Meteorological conditions important to consider

Recent data analyses indicate that high elevation sites throughout the Western US (cf. Jaffe, 2010 and Bytnerowicz et al., 2013) will exceed a lowered O3 standard. An additional concern is that the secondary standard, currently equivalent to the primary standard, is not adequate for protection of vegetation (cf. Miller, 2011 and Ashmore, 2005). Much of the Western USA is rural and home to protected ecosystems designated as National Forests, National Parks, Wilderness Areas, and rangelands. Work in the 1980's focused on the effect of Los Angeles pollution on local forests showed that Jeffrey and Ponderosa Pines were most sensitive (Miller et al., 1983). These are important 1-NM-PP1 across western ecosystems. Earlier work by Miller et al. (1963) showed an average daily peak O3 concentration of 90 ppbv associated with stands of declining Ponderosa Pine. Takemoto et al. (1997) showed exposures of 40 to 60 ppbv could yield increased chlorotic mottle and premature senescence of Ponderosa Pine needles. The effective dose or commonly used threshold for causing either a reduction in growth or yield or visible injury is 40 ppb (cf. Cape, 2008). One standard that has been considered for a secondary standard is the cumulative exposure index W126 (cf. http://www.asl-associates.com/w126.htm). For additional information on O3 in the Western US and impacts on vegetation see the Supplemental information (SI).