Spatial and temporal patterns of ozone in high elevation ecosystems of Colorado

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Ozone is a strong oxidant in the atmosphere that is regulated by the US EPA to protect human health and welfare. High ozone is known to cause respiratory problems in humans, but the standards are also intended to provide protection to trees, native vegetation and crops. Currently, the standards are met if the fourth-highest maximum daily 8-hour ozone concentration per year, averaged over three years, is equal to or less than 75 ppb. While ozone does occur naturally in the troposphere, the majority of ozone at the surface today is related to anthropogenic activities. Ozone forms in the troposphere from interaction of nitrogen oxides and volatile organic compounds in the presence of sunlight.  Because these gases largely come from transportation and industrial activity, ozone has typically been thought of as an urban problem.  However, there is growing concern about increased ozone concentrations in rural areas.  One region with surprisingly high ozone concentrations is the high elevation ecosystems in the Front Range of Colorado. At two sites in the subalpine forest to the alpine tundra on Niwot Ridge (NWT) the median daily ozone mixing ratio were 49 and 53 ppb respectively over the last decade. At these relatively pristine sites, there are exceedances of the federal ozone standards every year. The ozone mixing ratio typically exceeds 100 ppb at some point every year.  A comparison with lower elevation sites suggests that ozone mixing ratios generally increase with elevation.  In addition, the diurnal and seasonal variability of ozone decreases with elevation.  Along a gradient from Boulder to NWT, the tundra site had the highest median values and the least variability of any site. The seasonal pattern at these high elevation sites is also distinct as the maximum mixing ratios occur in the spring in contrast to the summer maximum typically observed in urban areas. While there have been relatively small changes in concentration in the measured data record going back a few decades, modeling suggests that ozone mixing ratios have almost doubled over the last one hundred years in Colorado. A plethora of studies has shown that elevated ozone damages foliage, with sensitive species showing effect at levels exceeding 35-40 ppbv.  Since ozone levels at NWT are clearly above that, we believe that the NWT ecosystem has been and continues to be severely affected by elevated ozone. It is not well established which high elevation species are most sensitive to ozone, but the historical changes in ozone may have already altered ecosystem structure and function, such as species composition and carbon and nutrient cycling. That is, high ozone may be selecting for species composition that is more ozone tolerant.