Technology Transfer Network / NAAQS
Ozone Implementation
Ozone and Carbon Monoxide
Design Value Calculations
Note: EPA no longer updates this information, but it may be useful as a reference or resource.
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
June 18, 1990
MEMORANDUM
SUBJECT: Ozone and Carbon Monoxide Design Value Calculations
FROM: William G. Laxton, Director
Technical Support Division (MD-14)
TO: See Below
In discussions related to the Clean Air Act legislation, design
values for ozone and carbon monoxide are receiving particular
attention. Previously, it sufficed to designate areas as either
attainment or nonattainment but now areas will be further classified
into different categories based upon the magnitude of the appropriate
design value. This additional classification step places added
emphasis on the need to accurately determine these design values. The
classification will be done according to concentration cutpoints, and
on a schedule, specified in the legislation.
Obviously, once this process is set in motion we will be working
very closely with you to develop these design values. However, I
thought it would be appropriate to reiterate our design value
computation procedures in advance to help people anticipate the types
of data review questions that may arise. The computation procedures
stated here are consistent with our previous methods. There are
differences between the procedures for ozone and carbon monoxide
because the National Ambient Air Quality Standard (NAAQS) is
structured in terms of expected exceedances while the carbon monoxide
NAAQS uses the older "once per year" format. The most apparent
difference is that the CO design values are based upon 2 years of data
while design values for ozone use 3 years. Another difference is that
the ozone NAAQS uses the daily maximum ozone value while the CO NAAQS
considers running 8-hour averages so that, even though they must be
non-overlapping, it is possible to have more than one CO exceedance per
day. Because of these differences, it is convenient to discuss each
pollutant separately. With respect to terminology, you may hear the CO
design value approach referred to as "the highest of the second highs',
while the ozone design value is frequently simplified as "the fourth
high in 3 years."
One point to remember is that all locations within an area have to
meet the standard (NAAQS). Therefore, when we do our evaluations, we
look at each individual site to make sure that every site meets the
standard. A separate design value is developed for each site that does
not meet the NAAQS, and the highest of these design values is the
design value for the area.
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Carbon Monoxide
CO design values are discussed in terms of the 8-hour CO NAAQS,
rather that the 1-hour NAAQS, because the 8-hour NAAQS is typically the
standard of concern. However, a 1-hour design value would be computed
in the same manner. For 8-hour CO, we simply look at the maximum and
second maximum (non-overlapping) 8-hour values at a site for the most
recent 2 years of data. These values may be readily found on an AIRS
AMP450, "Quick Look", printout. Then we choose the highest of the
second highs and use this as our design value for that site. We then
look at all design values within an area and the highest of these
serves as the design value for the area. Note that, for each site,
individual years of CO data are considered separately to determine the
second maximum for each year - CO data are not combined from different
years. It is probably worth commenting on this. The CO NAAQS requires
that not more than one 8-hour average per year can exceed 9 ppm
(greater than or equal to 9.5 ppm to adjust for rounding). We evaluate
attainment over a 2-year period. If an area has a design value greater
that 9 ppm, it means there was a monitoring site where the second
highest (non-overlapping) 8-hour average was greater that 9 ppm in at
least 1 year. Therefore, there were at least two values above the
standard during 1 year at that site and thus the standard was not met.
Hypothetical Case (two CO sites in an area)
(8-Hour Averages)
MAX 2nd High
SITE 1 1987 14.6 8.9
1988 13.9 10.9
10.9 is the Design Value
for Site 1
(8-Hour Averages)
MAX 2nd High
SITE 2 1987 12.2 11.1
1988 10.8 10.4
11.1 is the Design Value
for Site 2
11.1 ppm would be the design value for the area.
Ozone
The form of the ozone NAAQS requires the use of a 3-year period to
determine the average number of exceedances per year. In its simplest
form, the ozone standard requires that the average number of
exceedances over a 3-year period, cannot be greater than 1.0. An area
with four exceedances during a 3-year period, therefore, does not meet
the ozone standard because four exceedances in 3 years averages out to
more than once per year. Now, if the fourth highest value was equal to
the level of the ozone standard, i.e. 0.12 ppm, then the area would
have no more than three exceedances during the 3-year period and the
average number of exceedances per year would not be greater 3
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than one. This assumes no missing data and is how the fourth high
value in 3-years came to be used as the design value. Actually, an
adjustment is specified in the ozone NAAQS to account for missing data
in determining the expected exceedances for ozone. Because of
considerations associated with control strategy modeling, the following
basic approach for ozone design values has been in use since 1981. If
there are 3 complete years of ozone data, then the fourth highest daily
maximum during the 3-year period is the data, then the fourth highest
daily maximum during the 3-year period is the design value for that
site. If only 2 complete years of data are available, then the third
highest is used and, if only one complete year is available, then the
second highest is used. In this approach, a year of ozone data is
considered complete if valid daily maximums are available for at least
75 percent of the ozone season. Note that because of the form of the
ozone NAAQS, data are combined over multiple years but they are not
combined from different sites.
Hypothetical Case (two O3 sites in an area, each year at
least 75% complete)
FOUR HIGHEST DAILY MAXIMUM VALUES
Max 2nd Hi 3rd Hi 4th Hi
SITE 1 1986 .127 .123 .122 .110
1987 .129 .124 .121 .116
1988 .142 .136 .134 .115
The design value for Site 1 is 0.129 ppm, the fourth highest
daily maximum value during the three year period.
FOUR HIGHEST DAILY MAXIMUM VALUES
Max 2nd Hi 3rd Hi 4th Hi
SITE 2 1986 .110 .100 .095 .090
1987 .110 .100 .095 .090
1988 .180 .175 .160 .110
The design value for Site 2 is 0.110, the fourth highest
value during the three year period.
0.129 ppm would be the design value for the area.
There are a few additional comments warranted on the ozone
example. First, note that data from each site was treated
independently in computing the design value for that site. Assuming no
missing data, the second site would meet the ozone NAAQS but the area
would not because the other site shows that the NAAQS is not being met.
Also, it should be noted that the high
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values for a year are considered even if the data for that year did not
satisfy the 75 percent data completeness criterion. For example, if a
site had 2 years of data that met the 75 percent data completeness
requirement and 1 year that did not, then the third highest value
during the 3-year period would be the design value because there were
only 2 complete years of data but the data from all 3 years would be
considered when determining the third highest value. This ensures that
valid high ozone measurements in a particular year are not ignored
simply because other data in that year were missing. When computing
data completeness, the number of valid days can be increased to include
days that may be assumed to be less than the standard level as stated
in the ozone NAAQS. Also, for new sites that have just come on line,
the 75 percent data completeness requirement for the start-up year may
be applied beginning with the first day of actual monitoring as long as
the data set is at least 75 percent complete for June through August.
A final practical complication that must be addressed in
determining ozone design values is the case where a site reports data
but has no year that meets the 75 percent data completeness
requirement. Admittedly, this is an unusual situation but, for the
sake of completeness, it needs to be addressed. At the same time,
however, the reason for this consistent data completeness problem
should be examined because ozone monitoring data completeness is
typically greater that 90 percent. In general, if a site has no
complete years of data and fewer than 90 days of data during the 3-year
period, the design value will be determined on a case by case basis.
In such cases, the data base is so sparse that it would be extremely
difficult to describe general rules that would apply and a careful
evaluation would have to be made to determine why this situation
occurred and what is the most appropriate way to use the data. For a
site without a single complete year of data but at least 90 days of
data during the 3-year period, the following steps are followed in
determining the ozone design value:
1. Divide the number of valid daily maximums during the 3-year
period by the required number of monitoring days per year.
As noted earlier, the number of valid days can be increased
by including the number of days that may be assumed to be
less than the standard level as specified in the ozone NAAQS.
2. Add 1.0 to the above total and then use the integer portion
of the result as the rank of the design value.
These steps are not as complicated as they may initially appear.
For example, suppose a site with a required ozone monitoring season of
214 days each year reports 0, 121, and 130 valid days of ozone data
during the 3-year period. Step 1 would give (0+121+130)/214=1.17. In
step 2, 1.0 is added to this total giving 2.17. The integer portion of
2.17 is 2 and so the design value is the second highest value during
the three year period. Again, this type of situation should not occur
that often and the reasons for the data completeness problems should be
identified.
When discussing data completeness for ozone, it is important to
recognize that monitoring sites are occasionally discontinued for valid
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practical reasons. In such cases, if data are available from another
site that is representative of the same situation, then data from the
discontinued site may be superceded by data from the other site. The
intent is to ensure that a single year of data from a monitor that was
discontinued 2 years ago, does not dictate the design value if data are
available from another, equally representative, site. This is not
intended to eliminate the missing data penalty when a site is
discontinued and there is no data available from a similar monitor.
I have not discussed certain basic data handling
conventions, such as computing 8-hour CO averages with missing
data, determining the non-overlapping second maximum 8-hour
average, or the definition of a valid daily maximum 1-hour ozone
daily maximum. All of these conventions have been in place since
the 1970's and are routinely incorporated into AIRS outputs so I
have not bothered to discuss these points.
Addressees:
Director, Environmental Services Division, Regions I-VIII, X
Director, Office of Policy and Management, Region IX
Director, Air Management Division, Region III
Director, Air and Waste Management Division, Region II
Director, Air, Pesticides and Toxics Management Division, Regions
I and IV
Director, Air and Radiation Division, Region V
Director, Air, Pesticides and Toxics Division, Region VI
Director, Air and Toxics Division, Regions VII, VIII, IX, and X
cc: J. Calcagni (MD-15)
R. Campbell (MD-10)
T. Curran (MD-14)
D. DeVoe (ANR-443)
J. Farmer (MD-13)
T. Helms (MD-15)
W. Hunt (MD-14)
S. Meiburg (MD-11)
R. Ossias (LE-132A)
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Note to reader:
This copy of the Laxton memo is a retyped version of the original. As
a result, the page breaks had to be forced after the last word on each
page on the original document.