A modified K model for chemically reactive species in the planetary boundary layer

A modified K model is proposed for the diffusion of chemically reactive species. Eddy diffusivity is modified due to chemical reactions. The effect of cross diffusion is introduced; that is, the scalar flux of a species depends on the scalar gradients of other species. One‐dimensional calculations are carried out for the convective boundary layer by using a K model, a modified K model, and a second‐order model. Three types of simple chemical reactions are investigated. It is shown that results of the modified K model agree well with those of the second‐order model. Profiles of mean mixing ratios and scalar fluxes are compared between the K model and the modified K model. For the simplest chemical reaction the difference between the two models is large when the chemical reaction time scale is shorter than the turbulent time scale. For the reaction of NO‐NO 2 ‐O 3 triad, profiles of the scalar fluxes are shown to be very different between the two models.