Assessing Sources and Fluxes of Reactive Nitrogen Deposition to Urban Landscapes Using Ion Exchange Resins

Rebecca Forgrave1, Kassia Groszewski2, Elizabeth Boyer3 and Emily Elliott4

Previous research documents that urban reactive nitrogen deposition is both higher than rural areas and spatially variable, highlighting key knowledge gaps in understanding the sources, dynamics, and overall fluxes of reactive nitrogen deposition in urban areas. Traditional methods used by the NADP NTN and US EPA CASTNET, for wet and dry deposition fluxes respectively, require considerable cost to establish and maintain, thus making it difficult to establish enough sites to fully explore this phenomenon or spatial variability.  Ion exchange resins, however, offer a robust alternative as they are relatively inexpensive, integrate fluxes over many weeks without requiring exact precipitation volume, and can be used for isotopic analyses of resin eluents without fractionation.  In this project, ion exchange resin columns selectively targeting nitrate (NO3-) or ammonia (NH4+) were deployed at 7 monitoring sites (5 urban and 2 rural) around Pittsburgh and the nearest NTN-CASTNET site and were exchanged every 1-2 months over a one-year period. Column eluents were analyzed for concentrations of NO3- and NH4+ to calculate deposition fluxes, as well as dual nitrate isotopes (δ15N, δ18O) to examine emission sources and oxidation chemistry.  Fluxes were compared to those recorded by NTN and CASTNET at the nearest rural national monitoring sites to verify that the ion exchange resins are capturing the same signal as the established networks. Contrary to previous research, preliminary results indicate no significant differences in deposition fluxes between urban and rural sites for either NO3- or NH4+. δ15N and δ18O values of eluted NO3- were highly variable among network sites where both δ15N and δ18O values are generally higher in winter than summer. The range in δ15N and δ18O values and the seasonal patterns are consistent with prior reports linking temporal patterns in δ15N with increased power plant emissions during the winter months and δ18O value with seasonal changes in oxidation chemistry. This research seeks to narrow existing knowledge gaps regarding the fluxes of reactive nitrogen deposition to landscape, and how the high degree of spatial variability observed in urban systems is related to NOX and NH3 emission sources.  The preliminary results reported here demonstrate the need for finer spatial scale sampling to further understand causes of site variability, as well as the potential for ion exchange resins to be used as a tool for this type of analysis.


1University of Pittsburgh,
2University of Pittsburgh,
3Penn State University,
4University of Pittsburgh,