In late 2005, EPA promulgated AERMOD as a replacement for ISCST3. The new model includes state-of-the-art boundary layer parameterization techniques, convective dispersion, plume rise formulations, complex terrain/plume interactions as well as an improved building downwash algorithm. The new building downwash algorithm was originally developed by EPRI and was referred to as the PRIME model. Subsequently, the PRIME building downwash model was included in AERMOD. As with all models, their predictive ability is limited by the accuracy of the inputs (e.g., the old GIGO adage). For a complex site where building (or terrain) downwash dominates the dispersion, accurate building dimension inputs are essential for obtaining accurate concentration estimates. Inaccurate concentration estimates can either add significant operating costs for a facility or can result is air quality problems after a project is constructed.

The frequently used method for obtaining the building dimension inputs is through the use of BPIP (Building Profile Input Program). A site plan with building footprints and roof tier heights is input into BPIP and then an output file is generated that gives the building height, width, length and position for 36 wind directions. BPIP uses various assumptions to determine when two buildings combine, which tier height dominates the dispersion and where the building is positioned relative to the stack. What is really needed are the building dimensions that model the site dispersion accurately. These dimensions are referred to as “Equivalent Building Dimensions” (EBD) and are obtained through the use of wind tunnel modeling. This talk describes the process involved in determining EBD and compares the building dimension inputs obtained using BPIP with EBD values. The talk shows how BPIP tends to overestimate building size for sites with cylindrical structures, lattice structures or even complex rectangular structures. For a few shapes, BPIP underestimates the building dimensions.

Upwind terrain also creates wakes and eddies that can adversely affect plume dispersion. Currently, this effect is neglected in most permitting and dispersion modeling applications. This paper will also discuss how the EBD approach can provide building dimension inputs into AERMOD to account for the effect of upwind terrain.

AERMOD predictions with BPIP and EBD building dimension inputs are compared for various sites. The concentration estimates vary significantly depending upon the approach utilized. This variation points out that more accurate inputs may lead to different expansion, equipment and/or emission control decisions.