7 hourly averages are available, the 8-hour average shall be 

computed on the basis of the hours available using 6 or 7 as the divisor. 8-hour periods with three or 

more missing hours shall be considered valid also, if, after substituting one-half the minimum 

detectable limit for the missing hourly concentrations, the 8-hour average concentration is greater 

than the level of the standard. The computed 8-hour average O3 concentrations shall be reported to 

three decimal places (the digits to the right of the third decimal place are truncated, consistent with 

the data handling procedures for the reported data).

Daily maximum 8-hour average concentrations. (a) There are 24 possible running 8-hour average O3

concentrations for each calendar day during the O3 monitoring season. The daily maximum 8-hour 

concentration for a given calendar day is the highest of the 24 possible 8-hour average 

concentrations computed for that day. This process is repeated, yielding a daily maximum 8-hour 

average O3 concentration for each calendar day with ambient O3 monitoring data. Because the 8-

hour averages are recorded in the start hour, the daily maximum 8-hour concentrations from two 

consecutive days may have some hourly concentrations in common. Generally, overlapping daily 

maximum 8-hour averages are not likely, except in those non-urban monitoring locations with less 

pronounced diurnal variation in hourly concentrations.

(b) An O3 monitoring day shall be counted as a valid day if valid 8-hour averages are available for at 

least 75% of possible hours in the day (i.e., at least 18 of the 24 averages). In the event that less 

than 75% of the 8-hour averages are available, a day shall also be counted as a valid day if the daily 

maximum 8-hour average concentration for that day is greater than the level of the standard.

2.2Primary and Secondary Standard-related Summary Statistic

The standard-related summary statistic is the annual fourth-highest daily maximum 8-hour O3

concentration, expressed in parts per million, averaged over three years. The 3-year average shall 

be computed using the three most recent, consecutive calendar years of monitoring data meeting 

the data completeness requirements described in this appendix. The computed 3-year average of 

the annual fourth-highest daily maximum 8-hour average O3 concentrations shall be reported to 

three decimal places (the digits to the right of the third decimal place are truncated, consistent with 

the data handling procedures for the reported data).

2.3Comparisons with the Primary and Secondary Ozone Standards

(a) The primary and secondary O3 ambient air quality standards are met at an ambient air quality 

monitoring site when the 3-year average of the annual fourth-highest daily maximum 8-hour average 

O3 concentration is less than or equal to 0.075 ppm.

(b) This comparison shall be based on three consecutive, complete calendar years of air quality 

monitoring data. This requirement is met for the 3-year period at a monitoring site if daily maximum 

8-hour average concentrations are available for at least 90% of the days within the O3 monitoring 

season, on average, for the 3-year period, with a minimum data completeness requirement in any 

one year of at least 75% of the days within the O3 monitoring season. When computing whether the 

minimum data completeness requirements have been met, meteorological or ambient data may be 

sufficient to demonstrate that meteorological conditions on missing days were not conducive to 

concentrations above the level of the standard. Missing days assumed less then the level of the 

standard are counted for the purpose of meeting the data completeness requirement, subject to the 

approval of the appropriate Regional Administrator.

(c) Years with concentrations greater than the level of the standard shall be included even if they 

have less than complete data. Thus, in computing the 3-year average fourth maximum 

concentration, calendar years with less than 75% data completeness shall be included in the 
computation if the 3-year average fourth-highest 8-hour concentration is greater than the level of the 

standard.

(d) Comparisons with the primary and secondary O3 standards are demonstrated by examples 1 and 

2 in paragraphs (d)(1) and (d)(2) respectively as follows:

Example 1—Ambient Monitoring Site Attaining the Primary and Secondary O3 Standards

Year

Percent valid days 

(within the 

required 

monitoring 

season)

1st Highest 

daily max 8-

hour Conc. 

(ppm)

2nd Highest 

daily max 8-

hour Conc. 

(ppm)

3rd Highest 

daily max 8-

hour Conc. 

(ppm)

4th Highest 

daily max 8-

hour Conc. 

(ppm)

5th Highest 

daily max 8-

hour Conc. 

(ppm)

2004 100 0.092 0.090 0.085 0.079 0.078

2005 96 0.084 0.083 0.075 0.072 0.070

2006 98 0.080 0.079 0.077 0.076 0.060

Average98 0.075

(1) As shown in Example 1, this monitoring site meets the primary and secondary O3 standards 

because the 3-year average of the annual fourth-highest daily maximum 8-hour average O3

concentrations (i.e., 0.075666 * * * ppm, truncated to 0.075 ppm) is less than or equal to 0.075 ppm. 

The data completeness requirement is also met because the average percent of days within the 

required monitoring season with valid ambient monitoring data is greater than 90%, and no single 

year has less than 75% data completeness. In Example 1, the individual 8-hour averages used to 

determine the annual fourth maximum have also been truncated to the third decimal place.

Example 2—Ambient Monitoring Site Failing to Meet the Primary and Secondary O3 

Standards

Year

Percent valid days 

(within the 

required 

monitoring 

season)

1st Highest 

daily max 8-

hour Conc. 

(ppm)

2nd Highest 

daily max 8-

hour Conc. 

(ppm)

3rd Highest 

daily max 8-

hour Conc. 

(ppm)

4th Highest 

daily max 8-

hour Conc. 

(ppm)

5th Highest 

daily max 8-

hour Conc. 

(ppm)

2004 96 0.105 0.103 0.103 0.103 0.102

2005 74 0.104 0.103 0.092 0.091 0.088

2006 98 0.103 0.101 0.101 0.095 0.094

Average89 0.096

As shown in Example 2, the primary and secondary O3 standards are not met for this monitoring site 

because the 3-year average of the fourth-highest daily maximum 8-hour average O3 concentrations 

(i.e., 0.096333 * * * ppm, truncated to 0.096 ppm) is greater than 0.075 ppm, even though the data 

capture is less than 75% and the average data capture for the 3 years is less than 90% within the 

required monitoring season. In Example 2, the individual 8-hour averages used to determine the 

annual fourth maximum have also been truncated to the third decimal place.

3. Design Values for Primary and Secondary Ambient Air Quality Standards for Ozone

The air quality design value at a monitoring site is defined as that concentration that when reduced 

to the level of the standard ensures that the site meets the standard. For a concentration-based 

standard, the air quality design value is simply the standard-related test statistic. Thus, for the 

primary and secondary standards, the 3-year average annual fourth-highest daily maximum 8-hour 

average O3 concentration is also the air quality design value for the site.

[73 FR 16511, Mar. 27, 2008]