Impacts of International Emissions on Lateral Boundary Conditions Used in U.S. Regional Air Quality Modeling Analyses

It is expected that future efforts to effectively manage ozone, PM, and regional haze concerns will require estimates of the impacts of international emissions on air quality in the U.S.  EPA has applied the Community Multiscale Air Quality (CMAQ) model over the Northern Hemisphere for the year of 2011 as part of an effort to begin to assess the impacts of international emissions on the lateral boundary conditions used in U.S. regional scale modeling analyses.  Utilizing a combination of the latest National Emissions Inventory (NEI) estimates for the U.S. and available emissions input data sets outside the U.S., along with meteorological inputs simulated by the Weather Research Forecast (WRF) model, an annual hemispheric CMAQ simulation has been completed and compared against pre-existing global modeling simulations and evaluated against observed data.  Importing the hemispheric CMAQ results into a 12-km, annual, regional scale modeling simulation at the lateral boundaries, results in improved model performance for surface level ozone especially along the Gulf Coast region.  A series of sensitivity tests were completed looking at the impacts of reductions in upstream emissions on U.S. air quality levels.  These results are discussed in the context of U.S. implementation strategies designed to effect attainment of the National Ambient Air Quality Standards (NAAQS).