Calculations of the two-dimensional species-independent mixing coefficients for two-dimensional chemical models for the troposphere and stratosphere are performed using quasi-geostrophic potential vorticity fluxes and gradients from 4 years of National Meteorological Center data for the four seasons in both hemispheres. Results show that Kyy values for the winter lower stratosphere are broadly consistent with those currently employed in two-dimensional models, but Kyy values in the northern winter upper stratosphere are much larger than those usually used.