When individual sources are added directly into a grid model, it is assumed that the emissions are instantaneously mixed into the grid where the source is located. In reality, stack gases are emitted in conical plumes that disperse and expand gradually as a function of stability, wind speed, and distance downwind of the stack. Thus, the assumption that individual sources are instantaneously mixed can cause over-dilution of the emissions into large grid cells. Also, when advection dominates as the transport mechanism for the emissions, the source's plume can be mixed too close to the source, when it should be mixed into air downwind of the source location.

Improvements have yielded a more realistic treatment of the subgrid-scale physical and chemical processes impacting individual point-source plumes. CCTM’s Plume Dynamics Model (PDM) treats plume rise, vertical/horizontal growth due to turbulence and sheer processes, and plume position. A Lagrangian Reactive Plume Module (LRPM) accounts for the relevant dynamic and chemical processes of the subgrid plumes and incorporates the plume material into the intercepted grid cells, when certain physical size or chemical criteria are met. PinG is designed for the larger scale grid-size simulations (36 km and 12 km), and it is not invoked for the smaller scale 4-km simulations. At fine resolutions, the emissions are released directly into the grid cells, and subgrid level plume resolution is not needed. It is assumed that instantaneous mixing occurs at this fine resolution.