| Abstract: |
Between 1985 and 2005, the simulation period of the Phase 5 Watershed Model, atmospheric deposition loads of nitrate have tended to decrease overall in the in the Chesapeake watershed. Over this 20 year period, nitrate loads have decreased by about 30%, however, there is considerable variability across the Phase 5 domain with the greatest reductions occurring in the northern and western portions. Much of the reduction has been due to point source air emission reductions, particularly from Electric Generating Units (EGUs). More rapid declines are expected in 2010 as the Clean Air Interstate Rule (CAIR) controls on power plant emissions and the air quality standards for ozone and particulate matter come into their enforcement deadlines. Reductions from mobile sources are another contributor to the downward trend. Reductions from mobile sources will continue past the year 2020 as large off-road diesel and marine diesel fleets are replaced.
Simulating the past and future projected nitrogen deposition is the Chesapeake Bay Program’s Airshed Model, which is a combination of a regression model of wet deposition developed by Penn State, and a continental-scale air quality model of North America called the Community Multiscale Air Quality Model (CMAQ) for estimates of dry deposition. The CMAQ Model provides estimates of nitrogen deposition resulting from changes in emissions from utility, mobile, and industrial sources due to management actions or growth. The CMAQ Model scenarios include estimated nitrogen deposition in 2010, 2020, and 2030. The future year scenarios reflect emissions reductions from national control programs for both stationary and mobile sources, including the Clean Air Interstate Rule, the Tier-2 Vehicle Rule, the Nonroad Engine Rule, the Heavy-Duty Diesel Engine Rule, and the Locomotive/Marine Engine Rule.
Atmospheric deposition inputs from the current and future Airshed Model scenarios are maintained in the CCMP data library for the Phase 5 Model. Inputs include daily inputs of oxidized and reduced nitrogen, and sulfate. These inputs are available for any model practitioner and may be useful in TMDLs for nitrogen or acidity. Another deposition input to be added to the data library is mercury deposition, which will be available once the CMAC Model completes development of the mercury simulation in 2011. |
