AERMOD has the capability of predicting concentration levels due to building downwash effects through the use of the PRIME building downwash algorithm. While the model has been shown to provide reasonably accurate (EPA, 2003) concentration estimates for many building configurations, some situations have not been validated against field data bases and may not be modeled accurately. These situations include: 1) very complicated multi-tiered building geometries, porous or lattice type structures and cylindrical structures; 2) building aspects ratios significantly different than those for which the model was developed and tested; and 3) wind directions where the stack and upwind building corners are in alignment (causes intensified downwash due to corner vortices).

The building dimensions (height, width, length and position) are calculated using the Building Profile Input Program (BPIP) for input into AERMOD. The ultimate goal for BPIP should be to try and find the building shape and position that places the stack of concern into the correct Snyder and Lawson (1994) data base flow region (i.e., the data base used to develop the PRIME downwash algorithm). For the situations discussed above, there is no assurance that the current BPIP algorithms do this. For the above situations BPIP can pick the wrong dominant building, merge structures together incorrectly, position the building incorrectly, not account for lattice and/or cylindrical structures, or ignore the corner vortex. If the building dimension inputs are flawed, the concentration predictions will also be flawed. For example, the PRIME building downwash algorithm in AERMOD was developed and tested using concentration and flow field measurements obtained in wind tunnel simulations for several generic building and source configurations (Schulman et al., 2000). However, the building configurations evaluated had a limited range of building aspect ratios. Hence, the building downwash algorithms in AERMOD are only appropriate for length (L) to height (H) ratios of 0.3 < L/H < 3.0. Furthermore the building dimensions used to develop and test the PRIME cavity and wake dimensions are limited to: L/H = 0 to 4 and H/W = 1 to 3. In addition, the building wake algorithms do not account for wind directions with increased downwash due to corner vortices. Currently, AERMOD is applied to any building aspect ratio when in fact the downwash equations have specific limits of applicability.

To overcome these limitations, wind tunnel testing can be conducted to determine Equivalent Building Dimensions (EBD) that can be used in place of BPIP determined building dimensions. The EBD are essentially the building dimension and position that places the stack in the flow region that approximately matches the Snyder and Lawson (1994) data base flow region. This approach can help ensure that the building dimension inputs and the theoretical basis for the PRIME downwash calculations are in sync and identify enhanced downwash regions due to corner vortices.

This paper will provide “real-world' applications for each of the unique building configurations discussed above. The use of EBD for these applications will also be discussed along with comparisons of AERMOD predictions using BPIP and EBD inputs.

REFERENCES

EPA, “AERMOD: Latest Features and Evaluation Results,” USEPA Office of Air Quality, Planning and Standards, Research Triangle Park, North Carolina, EPA-454/R-03-003, June, 2003

Petersen, R. L., J. Reifschneider, D. Shea, D. Cramer, and L. Labrie, “Improved building Dimension Inputs for AERMOD Modeling of the Mirant Potomac River Generating Station,” 100th Annual A&WMA Conference, Pittsburgh, PA, June 2007

Petersen, R. L., A. Beyer-Lout, and J. Mitchell, “Is AERMOD/PRIME Overpredicting for Short Buildings with a Large Footprint?” on,” 102th Annual A&WMA Conference, Detroit, MI, June 2009

Shea, D., O. Kostrova, A. MacNutt, R. Paine, D. Cramer, L. Labrie, “Model Evaluation Study of AERMOD Using Wind Tunnel and Ambient Measurements at Elevated Locations,” 100th Annual A&WMA Conference, Pittsburgh, PA, June 2007.

Schulman, L.L., D.G. Strimaitis, J.S. Scire, “Development and Evaluation of the PRIME Plume Rise and Building Downwash Model,” Journal of Air & Waste Management Assoc., V 50, pp 378-390, 2000.

Snyder, W.H. and R.E. Lawson, Jr.: “Wind Tunnel Measurements of Flow Fields in the Vicinity of Buildings”; 8th Joint Conference on Appl. of Air Poll. Met. With A&WMA; AMS, Boston, MA, 1994; pp. 244-250