At the interface between a turbulent boundary layer and a laminar boundary layer, laminar air is incorporated into the turbulent layer, but none of the turbulent air is incorporated into the laminar layer. This causes the turbulent layer to get thicker as it erodes the laminar layer. Such a process explains why turbulent boundary layers increase thickness (because laminar air from the free atmosphere and capping inversion are incorporated into the mixed layer) and why air pollutants are trapped in the mixed layer (because none of the turbulent mixed layer air can carry pollutants into the laminar air).