Many atmospheric releases of hazardous materials involve chemical reactions that occur within a few kilometers of the source. Such reactions can transform and potentially neutralize the original release compounds. In these cases, accurately resolving the air flow around nearby structures and over surrounding topography can be critical to correctly predicting material dispersion and, thus, the extent of the hazard. To approach this challenge, we have incorporated the reaction solver SMVGear II into our building-scale massively-parallel computational fluid dynamics code FEM3MP. In this talk, we describe the physical requirements for this coupling to make sense, detail the testing of the model, and list the steps from a set of chemical reactions to the simulation of a chemically evolving plume. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.