Appendix H to Part 50 - Interpretation of the 1-Hour Primary and Secondary National Ambient Air Quality Standards for Ozone
40:2.0.1.1.1.0.1.20.9 : Appendix H
Appendix H to Part 50 - Interpretation of the 1-Hour Primary and
Secondary National Ambient Air Quality Standards for Ozone 1.
General
This appendix explains how to determine when the expected number
of days per calendar year with maximum hourly average
concentrations above 0.12 ppm (235 µg/m 3) is equal to or less than
1. An expanded discussion of these procedures and associated
examples are contained in the “Guideline for Interpretation of
Ozone Air Quality Standards.” For purposes of clarity in the
following discussion, it is convenient to use the term “exceedance”
to describe a daily maximum hourly average ozone measurement that
is greater than the level of the standard. Therefore, the phrase
“expected number of days with maximum hourly average ozone
concentrations above the level of the standard” may be simply
stated as the “expected number of exceedances.”
The basic principle in making this determination is relatively
straightforward. Most of the complications that arise in
determining the expected number of annual exceedances relate to
accounting for incomplete sampling. In general, the average number
of exceedances per calendar year must be less than or equal to 1.
In its simplest form, the number of exceedances at a monitoring
site would be recorded for each calendar year and then averaged
over the past 3 calendar years to determine if this average is less
than or equal to 1.
2. Interpretation of Expected Exceedances
The ozone standard states that the expected number of
exceedances per year must be less than or equal to 1. The
statistical term “expected number” is basically an arithmetic
average. The following example explains what it would mean for an
area to be in compliance with this type of standard. Suppose a
monitoring station records a valid daily maximum hourly average
ozone value for every day of the year during the past 3 years. At
the end of each year, the number of days with maximum hourly
concentrations above 0.12 ppm is determined and this number is
averaged with the results of previous years. As long as this
average remains “less than or equal to 1,” the area is in
compliance.
3. Estimating the Number of Exceedances for a Year
In general, a valid daily maximum hourly average value may not
be available for each day of the year, and it will be necessary to
account for these missing values when estimating the number of
exceedances for a particular calendar year. The purpose of these
computations is to determine if the expected number of exceedances
per year is less than or equal to 1. Thus, if a site has two or
more observed exceedances each year, the standard is not met and it
is not necessary to use the procedures of this section to account
for incomplete sampling.
The term “missing value” is used here in the general sense to
describe all days that do not have an associated ozone measurement.
In some cases, a measurement might actually have been missed but in
other cases no measurement may have been scheduled for that day. A
daily maximum ozone value is defined to be the highest hourly ozone
value recorded for the day. This daily maximum value is considered
to be valid if 75 percent of the hours from 9:01 a.m. to 9:00 p.m.
(LST) were measured or if the highest hour is greater than the
level of the standard.
In some areas, the seasonal pattern of ozone is so pronounced
that entire months need not be sampled because it is extremely
unlikely that the standard would be exceeded. Any such waiver of
the ozone monitoring requirement would be handled under provisions
of 40 CFR, part 58. Some allowance should also be made for days for
which valid daily maximum hourly values were not obtained but which
would quite likely have been below the standard. Such an allowance
introduces a complication in that it becomes necessary to define
under what conditions a missing value may be assumed to have been
less than the level of the standard. The following criterion may be
used for ozone:
A missing daily maximum ozone value may be assumed to be less
than the level of the standard if the valid daily maxima on both
the preceding day and the following day do not exceed 75 percent of
the level of the standard.
Let z denote the number of missing daily maximum values that may
be assumed to be less than the standard. Then the following formula
shall be used to estimate the expected number of exceedances for
the year:
(*Indicates multiplication.)
where: e = the estimated number of exceedances for the year, N =
the number of required monitoring days in the year, n = the number
of valid daily maxima, v = the number of daily values above the
level of the standard, and z = the number of days assumed to be
less than the standard level.
This estimated number of exceedances shall be rounded to one
decimal place (fractional parts equal to 0.05 round up).
It should be noted that N will be the total number of days in
the year unless the appropriate Regional Administrator has granted
a waiver under the provisions of 40 CFR part 58.
The above equation may be interpreted intuitively in the
following manner. The estimated number of exceedances is equal to
the observed number of exceedances (v) plus an increment that
accounts for incomplete sampling. There were (N-n) missing values
for the year but a certain number of these, namely z, were assumed
to be less than the standard. Therefore, (N-n-z) missing values are
considered to include possible exceedances. The fraction of
measured values that are above the level of the standard is v/n. It
is assumed that this same fraction applies to the (N-n-z) missing
values and that (v/n)*(N-n-z) of these values would also have
exceeded the level of the standard.
[44 FR 8220, Feb. 8, 1979, as amended at 62 FR 38895, July 18,
1997]