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)

------------------------- end of original document -----------------

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.

Source: http://www.epa.gov/ttn/naaqs/ozone/rto/ozonetech/laxton.htm last checked August 15, 

2011.