UNITED STATES ENVIRONMENTAL PROJECTION AGENCY OfficeofAir Ouality Planning and Standards Research Triangle Park. North Carolina 27711 JUN 1 8 1990 I \:+

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FROM: SUBJECT: MEMORANDUM 

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### (MD-14)/~ TO: See Below

Indiscussions related to the Clean AirAct 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 befurther classified into different categories basedupon 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 bedone according to concentration cutpoints, and on a schedule, specified in the legislation. Obviously, once this process is set in motion we will beworking very closely withyou to develop these design values. However, I thought it would beappropriate to reiterate our design value computation procedures in advance to help peopleanticipate the types of data reviewquestions 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 expectedexceedances while the carbon monoxide NAAQS uses the older "once per year" format. The most.apparent difference is that the CO design values are basedupon 2 years of data while design values for ozone use 3years. 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 eachpollutant separately. With respect to terminology, you may hear the CO design value approach referred to as lithehighest 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 lookat 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-hourCO NAAQS, rather than the I-hour NAAQS, because the 8-hourNAAQS is typically the standard of concern. However, a I-hour design value would becomputed in the same manner. For 8-hourCO, we simply lookat the maximum and second maximum (nonoverlapping) 8-hour values at a site for the most recent 2 years ofdata. These values may bereadily found on anAIRS AMP450, "Quick look", printout. Then we choose the highest of the second highs and usethis as our design value for that site. We then lookat 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 probablyworth commenting on this. The CO NAAQS requires that notmore than one 8-hour average per year can exceed 9ppm (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 9ppm, it means there was a monitoring site where the second highest (non-overlapping) 8-hour averagewas greater than 9ppm in at least 1year. Therefore, there were at least two values above the standard during 1 year at that site and thus the standard was notmet. 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 averagenumber of exceedances peryear. In its simplest form, the ozone standard requires that the average number of exceedances over a 3- year period cannot begreater than 1.0. An area with four exceedances during a3-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 valuewas 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 averagenumber of exceedances per year would not begreater3 than one. This assumes no missing data and is how the fourth high value in 3- years came to beused 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 ofconsiderations associated with control strategy modeling, the following basic approach for ozone design values hasbeen in use since 1981. If there are 3complete years of ozone data, then the fourth highest daily maximum during the 3-year period is the design value for that site. Ifonly 2complete 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. Inthis approach, a year ofozone 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 overmultiple years but they are 'notcombined 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 3rdHi 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 0.129ppm, the fourth highest daily maximum value during the three year period.

FOUR HIGHEST DAILY MAXIMUM VALUES Max 2nd Hi 3rdHi 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 ppmwould be the desjgn value for the area. There are afew additional comments warrantedon the ozone example. First, note that data from each site was treated independently in computing the design value for that site. Assuming no missingdata, 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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4 values for a year are considered even ifthe data for that year did not satisfy the 75 percent data completeness criterion. For example, ifa site had 2 years of data that met the 75 percent data completeness requirementand 1 year that did not, then the third highest value during the 3-year period would be the design valuebecause there were only 2complete years of data but the data from all 3 years would be consideredwhen determining the third highest value. This ensures that valid highozone measurements in a particularyear are not ignored simplybecause other data in that yearwere

. missing. When computing data completeness, the number of valid days can be increased to include daysthat may beassumed 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 Junethrough August

.. Afinal 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 beexamined becauseozone 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 bedetermined on acase by case basis. In such cases, the data base is so sparse that it would be extremelydifficult to describe general rules that would applyand acareful evaluation would have to bemade to determinewhy 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-yearperiod by the required number of monitoring days per year. As noted earlier, the number of valid dayscan be increased by includ1ng the number ofdays that may beassumed to beless 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 theymay initially appear. For example, suppose a site with arequired ozone monitoring season of 214 days each year reports 0, 121, and 130 valid days ofozone data during the 3-year period. Step 1would 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 2and 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 valid5 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 besuperceded by data from the other site. The intent is to ensure that a single year of data from amonitor that was discontinued 2 years ago,' does notdictate the design value ifdata 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 similarmonitor. I have not discussed certain basic data handling conventions, such as computing 8-hourCO averages with missingdata,-determining the nonoverlapping second maximum8-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, Airand 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)