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Email concerning the attainment demonstration part of the SIP Suit.

Hi Kevin,                        Feb. 19, 2000

 

            Here's an important SIP Suit issue that revolves around how DEQ calculated ExEx and the actual ExEx.

 

EPA's ExEx and DEQ's ExEx

            ExEx is defined in the Guidance, ('Guidance on use of modeled results to demonstrate attainment of the ozone NAAQS'), page 12, as;

ExEx = (Rank - 1)/(# of years used for ranking)

 

DEQ didn't use this ExEx but instead calculated the number of times a certain set of meteorological conditions is expected to occur in any year.  This is stated in SIP Appendix N on page 7-25, para. 3.

"Using the results of the CART analysis, ExEx was calculated for each of the primary episode days by dividing the number of days within each terminal node by the number of years (9) for which the analysis was performed."

            This is a ridiculous attempt to make sure all ozone exceedance days have an ExEx of 2 or less.  This approach can be carried to its illogical extreme by noting that 27 terminal nodes were formulated in the CART analysis, only nine of which were associated with ozone forming potential.  So, following the DEQ approach quoted above, one could take a terminal node containing 18 days and no ozone exceedances, divide it by nine and find an ExEx of less than 2 for days with no ozone exceedances???? 

            DEQ doesn't give the complete CART results, but there were undoubtedly many nodes with no associated ozone exceedances that have an ExEx of less than 2.

 

The Actual ExEx

            Fortunately, DEQ has provided enough information to calculate the actual ExEx.  First, the rank must be found.  For 19 August, 1993, the data is given in Appendix N, page 7-31, para. 5.

"In terms of ozone forming potential, as defined above, this node was ranked thirteenth.  Approximately 15 percent of the days within the CART dataset were estimated to have greater ozone forming potential."

            The total number of days in the dataset is 9 years times 245 days per year.  (See page 7-22, para. six, which states that the analysis was based on data for each year from "April through October"). 

Total  number of days = (9 years) x (245 days/year) = 2,205 days

 

            Of these 2,205 days, approximately 15% have greater ozone forming potential than 19 August, 1993.  So, the rank for 19 August, 1993 is;

Rank (19 August, 1993) = (2,205 days) x ( 0.15) =  330

 

            The ExEx for 19 August can now be calculated from the equation for ExEx as;

ExEx (19 August) = (Rank - 1)/(# of years used for ranking) = (330 - 1)/9 = 25.4

 

            This is the actual ExEx for 19 August, not the 1.9 that DEQ used.  Another important point that demonstrates 19 August was not a severe day is that of the 17 days put into this CART node only 19 August was an exceedance. This means that the meteorological conditions associated with this node rarely create an ozone exceedance, i.e. only once in nine years and only once in seventeen occurances.

            Also note, since this node was ranked thirteenth and only nine nodes "were expected to contribute to maximum ozone concentrations greater than 12.0 pphm", DEQ recognizes that this node is not associated with meteorological conditions that promote ozone formation. (See page 7-24, bottom paragraph).

            The same analysis can be done for the other two days.  For 16 August 1989 the CART analysis found that 6% of the days have greater ozone forming potential. (See page 7-29, para. 5)  The Rank is then;

Rank (16 August, 1989) = (2,205 days) x (0.06) = 132.3

ExEx (16 August) = (132 - 1)/9 = 14.5      NOT A SEVERE DAY.

 

Rank (25 May, 1990) = (2,205 days) x (0.002) = 4.4

ExEx (25 May) = (4-1)/9 = 0.33               A SEVERE DAY.

 

Conclusions

            Statistical Benchmark 1 was failed three ways.  First, recognizing that only one of the three primary days is categorized as serious (ExEx < 2.0), N is one and N-1 = (1-1) = 0.  This means that there can zero exceedances in any Defined Subregion.  Thus, Benchmark 1 is failed as many Defined Subregions have ozone exceedances and one Subregion has two.

            Also, no exceedances are allowed on primary episode days with ExEx > 2.0.  The DEQ attainment demonstration has two primary episode days with ExEx > 2.0 that have exceedances.  Thus, Benchmark 1 fails twice more on this issue.

            Statistical Benchmark 2 was failed three times as well.  Statistical Benchmark 3 didn't have to be applied, but will fail in one of the three primary episode days.

           

Gary