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equivalent to a W 126 of 13 ppm-hrs. Note that these W 1261evels -based on NCLAN data collected in the 1980s -are 84 to 87% of their "equivalent" SUM06 levels, respectively. An alternative indication of "equivalency" is displayed in the Figure 7-4 scatter plot on page 7-29 of the Staff Paper. Based on much more recent ozone measurement data from 2001, the 2 indices are very highly correlated across all monitoring locations (RZ=0.98) but the current slope is more like 0.75 (not 0.85). As shown in the box at lower left of this figure, a SUM06 of 25 ppm-hrs is - in recent years - "equivalent" to a W 126 of 19 (not 21) ppm hrs. A reason for the differences in khese calculations of equivalency may very well be a large (downward and broadening) shift in the distribution of ozone concentrations since the 1980s when the NCLAN studies were conducted. This shift in the relationship between ozone indices over time further emphasizes on the importance of establishing a separate secondary standazd, and cautions against concluding that the relationship between primary and secondary standard metrics will remain constant in the future. Thus if an upper SUM06 bound of 25 ppm-hrs is intended, the equivalent W 126, based on the current US distribution ofmid-summer ozone concentrations would be 19 ppm-hrs. If a multi- year form is being considered, this upper end of 19 ppm-hrs should be further reduced to assure it isn't substantially exceeded in individual years. If accommodations were also made to account for effects during hours of day or months of growing seasons which are not considered by the current proposal, then the upper bound level of the secondary W 126 standard range could very well be adjusted downward from 19 toward 15 ppm-hrs -the approximate equivalent of the 20 ppm-hrs. upper bound SUM06 recommended in the last CASAC review. G. ~ ~',{. C-27