Isotopic Investigation of Reactive Nitrogen Deposition along a Highway Road Gradient

Katherine Middlecamp* and Emily M. Elliott

Though it is well understood that mobile emissions on highways create “hotspots” of air pollution, there is limited understanding of the effects of these emissions on the surrounding environment and human health. Because vehicle emissions constitute about half of Eastern U.S. NOx emissions, it is critical to identify the fate and impact of automobile emissions on near-road ecosystems. Previous studies have indicated that nitrogen emissions from automobiles are deposited locally, in the form of particulates and aerosols in dry deposition. Unlike emissions from more regional air pollution sources (e.g., smoke stacks) dry deposition from vehicles can deposit within 10’s to 100’s of meters from roadways. This spatial pattern of concentrated nitrogen deposition has important implications for near-road environments. For example, storm water infrastructure likely channels near-road deposition directly into surface waters. In addition, excess nitrogen can have adverse affects on near-road plant communities; studies have documented defoliation and changes in community structure due to nitrogen pollution near roadways.

This study uses stable isotopes of nitrogen in plant tissue and dry nitrogen deposition to examine the extent of nitrogen deposition along a gradient perpendicular to a major highway. The gradient spanned 400 meters perpendicular to a heavily trafficked section (33,000 cars and trucks per day) of I-76 in Rector, Pennsylvania. Passive samplers were deployed monthly for five months at six sites along the gradient to collect dry deposition of three N species: NH3, HNO3 and NO2. The passive samplers were analyzed for concentration and isotopic composition. Additionally, we used two types of grasses (Agrostis perennans and Panicum virgatum) as biomonitors to examine the relative sources of nitrogen to plant tissue along the road gradient. The plants were grown in pots with similar potting soil, deployed along the gradient, and sampled once a month for five months for isotopic analysis.

Initial results indicate that concentrations of the three N species were significantly higher within 20 meters of the road. Furthermore, we expect that isotopic signatures of the dry deposition and plant tissue will reflect an automobile source near the highway, and a biogenic source at the far end of the gradient.

* Corresponding author- University of Pittsburgh Department of Geology and Planetary Science 4107 O'Hara Street SRCC, Room 200 Pittsburgh PA 15260-3332, , (412)779-2693