Air monitoring data at the water tower monitor (WTM) in Rhinelander, WI shows SO2 concentrations exceeding the 1-hour SO2 NAAQS and Expera Rhinelander Mill’s 63 m tall cyclone boiler stack is the primary contributor to the monitored exceedance. Making matters more complicated, the AERMOD predicted “design value” concentration at the WTM is in compliance and more than a factor of two lower than observations. Hence, a standard AERMOD modeling approach cannot be used to determine a compliance solution. After investigating the building geometry, it was noticed that the 38 m high Boiler 7 building corner is directly upwind of the stack when the wind blows toward the WTM. This results in the formation of corner vortices that enhance building downwash, an effect that is not accounted for in AERMOD. To develop a compliance solution, a multi-phased approach was used. First, wind tunnel modeling was conducted to determine an EPA approved 90 m GEP stack height that is taller than the 75 m formula GEP stack height. Next, compliance at the 90 GEP stack height was assessed using two alternate methods. Method 1 employed an alternate model, HYWINMOD, a validated hybrid wind tunnel/numerical model. Method 2 utilized AERMOD in an approved non-standard manner. AERMOD was run without building downwash affects but the results were adjusted to account for building downwash affects using wind tunnel modeling. Both methods provided very similar and manageable compliance solutions.