Total Deposition of Nitrogen and Sulfur in the United States

Gary Lear1 and Donna Schwede2

Atmospheric deposition of nitrogen and sulfur causes many deleterious effects on ecosystems including acidification and excess eutrophication.  Assessments to support development of strategies to mitigate these effects require spatially and temporally continuous values of nitrogen and sulfur deposition.   In the U.S., national monitoring networks exist that provide values of wet and dry deposition at discrete locations.  While wet deposition can be interpolated between the monitoring locations, dry deposition cannot.  Additionally, monitoring networks do not measure the complete suite of chemicals that contribute to total sulfur and nitrogen deposition.  Regional air quality models provide spatially continuous values of deposition of monitored species as well as important unmeasured species.  However, air quality modeling values are not generally available for an extended continuous time period.   Air quality modeling results may also be biased for some chemical species.  We developed a novel approach for estimating dry deposition using data from monitoring networks such as the Clean Air Status and Trends Network (CASTNET), the National Atmospheric Deposition Program (NADP) Ammonia Monitoring Network (AMoN), and the Southeastern Aerosol Research and Characterization (SEARCH) network and modeled data from the Community Multiscale Air Quality (CMAQ) model.  These dry deposition values estimates are then combined with wet deposition values from the NADP National Trends Network (NTN) to develop values of total deposition of sulfur and nitrogen.  Data developed using this method are made available via the CASTNET website.  Future plans include the use of CMAQ 5.0 with the ammonia bidirectional flux module, data from 1-in-3 monitoring networks (e.g., IMPROVE and CSN), and additional evaluations and comparisons with other estimates of total N and S deposition.


1Clean Air Markets Division, Office of Atmospheric Programs, U.S. Environmental Protection Agency,
2Atmospheric Modeling and Analysis Division, National Exposure Research Laboratory, USEPA,