You are absolutely correct. However, in order to model the potential plume transport, I specified a persistent chemical to represent the mix. Excluding deposition (both wet and dry) and complex chemical interactions, this model shows where the atmosphere may have transported the plume during the dispersion process. It's not perfect, but it does provide insight into who may have been affected downwind of the chemical disaster.

HYSPLIT does provide the ability to specify deposition if you check out the model for yourself, but I didn't feel that there was enough information to specify that variable. Thanks for your scientific inquiry; hopefully government officials release an official dispersion map at some point.