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contributing most to the seasonal index will occur during daylight hours in most low elevation agricultural areas. But this will not be the case for sensitive vegetation at higher elevations or near large water bodies, where a lengthening of the "daytime" window -even by a few hours -would result in a higher seasonal accumulation relative to a standard at any level, and especially For the W 126 index. Even if the intent were to ignore species sensitive to nocturnal effects, the average length of"daylight" during the 3-month (or 4 or 5-month) "summer growing season" ranges from > 13 hours at southerly latitudes to > 14 hours at northerly latitudes (not 12 hours) across most of the continental US. In a similar way, CASAC advice to include "at least" the 3 maximum summer months was based on concerns that although high ozone and active plant growth would tend to co-occur most frequently and intensively during mid-summer, there are certainly regions or years when elevated ozone and vegetation damage can occur over longer "growing seasons". In my home town of Burlington, VT, just south of the 45`" parallel and Canadian border, the "grow.ing season" (time between last spring frost and first autumn frost) has not been shorter than 4 months during any of the past 50 years, and has been greater than 5 months in 90% of those years. Substantially longer "growing seasons" exist throughout most of the rest of the country, where both higher ozone levels and longer ozone seasons are likely. So limiting the period for W 126 accumulation to only 3 months makes a seasonal standard at any level less stringent and/or protective than if a longer, more realistic growing season were used. The staff paper suggests (page 8-22) that a 3-year average form - as is used for the primary standazd = should be considered "given the legitimate policy interest in having a more stable standazd form." Again, as with the 12-hour daily and 3-month seasonal windows, this has the effect of making a seasonal standard at any level less protective than if an annual form were used. There's also an important conceptual distinction between a "stable" daily standard and a stable seasonal standard. The 8-hour daily standard depends on only 4 days - ormore precisely the level of the specific 8-hour maximum on the single 4°i highest day. So an exceedance could be due to "a few anomalously bad days" (on which sensitive individuals maybe able to take action to avoid maximum exposure). High levels of a seasonal index are not based on a few days but indicate cumulative effects over an entire "very bad summer" when substantial damage is likely to occur to sensitive vegetation. As the SP indicates (page 8-16) "Plants, unlike people, are exposed to ambient air 24 hours a day, every day for their entire life". So (standard-diluting) multi-year averaging is much less appropriate for a seasonal standard. If employed for "stability" purposes, the level of the standard should be adjusted downwazd to assure that the desired threshold is not exceeded in individual years. Another reason to lower the upper end of the proposed range relates to uncertainty in the "conversion factor" for relating a level of the seasonal SUM06 to an "equivalent" level of seasonal W 126. As indicated on page 7B-2 of Appendix 7, "there is no standard method for calculating equivalent levels between metrics". The method employed here and described on the same page is based on equations for similar projected crop losses based on the NCLAN data. In the given example, 50% of crop cases were estimated to be protected from a relative yield loss of 10% at a SUM061eve1 of 25 ppm-hrs. A similar level of protection is estimated at a W 126 level of 21 ppm-hrs, and thus a W 126 of 21 ppm-hrs is considered equivalent to a SUM06 of 25 ppm-hrs. In a similar way a SUM06 of I S ppm-hrs is estimated to be (S' ~ , ~' C-26