AG 2007 10 15 - Laserfiche WebLink

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(2) "It is not appropriate to continue to use an 8-hr averaging time for the secondary standard 03 standard. The 8-hr average form should be replaced with a cumulative, seasonal, concentration weighted form.... staff concludes that the W 126 form is more appropriate than the SUM06 form recommended in the last review." (3) "It is appropriate to consider the maximum consecutive 3 month period within the 03 season as the seasonal averaging time over which cumulative Os exposures for the daily 12-hr daylight (8 am to 8 pm) window. Though the length of time in the growing season varies significantly between species, staff concludes that the 3-month of maximum 03 exposure generally coincides with the maximum biological activity for most plants. Staff notes that for certain welfare effects of concern (e.g., foliar injury, yield loss for annual crops, growth effects on other annual vegetation and potentially tree seedlings), an annual standard form may be more appropriate, while for other welfare effects (e.g., mature tree biomass loss), a 3-year average form maybe more appropriate [especially because of carry over effects from one year to the next]. Staff concludes it is appropriate to consider both the annual and 3-year average forms." (4) "It is appropriate to consider a range of levels when making a determination regarding what is requisite [for] public welfare protection. Staff concludes that an appropriate upper bound of this range is 21 ppm-hrs, expressed in terms of the W 126 index, which is roughly equivalent to that proposed by the Administrator in the last review as able to provide a requisite level of protection to vegetation. Our analyses indicate that this level will provide protection against 03-related adverse impacts on vegetation such as tree growth and crop yield beyond that afforded by the current 8-hr standard. In large part, the basis for selecting the level in the last review was a judgment as to what was an appropriate level of protection against crop yield loss. Though crop data are still useful as a potential indicator of risk to other sensitive herbaceous plants, staff recognizes that agricultural systems are heavily managed. In addition, the annual productivity of agricultural systems is vulnerable to disruption from many other stressors (e.g., weather, insects, disease), who's impact in any given year greatly outweigh the direct reduction in annual productivity resulting from elevated 03 exposures. On the other hand, 03 can also more subtly impact crop and forage nutritive quality and indirectly exacerbate the severity of the impact from other stressors.... Taking all of the above considerations into account, staff concludes that from a public welfare perspective, greater concern should be placed on the impacts of 03 exposures on vegetation in less heavily managed and unmanaged ecosystems such as tree seedlings, mature trees, and forest ecosystems in general. Thus staff concludes that the lower end of the range should incorporate the lower end of the range expressed by CASAC - a 3-month 12-hr W 126 approximately equal to 7 ppm- hrs. This lower level will increase protection for the most sensitive tree species and the ecosystems where they are found." C-4