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Volume 30
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Fall 2013
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Photo of automobile traffic at the Desert View entrance to Grand Canyon National Park, Arizona Research Reports
Cars and canyons: Understanding roadside impacts of automobile pollution in Grand Canyon National Park
By Julie A. Kenkel, Thomas Sisk, Kevin Hultine, Steven Sesnie, Matthew Bowker, and Nancy Collins Johnson
Published: 4 Sep 2015 (online)  •  14 Sep 2015 (in print)
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Photo of sunset at Grand Canyon National Park, Arizona, near the main park visitor center

Julie Kenkel

A sunset view of Grand Canyon near the main park visitor center.

Each year millions of visitors from around the world come to Grand Canyon National Park, in Arizona to witness awe-inspiring views and to observe over 60 million years of Earth’s history preserved in the strata displayed in the canyon walls. Some days, visitors are disappointed to find these canyon views veiled by haze, reducing visibility to a fraction of the canyon’s legendary vistas (NPS 2013). Disappointed visitors understandably ask what causes this haze and how air pollution affects the Grand Canyon. How can clean air and breathtaking views be restored at Grand Canyon?

Atmospheric haze is composed of a mixture of chemicals, including nitrogen oxides (NOx), ozone (O3), peroxyacetyl nitrate (PAN), and sulphate (SO42–) (Jimoda et al. 2011). Two of these components, O3 and NOx, not only compromise Grand Canyon views, but also damage plant tissues and have impacts on ecosystem processes (Fenn et al. 2003; Bobbink et al. 2003), with potentially wide-ranging effects. Under certain weather conditions, urban air pollution and haze from Las Vegas, Phoenix, and Los Angeles, plus industrial pollution from power plants and copper smelters, accumulate in the canyon (Macias et al. 1981; Eatough et al. 1997; Eatough et al. 2001). In addition to visitors, automobiles bring air pollution to the park in the form of exhaust that may also reduce air quality at Grand Canyon. Every year, approximately one million automobiles pass through the south entrance of Grand Canyon National Park; nearly 200,000 pass through the east entrance at Desert View (NPS 2013; fig. 1).

Even though 78% of the atmosphere is composed of N2 gas, this form of N is unavailable for plant use. Consequently plant growth in natural ecosystems is often limited by a shortage of N. Plants acquire some organic forms of N along with ammonia (NH3+) and nitrate (NO3) from the soil. The process of transforming N2 gas into plant-available NH3+ or NO3 requires a lot of energy, and occurs naturally through nitrogen-fixing microbes and lightning, or through industrial processes related to fertilizer production, and fossil fuel combustion for transportation and the production of electricity.

Since the industrial and agricultural revolutions, human activities have doubled the amount of bioavailable N on the planet, overloading some ecosystems (Vitousek et al. 1997). Even low levels of persistent NOx pollution may cause the buildup of excess N, which can result in the ecological equivalent of “too much of a good thing.” Critical loads are defined as the amount of pollutants below which there are no adverse ecological effects (Fisher et al. 2007; Burns et al. 2008). Nitrogen inputs that surpass critical loads create undesirable effects on natural communities and ecosystems, including changes in soil fertility that disrupt native plants and their consumers (Galloway et al. 2003; Aber et al. 1989; Weiss 1999).

Since the industrial and agricultural revolutions, human activities have doubled the amount of bioavailable N on the planet, overloading some ecosystems.

Automobile emissions in our national parks may damage sensitive roadside vegetation, such as conifers, and favor a few dominant species, including invasive grasses (Trahan and Peterson 2007; Angold 1997). For example, in Rocky Mountain National Park, Colorado, N inputs from highways and long-range transport from urban and agricultural centers contribute to plant community shifts and establishment and spread of invasive grasses (Bowman 2000; Bowman et al. 2012). Similarly, roadside N enrichment in Grand Canyon National Park may provide too much N to organisms accustomed to otherwise N-limited plant communities, changing nutrient cycling processes and altering plant community composition. Currently we do not know the effects of N enrichment on Grand Canyon ecosystems; however, we do know that increasing population and industrialization in the region continue to challenge park staff’s ability to mitigate the effects of air pollution originating both in and outside of park jurisdiction. How can clean air be restored in the Grand Canyon? The first step is to determine the amount and sources of N pollution, the next step is to find out how much N is too much (critical loads), and the final step is to develop strategies to reduce these inputs. Our research is focused on the first step. To address this, we explored potential indicators of N enrichment and the origins of pollution in Grand Canyon National Park.

The relative abundance of the stable isotope 15N can be used to quantify inputs of atmospheric N and its incorporation into plant tissues. Isotopes are variants of elements that differ in the number of neutrons in their nucleus, with 15N having one more neutron than the more common isotope 14N. Stable isotope analysis offers a way to trace pollution in the environment to its source since the ratio of N isotopes differs in predictable ways. Isotope ratios are expressed using δ (delta) notation in parts per thousand (‰). δ15N = [(Rsamplesub>/Rstandard) – 1] times 1000, where R is the molar ratio of the heavier to the lighter isotope (15N/14N) for the standard or sample (Evans and Ehleringer 1993). The additional neutron makes 15N heavier and this slightly changes its physical and chemical behavior. Differences in δ15N ratios of exhaust from power plants, vehicular emissions, and natural sources can help identify the origin of N pollution (Elliott et al. 2007). d15N ratios in NOx from automobiles with catalytic converters generally range from +3.4 to +5.7‰, compared to background atmospheric d15N signatures (Ammann et al. 1999; Pearson et al. 2000) of around 0.003‰. Plant tissue d15N signatures have been found to be 10% higher (more positive) in highly trafficked roads than in remote areas (Pearson et al. 2000). Based on these findings, we hypothesized that trees close to roadsides and high traffic areas in Grand Canyon National Park should have higher d15N ratios than trees that are farther from automobile emissions.

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