Appendix P to Part 50 - Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Ozone
40:2.0.1.1.1.0.1.20.17 : Appendix P
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
2.1 Data 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.
2.2 Primary 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).
2.3 Comparisons 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 |
Average |
98 |
|
|
|
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 |
Average |
89 |
|
|
|
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]