(121b) Multi-Pollutant Plume-in-Grid Modeling

“One-atmosphere” grid models are now being widely used to predict the impacts of emission controls on the concentrations and depositions of pollutants such as ozone (O₃), fine particulate matter (PM_2.5), mercury (Hg), and air toxics. A major limitation of grid models is their inability to correctly represent the near-source transport and chemistry of sub-grid scale emissions, e.g., elevated point source emissions or roadway emissions. The horizontal resolution of grid models, typically from a few kilometers to tens of kilometers, is inadequate to resolve these emissions and their sub-grid scale impacts. A common approach to address this limitation is to use Plume-in-Grid (PinG) modeling, in which a sub-grid scale representation of the releases from the sources of interest is incorporated within the 3-D model. This approach has traditionally been used in O₃ modeling studies. Here, we describe the evolution of such a PinG model, initially developed for O₃, and subsequently extended to treat PM_2.5, mercury, and most recently, air toxics from roadway emissions. The model consists of a host grid model and an embedded plume model. The host grid model is the U.S. EPA Community Multiscale Air Quality (CMAQ) model. The embedded plume model is the Second-order Closure Integrated puff model (SCIPUFF) with CHEMistry (SCICHEM). Both the grid model and the plume model use identical treatments for gas-phase chemistry, PM processes, and aqueous-phase chemistry. The PM treatment is based on the Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID). We present typical results from a number of studies with the model to illustrate the importance of plume-in-grid modeling for a variety of sources and pollutants.