Lessons learned from the Bay Region Atmospheric Chemistry Experiment (BRACE) and implications for nitrogen management of Tampa Bay

Results from air quality modeling and field measurements made as part of the Bay Region Atmospheric Chemistry Experiment (BRACE) along with related scientific literature were reviewed to provide an improved estimate of atmospheric reactive nitrogen (N) deposition to Tampa Bay, to apportion atmospheric N between local and remote sources, and to assess the impact of regulatory drivers on N deposition to Tampa Bay. Simulations using the Community Multiscale Air Quality model v4.4 modified with the University of California Davis aerosol module (CMAQ-UCD) provided a framework for this review. For 2002, CMAQ-UCD modeled atmospheric loading rates were 6910 metric tons N to the land surface of the watershed and 548 metric tons N to bay surface of the watershed, respectively. If an 18% transfer rate of atmospherically-deposited N from watershed to bay is assumed, then the corresponding atmospheric loading to Tampa Bay was 1790 metric tons N or 57% of the total N loading to the bay. From CMAQ-UCD modeling, oxidized N sources both within and outside Tampa Bayʹs watershed were important contributors to atmospheric N loading to the bay. Within the watershed, oxidized N emissions from mobile sources had a disproportionately larger impact than did power plant sources on atmospheric N loading. Predicted decreases in atmospheric N deposition to Tampa Bay by 2010 due to regulatory drivers were significant, and plausibly evident in recent declines in ambient air NOx concentrations in urban Tampa and St. Petersburg.