COS 6-1
A method to trade air nitrogen emission reductions for watershed nitrogen load reductions
The Chesapeake Bay TMDL sets limits on the load that can be delivered from tributaries and the air to the Bay. TMDL implementation measures take into account nitrogen deposition reductions to the watershed from current national air rules (such as CAIR) and resultant reductions in load to the Bay. For efficient management, due to expense, Bay states would like to take advantage of air emissions reductions of nitrogen oxides (NOx) that would occur in addition to national air rules and use these extra reductions in air-water trading. To support such trading, an approach based on modeling has been developed to define transfer coefficients that can be used to convert changes in air emissions to changes in air deposition and subsequently to changes in loads delivered to the Bay. The approach uses a special instrumented version of the Community Multiscale Air Quality (CMAQ) model that quantitatively attributes watershed and Bay oxidized nitrogen deposition for 12kmx12km grids back to the six individual Bay state NOx emissions, defining a transfer function. The Bay watershed model is used to estimate how the deposition change to the watershed converts to a change in nitrogen load to the Bay.
Results/Conclusions
The transfer functions (kg-N deposited/ton-N emitted) associating kg-N deposited to the watershed per ton-N emitted varied across the six states in relation to their position to the watershed: NY at 43, DE at 76, WV at 82, PA at 122, VA at 142 and MD at 165. The transfer functions estimating deposition to the Bay itself per ton emitted in a state vary, again in relation of proximity to the Bay: NY at 2.2, WV at 3.5, PA at 4.5, VA at 11, and MD at 12. Calculations were performed for a Virginia consent decree to further reduce annual emissions of NOx by 5,700 tons beyond national rules, which is 2.7% of the Virginia NOx emissions. The change in load to the Bay from deposition to the watershed and from direct deposition to the Bay was 17,600 and 18, 800 kg-N per year, respectively. The magnitude of potential load reductions was found to be sufficient to be of interest to the Bay states to trade air control for water control.