UNITED STATES ENVIRONMENTAL PROJECTION AGENCY

Office of Air Ouality Planning and Standards

Research Triangle Park. North Carolina 27711

JUN 1 8 1990 I \ :+... '.J \ ~,' ., •

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FROM:

SUBJECT:

MEMORANDUM i • '\ ' '

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~~:::~::o:es ign V~ueO pa~~ 'n'i'l ,;~, 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 ozone 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 lithe highest of the

second highs", while the ozone design value is frequently simplified as lithe

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. Aseparate

design value is developed for each site that does not meet the NAA(fit~apWct.r~~~:-'·-1~:=·'

highest of these design values is the design value for the area. ~~~~~~

d • '- ~'-/
SITE 1

2

Carbon Monoxide

CO design values are discussed in terms of the 8-hour CO NAAQS, rather

than the I-hour NAAQS, because the 8-hour NAAQS is typically the standard of

concern. However, a I-hour design value would be computed in the same manner.

For 8-hour CO, we simply look at the maximum and second maximum (nonoverlapping) 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 than 9 ppm, it means there was a

monitoring site where the second highest (non-overlapping) 8-hour average was

greater than 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)

(a-Hour Averages) MAX 2nd High 1987 14.6 8.9

1988 13.9 10.9

10.9 is the Design Value for

Site 1

SITE 2

(8-Hour Averages) MAX 2nd High

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.

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

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

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 ar~ 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 03 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 .1l0

1987 .129 .124 .121 .1l6

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

1987 .110

1988 .180

.100

.100

.175

.095

.095

.160

.090

.090

.1l0

The design value for Site 2 is 0.110, the fourth highest value

during the three year period .

... 0.129 ppm would be the desjgn 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 Li~e, however, the reason for this consistent data completeness problem should be examined because ozone monitoring data completeness is

typically greater than 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 includ1ng 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 9ive {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
5

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 nonoverlapping second maximum 8-hour average, or the definition of a valid daily maximum I-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 Managem~~t 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-I0)

T. Curran (MD-14)

D. DeVoe (ANR-443)

J. Farmer (MD-13)

T. Helms (MD-15)

W. Hunt (MD-14)

S. Meiburg (MD-ll)

R. Ossias (lE-132A)