Surface observations alone offer a limited scope of the influence

In 2002, Bytnerowicz et al. first reported concentrations (48 to 72 ppbv) measured in the summer at Sequoia National Park, located on the eastern slope of the Sierra Nevada Mountains above the San Joaquin Valley of California, were associated with forest decline syndrome. Values reported for high elevations across Nevada, Yosemite National Park, and the White Mountains fall into and exceed the range reported by Bytnerowicz et al. (2002). Maximum values measured in Nevada and at Yosemite are higher than reported in Burley and Ray (2007) for Yosemite collected in 2003 to 2005 (for details see the SI). This suggests that O3 exposure concentrations are increasing in remote/rural areas and there are forest exposures of concern in the Western US. Exposure to these concentrations would be problematic early in the growing season when plants are physiologically active (spring) and their Isoprenaline are open (Panek and Goldstein, 2001). The range in values of the W126 metric considered for adoption as the secondary standard for O3 was 7 to 15 ppm h ( For discussion of calculation of this value see the SI. Great Basin National Park has exceeded this proposed standard and the values have varied from year-to-year. The W126 at Great Basin National Park has ranged from 1.4 to 20.4 from 1993 to 2013 with a skewness of –0.64, indicating a trend towards higher values (NPS, 2009). Overall impacts on vegetation are of significant concern for this affects economic development, reduces ecosystem and agricultural productivity, and impacts ecosystem health and aesthetics. The most recent IPCC report also notes that vegetation exposure to increased O3 can reduce net primary productivity which would decrease uptake of CO2 by vegetation (5th IPCC Assessment Report, 2013). Work with Arabidopsis (common name: rockcress) has shown that chronic exposure decreases photosynthesis, promotes early senescence, and reduces productivity ( Ainsworth et al., 2012).