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Detecting Past and Future Trends in Nitrogen Deposition: A Modeling Study to Guide the
Placement of Future Monitoring Sites

Robert W. Pinder, Alice B. Gilliland, Robin L. Dennis
In partnership with NERL, USEPA


Previous studies have reported large changes in concentrations and deposition fluxes observed in the national monitoring networks (NADP, CASTNet, and STN). However, as the focus of regulated emission reductions turns from SO2 to NOx, it is important to re-assess the location of the network monitoring sites to ensure sufficient coverage in order to evaluate the effectiveness of the regulated emission changes. This research examines the sensitivity of monitored concentrations and deposition fluxes to emission changes for two cases: (1) previous emission reductions during the NOx SIP call (2002 – 2005) and (2) future projected emission changes planned for the Clean Air Interstate Rule (2010 and 2020). To estimate this sensitivity, we use the Community Multiscale Air Quality (CMAQ) model. First, using the well-known emission changes of the NOx SIP Call, we evaluate the CMAQ model predicted sensitivity using monitoring network observations of concentration and deposition for that time period. Second, we apply CMAQ to estimate concentrations and deposition for the future years of 2010 and 2020 in response the Clean Air Interstate Rule. By the year 2020, after substantial reductions in SO2 and NOx emissions, we estimate large changes in the composition and spatial extent of inorganic aerosol. This causes a highly variable change in the spatial distribution of reduced nitrogen (NH3 + NH4 +) deposition. Given only current monitoring network locations and speciation, this change will not be well quantified.

DISCLAIMER: The research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.