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Nitrogen Deposition
Research Programs
Targeted applications that link the state-of-the-art models of nitrogen deposition with watershed and estuarine models in support of assessments provide an important test bed for learning what capabilities are required of the atmospheric models to support multimedia assessments. Past linkage work has shown that it is important that atmospheric deposition be able to be related to climatological averages. This work has also shown atmospheric models fill a critical gap for provision of dry deposition estimates for both oxidized and reduced nitrogen to provide the needed input of total, wet + dry, atmospheric deposition. This integrated experience contributes to multimedia science and helps define better predictive multi-stressor, open architecture approaches.
Airsheds: Oxidized-Nitrogen Deposition into Coastal Estuaries
Multimedia Modeling
Using the procedure developed for the Chesapeake Bay and outlined in Dennis (1997), airsheds for 20 coastal watersheds along the East and Gulf Coasts were developed. These oxidized nitrogen airsheds are expected to be available on the Division’s multi-media web site. This work is presented in the NOAA assessment of atmospheric deposition to coastal estuaries (Paerl et al., 2001).
Tables
Cheseapeke Bay
Atmospheric deposition of nitrogen to the Chesapeake Bay watershed and Bay surface is important. The atmosphere is estimated to contribute a quarter to a third of the nitrogen loading to the Bay, affecting current conditions and needing to be addressed in Bay restoration efforts. Chesapeake Bay has been placed on EPA’s list of impaired waters, with a TMDL plan required in 2011. The Chesapeake 2000 agreement calls for preempting the need for a TMDL plan by cleaning up the Bay by 2010. The Bay 2007 Re-evaluation is a critical step in the process towards the 2010 cleanup and delisting. To provide the best modeling science for the 2007 Re-evaluation, a major upgrade of the Chesapeake Bay Watershed model will be used as well as the latest version of CMAQ and the upgraded Aggregation method to create climatological average deposition. This atmospheric modeling will be a major update from earlier use of the Extended RADM and the original Aggregation method. The CMAQ modeling for the 2007 Re-evaluation has two major foci: (1) development of scenarios estimating the deposition reductions expected by 2010 and 2015 due to Clean Air Act regulations, such as the Clean Air Interstate Rule, and (2) estimation of the relative contribution the nitrogen oxide (NOx) emissions from each Bay state make to the atmospheric deposition of oxidized nitrogen to the Bay watershed and Bay surface.
Tampa Bay
Atmospheric deposition is extremely important to Tampa Bay’s nitrogen loading. Direct deposition to Tampa Bay is central and is estimated to be second only to storm water runoff, but a portion of storm water runoff is due to atmospheric deposition (wet and dry). Tampa is an excellent example of a coastal bay where the existence of sea salt is a determining factor in the rate of local nitrogen dry deposition. Tampa Bay is unusual in that a majority of the oxidized nitrogen deposition to Tampa Bay is estimated to come from local sources (approximately 60%). Two of the largest utility emitters of NOx emissions in the country are located at the edge of Tampa Bay. They have, through a consent decree, agreed to reduce their NOx emissions by up to 95%. A science version of CMAQ, CMAQ-AIM incorporates sea salt. The Florida Department of Environmental Protection (FDEP) organized, with EPA/ORD help, the Bay Regional Air Chemistry Experiment (BRACE) field study that took place in Tampa during May 2002. One key objective of BRACE is to provide field data to evaluate CMAQ-AIM. The three major thrusts of the Tampa Bay Model Evaluation and Application task are: (1) to evaluate CMAQ-AIM against the BRACE May 2002 data and make any model refinements that may be required, (2) to assess the relative contributions from the different emissions sectors, particularly mobile sources and utilities, to the annual oxidized nitrogen deposition to Tampa Bay, and (3) to assess the change in annual deposition to Tampa Bay that could be attributed to the NOx emissions reductions by the two power plants on its shores. The Tampa Bay assessment will be conducted in concert with FDEP and the Tampa Bay Estuary Program.