Improved reduced nitrogen deposition estimates in the United States: Spatial variability of ammonia

Melissa Puchalski1, Donna Schwede2, John T. Walker3, Kristen Foley2, Gary Lear1, Gregory Beachley1, Richard Haeuber1

1 US Environmental Protection Agency, Office of Air Programs, 1200 Pennsylvania Ave NW, Washington D.C. 20460,, Email:

2US Environmental Protection Agency, National Exposure Research Laboratory, 109 T.W. Alexander Dr, Research Triangle Park, N.C. 27709

3US Environmental Protection Agency, National Risk Management Research Laboratory, 109 T.W. Alexander Dr, Research Triangle Park, N.C. 27709


The Clean Air Status and Trends Network (CASTNET) is the only long-term monitoring network in the United States that provides estimates of dry deposition for sulfur and nitrogen species. CASTNET measures ambient concentrations that are combined with modeled deposition velocities to estimate dry deposition at more than 90 sites. Until recently, concentrations of NH3 were not routinely measured at CASTNET sites, resulting in a significant gap in the total nitrogen budget. Between 2008 and 2015, more than 60 National Atmospheric Deposition Program (NADP) Ammonia Monitoring Network (AMoN) sites were deployed at CASTNET sites, providing ambient NH3 concentrations.

Estimates of total (wet + dry) deposition are provided by combining measured ambient concentrations and wet deposition fluxes with modeled estimates of dry deposition velocities and fluxes for unmeasured species (Schwede and Lear, 2015). The NADP’s total deposition (TDEP) hybrid method combines data from several routine monitoring networks and the Community Multi-scale Air Quality (CMAQ) model. The deposition fluxes are interpolated to create a continuous surface using inverse distance weighting (Figure 1). NH3 may not be well characterized in the model and influences from emissions sources may impact the radius of influence between sites. This paper describes results from an NH3 spatial variability study designed to improve the spatial interpolation methods used in the TDEP maps. During this study, supplemental passive NH3 monitoring sites were randomly distributed around the Bondville, IL (IL11) and Fort Collins, CO (CO13) AMoN sites for one year. Results from this analysis will improve future gridded total deposition estimates.