In late summer of 2019, EPA released AERMOD version 19191 which included alpha building downwash options which are enabled by the following keywords: ORD_DWNW and AWMA_DWNW. The first option is based on research led by the EPA’s Office of Research and Development (ORD) and the second is based on the PRIME2 research effort funded by an industry group and reviewed by AWMA’s APM committee. Both options have been evaluated against field observations and generally good agreement has been found for PRIME2 as previously reported on by Guerra and Petersen. The PRIME2 model evaluation did however find a tendency for PRIME2 to overpredict concentration levels for the Bowline Point and Alaska North Slope field data bases. Subsequently, the plume rise algorithms in AERMOD/PRIME were evaluated and it was determined that the plume rise may be underpredicted which would account for the PRIME2 concentration overprediction tendency. A new entrainment constant was found that gave better agreement with plume rise observations and with the Alaska and Bowline Point field concentration observations. A new Alpha option switch, AWMAEntrain was added then to AERMOD. In addition, the PRIME2 building downwash theory implementation was improved so that the PRIME plume rise is used for all computations versus downwind distance instead of a representative PRIME plume rise which is used in the current PRIME2 version. Another new Alpha option switch AWMAUTURBHX was added to make use of this improvement. It is anticipated that these new switches will be available in the next AERMOD release. This paper discusses the new Alpha option switches that have been added to AERMOD as well as an evaluation of new improvements against the Colorado, Bowline Point, Alaska North Slope and PRCI Balko Oklahoma field observations. Q-Q plots were developed and predicted and observed Robust Highest Concentrations (RHC) were computed for all cases. With improved plume rise included, the new PRIME2 tended to provide the overall best agreement against field observations.
