This is a post-processing artifact. In the EDC model, reaction rates are computed by the following equation: ———– R=C*(C_YI(c,t,i)-C_YI_EDC(c,t,i))/time_scale. ———– where C_YI is the result of species mass fraction from solving the transport equation and C_YI_EDC is the result from the chemistry solver. C_YI and C_YI_EDC are always the same if there is no reaction. These rates are not saved, however, and are calculated on-the-fly for post-processing. C_YI_EDC(c,t,i) is the result from the previous iteration while C_YI(c,t,i) is the result of the current iteration. These are not the same before solution is converged. As the solution gets converged, C_YI_EDC(c,t,i) and C_YI(c,t,i) get closer, and eventually the artifact will disappear.