Eulerian chemical transport models (e.g. EPA’s CMAQ) cannot clearly show the source-receptor of polluted areas. Furthermore, Lagrangian back-trajectory analysis (e.g. NOAA HYSPLIT) cannot clearly show the polluted air mass; it could be polluted or clean air mass. Trajectory Grid (TG) by Choke et al. (2005) is a Lagrangian advection algorithm. It removes the numerical diffusion caused by Eulerian advection algorithms and also transports all species at once in each “packet” so saves execution time. TG also can help in the interpretation of back-trajectories as a direct clue for aerosols variation in Eulerian models by introducing “Back Trajectory and Concentration Output.” We implemented TG into EPA’s Community Multiscale Air Quality model version 5.2.