Estimating the Importance of Near-Source Deposition of Nitrogen from Vehicle Emissions along Roadside Gradients

Roxanne Marino1, Robert W. Howarth1, Neil D. Bettez2, Eric A. Davidson3, Thomas J. Butler1,2, Marina Molodovskaya1 and Michael Horowitz1

Recent evidence suggests that total nitrogen deposition in urban and suburban areas can be substantially higher than estimated using data from national monitoring stations, which are located purposefully away from such areas, and which do not measure dry deposition of some important gaseous components. We are using passive samplers to estimate near-source dry deposition of the primary nitrogen gases (NOx, NH3) associated with vehicle emissions. At our primary study site on Cape Cod, MA, we examined summertime patterns of nitrogen pollution and estimated deposition along 5 to 150 meter gradients away from two roads (16,000 to 20,000 vehicles/day average), using gas concentration measurements and literature-based depositional velocities. Gaseous nitrogen deposition is highest near the road and falls exponentially with distance, decreasing 60 to 80% over 20 meters. Concurrent measurements of nitrogen in bulk (open field) and through-fall collectors were made along the same gradient, and the patterns of nitrogen concentration correlate well with the passive sampler data (see abstract and oral presentation by N. Bettez et al.). Our results suggest that vehicle traffic is likely a significant source of nitrogen deposition to high-traffic, suburban coastal areas. We estimate that within 50 meters of the road, gaseous dry nitrogen deposition may represent half of total nitrogen deposition at these sites, with two thirds as NH3, and one third as oxidized nitrogen. We have recently begun making air concentration measurements and gaseous dry deposition estimates for NH3 in two urban areas in New York State (New York City, Binghamton); preliminary data may be presented.

1 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY; 607-255-3572
2 Cary Institute of Ecosystem Studies, Millbrook, NY 845-677-7600
3 Woods Hole Research Center, Woods Hole, MA, 508-540-9900