Appendix U to Part 50 - Interpretation of the Primary and Secondary National Ambient Air Quality Standards for Ozone
40:2.0.1.1.1.0.1.20.22 : Appendix U
Appendix U 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 primary and
secondary national ambient air quality standards (NAAQS) for ozone
(O3) specified in § 50.19 are met at an ambient O3 air quality
monitoring site. Data reporting, data handling, and computation
procedures to be used in making comparisons between reported O3
concentrations and the levels of the O3 NAAQS are specified in the
following sections.
(b) Whether to exclude or retain the data affected by
exceptional events is determined by the requirements under §§ 50.1,
50.14 and 51.930.
(c) The terms used in this appendix are defined as follows:
8-hour average refers to the moving average of eight
consecutive hourly O3 concentrations measured at a site, as
explained in section 3 of this appendix.
Annual fourth-highest daily maximum refers to the fourth
highest value measured at a site during a year.
Collocated monitors refers to the instance of two or more
O3 monitors operating at the same physical location.
Daily maximum 8-hour average O3 concentration
refers to the maximum calculated 8-hour average value measured at a
site on a particular day, as explained in section 3 of this
appendix.
Design value refers to the metric (i.e.,
statistic) that is used to compare ambient O3 concentration data
measured at a site to the NAAQS in order to determine compliance,
as explained in section 4 of this appendix.
Minimum data completeness requirements refer to the
amount of data that a site is required to collect in order to make
a valid determination that the site is meeting the NAAQS.
Monitor refers to a physical instrument used to measure
ambient O3 concentrations.
O3 monitoring season refers to the span of time within a
year when individual states are required to measure ambient O3
concentrations, as listed in Appendix D to part 58 of this
chapter.
Site refers to an ambient O3 air quality monitoring
site.
Site data record refers to the set of hourly O3
concentration data collected at a site for use in comparisons with
the NAAQS.
Year refers to calendar year.
2. Selection of Data for use in Comparisons With the Primary and
Secondary Ozone NAAQS
(a) All valid hourly O3 concentration data collected using a
federal reference method specified in Appendix D to this part, or
an equivalent method designated in accordance with part 53 of this
chapter, meeting all applicable requirements in part 58 of this
chapter, and submitted to EPA's Air Quality System (AQS) database
or otherwise available to EPA, shall be used in design value
calculations.
(b) All design value calculations shall be implemented on a
site-level basis. If data are reported to EPA from collocated
monitors, those data shall be combined into a single site data
record as follows:
(i) The monitoring agency shall designate one monitor as the
primary monitor for the site.
(ii) Hourly O3 concentration data from a secondary monitor shall
be substituted into the site data record whenever a valid hourly O3
concentration is not obtained from the primary monitor. In the
event that hourly O3 concentration data are available for more than
one secondary monitor, the hourly concentration values from the
secondary monitors shall be averaged and substituted into the site
data record.
(c) In certain circumstances, including but not limited to site
closures or relocations, data from two nearby sites may be combined
into a single site data record for the purpose of calculating a
valid design value. The appropriate Regional Administrator may
approve such combinations after taking into consideration factors
such as distance between sites, spatial and temporal patterns in
air quality, local emissions and meteorology, jurisdictional
boundaries, and terrain features.
3. Data Reporting and Data Handling Conventions
(a) Hourly average O3 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. Each hour
shall be identified using local standard time (LST).
(b) Moving 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 6 of the hourly
concentrations for the 8-hour period are available. In the event
that only 6 or 7 hourly concentrations are available, the 8-hour
average shall be computed on the basis of the hours available,
using 6 or 7, respectively, as the divisor. In addition, in the
event that 5 or fewer hourly concentrations are available, the
8-hour average shall be considered valid if, after substituting
zero for the missing hourly concentrations, the resulting 8-hour
average is greater than the level of the NAAQS, or equivalently, if
the sum of the available hourly concentrations is greater than
0.567 ppm. The 8-hour averages shall be reported to three decimal
places, with additional digits to the right of the third decimal
place truncated. Hourly O3 concentrations that have been approved
under § 50.14 as having been affected by exceptional events shall
be counted as missing or unavailable in the calculation of 8-hour
averages.
(c) The daily maximum 8-hour average O3 concentration for a
given day is 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 8-hour averages for 7:00 a.m. to
11:00 p.m.). Daily maximum 8-hour average O3 concentrations shall
be determined for each day with ambient O3 monitoring data,
including days outside the O3 monitoring season if those data are
available.
(d) A daily maximum 8-hour average O3 concentration shall be
considered valid if valid 8-hour averages are available for at
least 13 of the 17 consecutive 8-hour periods starting from 7:00
a.m. to 11:00 p.m. In addition, in the event that fewer than 13
valid 8-hour averages are available, a daily maximum 8-hour average
O3 concentration shall also be considered valid if it is greater
than the level of the NAAQS. Hourly O3 concentrations that have
been approved under § 50.14 as having been affected by exceptional
events shall be included when determining whether these criteria
have been met.
(e) The primary and secondary O3 design value statistic is the
annual fourth-highest daily maximum 8-hour O3 concentration,
averaged over three years, expressed in ppm. The fourth-highest
daily maximum 8-hour O3 concentration for each year shall be
determined based only on days meeting the validity criteria in
3(d). The 3-year average shall be computed using the three most
recent, consecutive years of ambient O3 monitoring data. Design
values shall be reported in ppm to three decimal places, with
additional digits to the right of the third decimal place
truncated.
4. Comparisons With the Primary and Secondary Ozone NAAQS
(a) The primary and secondary national ambient air quality
standards for O3 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 (i.e., the design value) is
less than or equal to 0.070 ppm.
(b) A design value greater than the level of the NAAQS is always
considered to be valid. A design value less than or equal to the
level of the NAAQS must meet minimum data completeness requirements
in order to be considered valid. These requirements are met for a
3-year period at a site if valid daily maximum 8-hour average O3
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 of at least 75% of the days within the O3 monitoring season
in any one year.
(c) 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
NAAQS. Missing days assumed less than the level of the NAAQS are
counted for the purpose of meeting the minimum data completeness
requirements, subject to the approval of the appropriate Regional
Administrator.
(d) Comparisons with the primary and secondary O3 NAAQS are
demonstrated by examples 1 and 2 as follows:
Example 1 - Site Meeting the Primary and
Secondary O3 NAAQS
Year |
Percent valid days within O3
monitoring season (Data
completeness) |
1st highest daily max
8-hour O3
(ppm) |
2nd highest daily max
8-hour O3
(ppm) |
3rd highest daily max
8-hour O3
(ppm) |
4th highest daily max
8-hour O3
(ppm) |
5th highest daily max
8-hour O3
(ppm) |
2014 |
100 |
0.082 |
0.080 |
0.075 |
0.069 |
0.068 |
2015 |
96 |
0.074 |
0.073 |
0.065 |
0.062 |
0.060 |
2016 |
98 |
0.070 |
0.069 |
0.067 |
0.066 |
0.060 |
Average |
98 |
|
|
|
0.065 |
|
As shown in Example 1, this site meets the primary and secondary
O3 NAAQS because the 3-year average of the annual fourth-highest
daily maximum 8-hour average O3 concentrations (i.e.,
0.065666 ppm, truncated to 0.065 ppm) is less than or equal to
0.070 ppm. The minimum data completeness requirements are also met
(i.e., design value is considered valid) because the average
percent of days within the O3 monitoring season with valid ambient
monitoring data is greater than 90%, and no single year has less
than 75% data completeness.
Example 2 - Site Failing to Meet the
Primary and Secondary O3 O3 NAAQS
Year |
Percent valid days within O3
monitoring season (Data completeness) |
1st highest daily max
8-hour O3
(ppm) |
2nd highest daily max
8-hour O3
(ppm) |
3rd highest daily max
8-hour O3
(ppm) |
4th highest daily max
8-hour O3
(ppm) |
5th highest daily max
8-hour O3
(ppm) |
2014 |
96 |
0.085 |
0.080 |
0.079 |
0.074 |
0.072 |
2015 |
74 |
0.084 |
0.083 |
0.072 |
0.071 |
0.068 |
2016 |
98 |
0.083 |
0.081 |
0.081 |
0.075 |
0.074 |
Average |
89 |
|
|
|
0.073 |
|
As shown in Example 2, this site fails to meet the primary and
secondary O3 NAAQS because the 3-year average of the annual
fourth-highest daily maximum 8-hour average O3 concentrations
(i.e., 0.073333 ppm, truncated to 0.073 ppm) is greater than
0.070 ppm, even though the annual data completeness is less than
75% in one year and the 3-year average data completeness is less
than 90% (i.e., design value would not otherwise be
considered valid).
[80 FR 65458, Oct. 26, 2015]