Abstract Methodologies that quantify numerical diffusion errors associated with advection algorithms are discussed. Diffusion errors in four flux-form advection schemes are analyzed, and the order of accuracy of each scheme is determined. The relationship between numerical diffusion and important parameters of air quality models such as the grid resolution, Courant number, and the wavelength of pollutant plume are described. Then, numerical diffusion introduced by the least diffusive advection scheme (among the four analyzed here) is compared with explicit horizontal diffusion introduced by two parameterizations, one for subgrid-scale diffusion and another for grid-scale diffusion. This quantitative analysis of numerical diffusion can be used for improving the formulation of horizontal diffusion as well as the treatment of subgrid-scale plumes in air quality models.