Figure D-2 is included to help the reader visualize the scenarios that are the end
points of the flowchart on Figure D-1. Figure D-2 contains more information
about important phenomena that must be addressed.
D.2.2 Dispersion Models. There is no "officially approved" model for the atmospheric
dispersion of hazardous chemical vapors. The two listed above, DEGADIS
(Reference 7) and SLAB (Reference 8), are in the public domain. The EPA has
developed a computer model, TSCREEN, that is suitable for several applications
in this area (Reference 9). Hanna et al. (Reference 10) evaluated 11 models
using data from nine field experiments. The EPA (Reference 11) and Touma
et al. (Reference 12) have also compared a number of heavy vapor models.
Each of the models mentioned in these references has various strengths and
weaknesses. The analyst must choose an appropriate model and justify its use.