**1 9** **th Conference on Air Quality** **Modeling – A&WMA AB-3 Comments on Building and** **Terrain Downwash Issues Presented by Ron Petersen, CPP Inc.** **1415 Blue Spruce Drive Fort Collins CO 80524** **970 221 3371 rpetersen@cppwind.com** **2 Comment Areas** **• Problems with BPIP • AERMOD/PRIME Problem for Short/Large** **Buildings • AERMOD/PRIME Underestimation For** **Corner Vortex • Terrain Wake Effects** 

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docket-customerservice@epa.gov**4 Ultimately, PRIME Needs the** **Building Shape and Position that Places Stack in the Correct** **Snyder/Lawson Data Base Flow Region (i.e., Data Base Used to** **Develop Downwash Algorithms) Sampling of Snyder Data Base** **Streamline figures from: Snyder, W.H. and R.E. Laws on, Jr.: Wind Tunnel Measurements of Flow Fields in** **the Vicinity of Buildings; 8** **th Joint Conference on Appl. of Air Poll. Met. With A&** **WMA; AMS, Boston, MA, 1994; pp. 244-250 5** **Other considerations • Building downwash algorithms in AERMOD are designe** **d for simple rectangular buildings.** **• Building downwash algorithms in AERMOD only approp riate for** **certain building aspect ratios • Use of wind tunnel testing to determine Equivalent** **Building Dimensions (EBD) has been used to help solve the pr** **oblem – EBD guidance provided in Tikvart July 1994 Memorand** **um -“Thus, the analysis is viewed as a source characterization stu** **dy which generally has been considered under the purview of the Region** **al Offices.” – All testing to determine EBD under neutral stratif** **ication, similar to assumptions in Prime Algorithms.** **– With AERMOD/PRIME building location is also a vari able and new** **methods may be appropriate and has been used on rec ent studies.** **6 Picking Dominant Structure Example** **BPIP Input Information 7** **BPIP Input for Entire Site 8** **BPIP Input without upwind Tower 9** **Input needed to model dispersion based on EBD Study** **10 Mirant Power Station – Recent** **Example • AERMOD with BPIP predicting high** **concentrations. • AERMOD with Equivalent Building** **Dimensions gave lower concentrations and ones that agreed better with field** **observations. • Petersen, R. L., J. Reifschneider, D. Shea, D. Cra** **mer, and L. Labrie, “Improved building Dimension Inputs for AERMOD Modeling of the Mi** **rant Potomac River Generating Station,” 100** **th Annual A&WMA Conference, Pittsburgh, PA, June 2007** **• Shea,D., O. Kostrova, A. MacNutt, R. Paine, D. Crame r, L. Labrie, “ Model Evaluation** **Study of AERMOD Using Wind Tunnel and Ambient Measure ments at Elevated** **Locations,” 100 th** **Annual A&WMA Conference, Pittsburgh, PA, June 2007. 11** **AERMOD/PRIME Problem for Short/Large Buildings** **• The wake algorithms have only been developed/tested for limited building** **aspect ratios • Short/large industrial facilities fall** **outside this range. 12** **Problems with BPIP Short/Large Industrial Facility** **Wind Direction** **BPIP Building Dimensions: H = 17 m** **L/H = 23 H/W = 0.02** **See Red Footprint Limitations in** **PRIME Equations: 0.3<L/H<3.0** **Building dimensions used to develop PRIME cavity** **and wake dimensions: W = H and L/H = 0 - 4** **W = L and H/W = 1 - 3 stack** **13 Problems with BPIP** **Wind Direction** **BPIP Building Dimensions: H = 17 m** **L/H = 53 H/W = 0.03** **See Red Footprint stack** **14 AERMOD/BPIP Problem for** **Short/Large Buildings 310.9** **302.4 310.8** **315.2 294.1** **Stack Exit Temperature** **T s** **(K) 85.0** **70.4 67.3** **65.7 15.9** **Flow Rate (m^3/s)** **1.2 17.9** **2.24 25.91** **4 0.2** **17.8 1.07** **23.16 1** **1.6 11.1** **2.74 26.33** **2 17.8** **11.4 Stack Exit** **Velocity V (m/s)** **2.46 2.74** **Stack Inside Diameter** **d (m) 1.9** **21.34 5** **1.6 26.33** **3 Emission** **Rate (g/s) Source** **Height H** **s (m)** **Source Number** **Source Input Parameters: Other Model Input:** **• Meteorology: one year of surface and upper air data from nearby Airport** **• Flat terrain • Uniform Cartesian grid with 3600 receptors** **• Regulatory default dispersion options 15** **AERMOD/BPIP Problem for Short/Large Buildings** **24 hr maximum ground-level concentrations BPIP** **Max = 173.7 ug/m^3 EBD** **Max = 43.7 ug/m^3 16** **AERMOD Corner Vortex Issue • Current building wake** **equations do not account for corner** **vortex • Corner vortex causes** **higher concentrations than currently** **predicted in AERMOD.** **17 Corner Vortex Issue** **H b** **= 39 m (1:2:1)** **H b** **= 39 m (1:4:1)** **H b** **= 39 m (1:2:1) rotated 45°** **AERMOD/ PRIME** Maximum Ground-Level Concentration02468101214160 100 200 300 400 500 600 700 800 900Distance (m)

C/Q (ug/m^3 per g/s)Hb=39 (1:2:1)Hb=39 (1:4:1)Hb=39 (1:2:1)rotated 45 deg.**Stack Height: H** **s = 48 m** **Stack Top Wind Speed: U** **s = 12.5 m/s** **Stack Inside Diameter: d = 2.4 m** **18 Corner Vortex Issue** **H b** **= 39 m (1:2:1)** **H b** **= 39 m (1:4:1)** **H b** **= 39 m (1:2:1) rotated 45°** Maximum Ground-Level Concentration01020304050600 100 200 300 400 500 600 700Distance (m)

C/Q (ug/m^3 per g/s)Hb=39 (1:2:1)Hb=39 (1:4:1)Hb=39 (1:2:1)rotated 45 deg.**Stack Height: H** **s = 48 m** **Stack Top Wind Speed: U** **s = 12.5 m/s** **Stack Inside Diameter: d = 2.4 m** **Wind Tunnel Measurements** **19 Terrain Wake Effects Neglected** **• The GEP stack height regulation defines nearby terrain for the purpose of limiting** **stack heights. • Past EPA research shows that the effect** **of upwind terrain can be significant. Currently this effect is neglected.** **• Recent study 1)** **showed concentrations increased by a nearly a factor of two** **when terrain wake effect is accounted for using Equivalent Building Dimensions in** **AERMOD. • A method should be developed to** **determine when upwind terrain wake effects should be considered.** 1) Petersen, R., J. Reifschneider, R. Paine, K Schmidt, “Useof Equivalent Building Dimensions (EBD) toCharacterize Upwind Terrain Wake Effects forAERMOD,” 100thAnnual A&WMA Conference,Pittsburgh, PA, June 2007.**20 Terrain Wake Effects Neglected** **Variation of terrain amplification factor for sources at or downwind of the** **reattachment point (Castro, Snyder, and Lawson, (Atmospheric Environment,** **Volume 22, 1988) Contours of constant terrain amplification** **factors over a two-dimensional ridge (Snyder Atmospheric Environment,** **Volume 24, 1990) EPA Research: Terrain Wake Amplification** **Factor, A, Ranges from 1.2 to 8. 21** **Recommendations • Continue research on ways to improve** **BPIP so input dimensions match assumptions in algorithms.** **• If needed, update guidance on use of EBD in place of BPIP for AERMOD/PRIME.** **• Develop algorithms for the corner vortex situation.** **• Develop method for accounting for upwind terrain wake effects.**