*** E.O. 12866 Review – Draft – Do Not Cite, Quote or Release During Review ***

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 O3 sensitivity 

of the other co-occurring species (O3 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 be 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 O3 standard (Heck and Cowling, 1997). In their consideration 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 regard 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 Report NPS/NRPC/ARD/NRR—2011/ 289. 

http://www.nature.nps.gov/air/Pubs/pdf/AQGuidance_2011-01-14.pdf
Page 482 of 702

Thus, based on her consideration of the full range of information as described above, the 

Administrator judges that ambient O3 concentrations 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 retaining 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 threemonth period yielding the highest seasonal value and with daily O3 exposures within a threemonth 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 13 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