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Page 361 of 702 forested ecosystems . These include: 1) the type of stand or community in which the sensitive species is found (i.e., single species versus mixed canopy); 2) the role or position the species has in the stand (i.e., dominant, sub

- dominant, canopy, understory); 3) the O 3 sensitivity of the oth er co - occurring species (O 3 sensitive or tolerant); and 4) environmental factors , such as soil moisture and others
. The lack of such established relationships complicates judgments as to the extent to which different amounts of tree seedling growth would b e significant to the public welfare and thus an important consideration in the level of protection for the secondary standard. The National Park Service has defined a major air quality impact as a W126 (12

- hr, maximum 3 month average) greater than 13 ppm

- hr
. 1 During the 1997 review of the secondary standard, views related to this issue were provided by a 1996 workshop of 16 then

- leading scientists in the context of discussing their views for a secondary O 3 standard (Heck and Cowling, 1997). In their consid eration of tree growth effects as an indicator for forest ecosystems and crop yield reduction as an indicator of agricultural systems, the workshop participants identified annual percentages, of RBL for forest tree seedlings and RYL for agricultural crops, considered important to their judgments on the standard. With regard to forest ecosystems and seedling growth effects as an indicator, the participants selected a range of 1

- 2% RBL per year “to avoid cumulative effects of yearly reductions of 2% .” With r egard to crops, they indicated an interest in protecting against crop yield reductions of 5% RYL yet noted uncertainties

1 NPS Air Resources Division Technical Guidance on Assessing Impacts to Air Quality in NEPA and Planning Documents. January 2011. Natural Resource Rep ort NPS/NRPC/ARD/NRR

— 2011/ 289. http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011

- 01

- 14.pdfPage 482 of 702

Thus, based on her consideration of the full range of information as described above, the Administrator judges that ambient O 3 concen trations in terms of a three

- year average W126 index value within the range extending from 13 ppm

- hrs to 17 ppm

- hrs would provide requisite public welfare protection. She further judges that it would be appropriate to achieve that level of air quality by r etaining the existing averaging time and form, and revising the level to within the range of 65 to 70 ppb. In recognition of CASAC’s recommendation and the PA conclusion with regard to a distinct secondary standard, the Administrator additionally solicits comment on the policy option of revising the form and averaging time for the secondary standard to a W126 index value, averaged across three years, with each year’s value identified as that for the three

month period yielding the highest seasonal value and with daily O 3 exposures within a three

month period cumulated for the 12

- hour period from 8:00 am to 8:00 pm, and a level within the range from 13 ppm

- hrs to 17 ppm

- hrs. F. Proposed Decision on the Secondary Standard The Administrator proposes to revise the level of the current secondary standard within the range proposed for the primary standard of 0.065 to 0.070 ppm
. The EPA solicits comments on this proposed revision of the secondary standard and on the adequacy of the hourly form of the primary metric to provide the requisite protection consistent with the cumulative seasonal W126 metric. . Further, the EPA solicits comments on the proposed conclusion that air quality in terms of a W126 index value, averaged across three consecutive years, within the range of 1 3 ppm - hrs to 17 ppm

- hrs would provide requisite protection against known or anticipated adverse effects to the public welfare. Additionally, the EPA solicits comments on alternative values for a