QUANTIFYING URBAN ATMOSPHERIC NITROGEN DEPOSITION FLUXES USING ION EXCHANGE RESINS

Atmospheric deposition is a significant, yet poorly characterized, flux of reactive nitrogen to terrestrial and aquatic ecosystems. Fossil fuel combustion in power plants and vehicle engines releases NO_x to the atmosphere that is subsequently oxidized to nitric acid (HNO₃) and then deposited to the Earth’s surface through precipitation or dry particulate deposition. Vehicle emissions in particular are concentrated in urban areas, however national monitoring networks (i.e., CASTNET for dry deposition, NTN for wet deposition) for reactive nitrogen deposition are generally located in rural areas far from urban sources.

In this project, we aim to quantify the total atmospheric nitrogen fluxes deposited across the city of Pittsburgh and compare those fluxes to those collected at the nearest rural national monitoring sites. We use ion exchange resins to collect wet and dry deposition. Ion exchange resins are small polyethylene polymer beads treated with counter ions to create charged exchange sites that selectively target anions or cations. These resins are packed into columns with funnels at the top to collect deposition. Dry particles settle on the funnel surface, and with precipitation, dry particles are washed into the column. Dissolved nitrate or ammonium in the precipitation replaces the counter ions and remains bound to the resin until eluted. Columns are placed at 8 sites for approximately one month, then eluted and analyzed for nitrogen concentrations.

Preliminary results indicate higher deposition fluxes in Pittsburgh relative to rural locations, as well as considerable intercity variability in nitrogen fluxes. Ion exchange resins are a useful method for field collection of atmospheric deposition that is relatively inexpensive, can integrate over many weeks without requiring precipitation volume, and impart no isotope fractionation effects. This research fills in knowledge gaps regarding local nitrogen deposition fluxes and variability and also allows for more accurate quantification of urban wet and dry nitrogen fluxes from the atmosphere.