Appendix I to Part 50 - Interpretation of the 8-Hour Primary and Secondary National Ambient Air Quality Standards for Ozone
40:2.0.1.1.1.0.1.20.10 : Appendix I
Appendix I to Part 50 - Interpretation of the 8-Hour Primary and
Secondary National Ambient Air Quality Standards for Ozone
1. General.
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
specified in § 50.10 are met at an ambient ozone air quality
monitoring site. Ozone is measured in the ambient air by a
reference method based on appendix D of this part. Data reporting,
data handling, and computation procedures to be used in making
comparisons between reported ozone concentrations and the level of
the ozone standard are specified in the following sections. Whether
to exclude, retain, or make adjustments to the data affected by
stratospheric ozone intrusion or other natural events is subject to
the approval of the appropriate Regional Administrator.
2. Primary and Secondary Ambient Air Quality Standards for
Ozone.
2.1 Data Reporting and Handling Conventions.
2.1.1 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 being
truncated. Running 8-hour averages shall be computed from the
hourly ozone concentration data for each hour of the year and the
result 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 not be ignored 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 ozone concentrations shall
be reported to three decimal places (the insignificant digits to
the right of the third decimal place are truncated, consistent with
the data handling procedures for the reported data.)
2.1.2 Daily maximum 8-hour average concentrations. (a)
There are 24 possible running 8-hour average ozone concentrations
for each calendar day during the ozone monitoring season. (Ozone
monitoring seasons vary by geographic location as designated in
part 58, appendix D to this chapter.) 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 ozone
concentration for each calendar day with ambient ozone 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 ozone 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 ambient standard.
2.2 Primary and Secondary Standard-related Summary
Statistic. The standard-related summary statistic is the annual
fourth-highest daily maximum 8-hour ozone 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 ozone
concentrations shall be expressed to three decimal places (the
remaining digits to the right are truncated.)
2.3 Comparisons with the Primary and Secondary Ozone
Standards. (a) The primary and secondary ozone 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 ozone concentration is less than or equal to 0.08
ppm. The number of significant figures in the level of the standard
dictates the rounding convention for comparing the computed 3-year
average annual fourth-highest daily maximum 8-hour average ozone
concentration with the level of the standard. The third decimal
place of the computed value is rounded, with values equal to or
greater than 5 rounding up. Thus, a computed 3-year average ozone
concentration of 0.085 ppm is the smallest value that is greater
than 0.08 ppm.
(b) This comparison shall be based on three consecutive,
complete calendar years of air quality monitoring data. This
requirement is met for the three year period at a monitoring site
if daily maximum 8-hour average concentrations are available for at
least 90%, on average, of the days during the designated ozone
monitoring season, with a minimum data completeness in any one year
of at least 75% of the designated sampling days. 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 than 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 not be ignored on the ground that 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 average
annual fourth maximum 8-hour concentration is greater than the
level of the standard.
(d) Comparisons with the primary and secondary ozone standards
are demonstrated by examples 1 and 2 in paragraphs (d)(1) and (d)
(2) respectively as follows:
(1) As shown in example 1, the primary and secondary standards
are met at this monitoring site because the 3-year average of the
annual fourth-highest daily maximum 8-hour average ozone
concentrations (i.e., 0.084 ppm) is less than or equal to
0.08 ppm. The data completeness requirement is also met because the
average percent of days with valid ambient monitoring data is
greater than 90%, and no single year has less than 75% data
completeness.
Example 1. Ambient monitoring site
attaining the primary and secondary ozone standards
Year |
Percent Valid Days |
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) |
1993 |
100% |
0.092 |
0.091 |
0.090 |
0.088 |
0.085 |
1994 |
96% |
0.090 |
0.089 |
0.086 |
0.084 |
0.080 |
1995 |
98% |
0.087 |
0.085 |
0.083 |
0.080 |
0.075 |
Average |
98% |
|
|
|
|
|
(2) As shown in example 2, the primary and secondary standards
are not met at this monitoring site because the 3-year average of
the fourth-highest daily maximum 8-hour average ozone
concentrations (i.e., 0.093 ppm) is greater than 0.08 ppm.
Note that the ozone concentration data for 1994 is used in these
computations, even though the data capture is less than 75%,
because the average fourth-highest daily maximum 8-hour average
concentration is greater than 0.08 ppm.
Example 2. Ambient Monitoring Site Failing
to Meet the Primary and Secondary Ozone Standards
Year |
Percent Valid Days |
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) |
1993 |
96% |
0.105 |
0.103 |
0.103 |
0.102 |
0.102 |
1994 |
74% |
0.090 |
0.085 |
0.082 |
0.080 |
0.078 |
1995 |
98% |
0.103 |
0.101 |
0.101 |
0.097 |
0.095 |
Average |
89% |
|
|
|
|
|
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 ozone standards, the 3-year average
annual fourth-highest daily maximum 8-hour average ozone
concentration is also the air quality design value for the
site.
[62 FR 38895, July 18, 1997]