The plume-in-grid modeling approach for treating the subgrid scale dispersion, transport, and chemistry of pollutants contained in plumes released from major point source stacks is described. A plume dynamics model provides the plume rise height, plume position, and the horizontal and vertical plume dispersions in the subgrid scale phase when the pollutants are to be simulated by the Lagrangian reactive plume module. A coupling exists between the models as the grid concentrations provide boundary conditions for entrainment to the subgrid plume sections, and a feedback ocurrs when the plume size and chemical criteria indicate a plume's pollutants are to be transferred to the grid system for further simulation. The methods for simulating the plume processes are also presented and the input/output data involved with these two components of the Models 3 plume-in-grid approach are discussed.