Two levels of model sophistication exist: screening and refined dispersion modeling.
| Screening models may be used to
eliminate more extensive modeling for either the preliminary analysis phase or the full
impact analysis phase, or both. However, the results must demonstrate to the satisfaction
of the permitting agency that all applicable air quality analysis requirements are met. Screening models produce conservative estimates of ambient impact in order to reasonably assure that maximum ambient concentrations will not be underestimated. If the resulting estimates from a screening model indicate a threat to a NAAQS or PSD increment, the applicant uses a refined model to re-estimate ambient concentrations (of course, the applicant can select other options, such as reducing emissions, or to decrease impacts). Some examples of screening models are CAL3QHC, CAL3QHCR, COMPLEX1, CTSCREEN, LONGZ, RTDM3.2, SCREEN3, SHORTZ, TSCREEN, VALLEY and VISCREEN. |
| A refined dispersion model
provides more accurate estimates of a source's impact and, consequently, requires more
detailed and precise input data than does a screening model. The guideline model selected for a particular application should be the one which most accurately represents atmospheric transport, dispersion, and chemical transformations in the area under analysis. For example, models have been developed for both simple and complex terrain situations; some are designed for urban applications, while others are designed for rural applications. Some examples of refined dispersion models are BLP, CALINE3, CDM2, CTDMPLUS, ISC, ISC-PRIME, OCD, RAM and UAM-IV. |
| In many circumstances the guideline models known as Industrial Source
Complex Model Short- and Long-term (ISCST and ISCLT, respectively) are acceptable for
stationary sources and are preferred for use in the dispersion modeling analysis. Other guideline models, such as the Climatological Dispersion Model (CDM), may be needed to estimate the ambient impacts of area and mobile sources. Under certain circumstances, refined dispersion models that are not listed in the Modeling Guideline, i.e., non-guideline models, may be considered for use in the dispersion modeling analysis. The use of a non-guideline model for a PSD permit application must, however, be pre-approved on a case-by-case basis by EPA. Close coordination with EPA and the appropriate State or local permitting agency is essential if a non-guideline model is to be used successfully. |