We introduced the chapter on partial differential equations modeling air quality, starting with a very simple model, using two assumptions: wind blows at constant speed in only one direction and with no diffusion.
Relieving the first assumption we introduced the del operator to arrive at a compact form of the "advection equation". Then we interpreted the diffusion process and used the "Laplacian" to write down the general PDE. We made the connection with the equation of diffusion, or the heat equation, one of the six big PDEs.