Model Estimated Components of Oxidized Nitrogen Dry Deposition Not Covered
By the Networks, with a Focus on Urban Deposition

Robin L. Dennis
NOAA/ARL, In Partnership with EPA/ORD
Atmospheric Modeling Division
US EPA, MD 243-04
Research Triangle Park, NC 27711

We already know we are missing a major fraction of reduced nitrogen deposition in our budget estimates by not measuring NH3 concentrations and not estimating NH3 dry deposition in the networks. We are also missing components of dry oxidized nitrogen (ox-N) deposition that are important to regional budgets and critical to urban deposition budgets. Wet deposition is more regional in character with no strong urban signal. Dry deposition shows a very strong urban signal. The degree to which we are missing deposition budgets will be illustrated with a high resolution (2 km grid) CMAQ study of Tampa and St. Petersburg, Florida, which have high population density. In regional and rural areas modeling suggests dry deposition of ox-N (stemming from NOX emissions) is modestly greater than wet deposition (factor of 1.5 for Chesapeake Bay watershed). In regional budgets ox-N deposition is primarily total-nitrate with a modest contribution from NOX, PANs and organic nitrate. In urban areas modeling suggests that dry deposition of ox-N is many times greater than wet deposition (factors of 2-10 and higher). In urban budgets, ox-N dry deposition is principally due to NOX deposition with a modest contribution from total-nitrate, even though the urban total-nitrate deposition is enhanced compared to its rural deposition. The relative contributions of NOX and total-nitrate to ox-N dry deposition are mirror images of each other across urban and regional space. Dry deposition budget numbers will be provided for the Tampa and St. Petersburg area to illustrate the effect of a strong urban signal and provide insight into the different sources of ox-N deposition. Currents networks are blind to this signal, both in terms of location of sites and species measured. With urban growth and fill-in, this signal will increase in importance and the delivery of N to coastal waters per unit of NOX emissions is expected to increase.