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Observations of Reactive Nitrogen Species in the Atmosphere and Nitrogen Deposition in the Rocky Mountains

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July 18, 2012
Katie Benedict
Hosted by Jeff Collett (advisor), Sonia Kreidenweis, Sue van den Heever, Jay Ham (Soil and Crop Sciences)


Many national parks are experiencing increased nitrogen deposition due to increased emissions of reactive nitrogen, especially in the western United States. Excess nitrogen deposition can adversely impact ecosystem function, in some cases leading to degradation of water quality and forest decline. One region of particular interest is the Rocky Mountains, where large increases in wet deposition of oxidized and reduced nitrogen have been observed in recent decades. Here we present results from several field campaigns in Rocky Mountain National Park (RMNP) and a field campaign in Grand Teton National Park (GTNP) focused on identifying important nitrogen deposition pathways and factors that are contributing to nitrogen deposition.

At both RMNP and GTNP, measurements included precipitation chemistry and atmospheric concentrations of gases (NH3, HNO3) and particles (NH4+, NO3-, organic nitrogen). A year-long measurement period took place from November 2008-November of 2009 in RMNP, and two different reactive nitrogen source regions of Colorado: the Front Range urban corridor and eastern Colorado agriculture. Additional observations were made in RMNP in 2006 when a network of sites operated across the state for 5 weeks in the spring and summer and in 2010 when measurements were made from April-September in RMNP. In GTNP a network of sites measured air quality and atmospheric deposition across the park from April-September 2011.

To understand nitrogen deposition in RMNP we focused on understanding the spatial variability of reactive nitrogen concentrations across the state of Colorado. We observed large gradients in the reactive nitrogen species that reflected the different source regions across the state. In eastern Colorado, home to large agricultural operations, we observed high concentrations of ammonia and ammonium. Concentrations decreased moving westward toward the Front Range urban corridor and the Rocky Mountains. Concentrations of nitric acid, an important oxidation product of nitrogen oxides emissions, were highest in the Front Range urban corridor. Concentrations of gaseous ammonia and nitric acid were much lower in RMNP than at the sites to the east. Particle concentrations of ammonium and nitrate were generally lower in RMNP as well; however, concentration gradients were sometimes not as strong as for the gas phase compounds.

Upslope (easterly) winds were observed to be important for transporting higher concentrations of ammonium and nitrate into RMNP. These upslope events often generated heavy mountain precipitation and were important contributors to RMNP wet deposition of ammonium and nitrate. More than 50% of wet nitrogen deposition occurred during upslope wind events.

Wet deposition of ammonium and nitrate were the two largest reactive nitrogen deposition pathways in RMNP, yielding inputs of 1.97 kg N·ha-1 or 56% of total nitrogen deposition. Dry deposition of ammonia and wet deposition of organic nitrogen were the next most important deposition pathways; together they accounted for 40% (1.37 kg N·ha-1) of annual total nitrogen deposition. These two pathways are of special interest because they have not historically been monitored as part of regional deposition budgets. The remaining deposition pathways (dry deposition of nitric acid, and PM2.5 ammonium, nitrate, and organic nitrogen) accounted for approximately 3% of total nitrogen deposition.

In GTNP there was a strong gradient in ammonia concentrations, with higher average concentrations to the west (0.6 µg/m3) and lower average concentrations to the east (0.3 µg/m3), consistent with the presence of large agricultural operations west of the park. Concentrations of nitric acid, nitrate, and ammonium did not exhibit any clear spatial trends. Ammonia concentrations were higher at GTNP than at RMNP while PM2.5 nitrate and ammonium concentrations were similar in the two regions. Average nitric acid concentrations were similar between the two parks as well, with the exception of one high elevation GTNP site where higher concentrations were observed. Wet deposition of ammonium and dry deposition of ammonia were the largest reactive nitrogen deposition pathways in GTNP followed by wet deposition of nitrate and wet deposition of organic nitrogen.

Previous ecological assessments have led to the establishment of a critical load for wet deposition of inorganic nitrogen to RMNP and GTNP. A critical load is the maximum level of nitrogen input that can be sustained by an ecosystem without irreversible damage. Our observations reveal that the critical load is currently being exceeded in both RMNP and GTNP. It is important to recognize that substantial additional inputs of reactive nitrogen are occurring in both parks through dry deposition of ammonia and wet deposition of organic nitrogen. Neither pathway is currently considered in the U.S. critical load framework.

In both RMNP and GTNP organic nitrogen was an important component of deposition. Organic nitrogen comprised on average between 14-29% of aerosol nitrogen and 12-25% of nitrogen in precipitation across all of the studies. In aerosol, concentrations of all nitrogen species were higher when influenced by biomass burning. The fraction of PM2.5 organic nitrogen was also higher during biomass burning episodes, climbing as high as 48%. Analysis of aerosol organic nitrogen using liquid chromatography with electrospray ionization and time-of-flight mass spectrometry led to the detection of more than 404 different organic nitrogen compounds. The majority of these compounds were identified as positive ions in the electrospray, suggesting important contributions from compounds with a high proton affinity such as alkaloids or organic bases. Identification of specific chemical structures and emission sources will require additional research.