Development, evaluation, and implementation of building downwash and plume rise enhancements in AERMOD.Recent research has pointed out many reasons why the building downwash formulation in AERMOD needs to be updated due to: overly simplified equations for building wake development; equations that do not account for porous or streamlined structures; a discontinuity in the streamline equation; and over predictions when compared to field observations for buildings with a large footprint.  The PRIME2 building wake equations for turbulence intensity increase and velocity deficit were shown to agree better with wind tunnel observations than the current PRIME equations in AERMOD.  The new equations were documented in a journal article and were added to AERMOD's PRIME subroutine.  A new version of AERMOD was then compiled with the new enhanced turbulence and wind speed equations (PRIME2) for evaluation.  This paper provides details on the implementation of the PRIME2 code into AERMOD, PRIME plume rise enhancements, the field databases used to evaluate PRIME2, and the evaluation of PRIME2 against three field databases.  The paper shows that AERMOD with the PRIME2 building downwash equations and other enhancements provides the overall best agreement with field observations.Implications: While AERMOD/PRIME is supposed to provide accurate and unbiased estimates (within a factor of two), recent research has identified several problems with the current building downwash theory in AERMOD and comparisons against field observations have shown significant under and overpredictions.  This could lead AERMOD to underpredict or overpredict ground-level concentrations.  This theory has been added to the PRIME building downwash algorithm in AERMOD and is currently included as an Alpha option in AERMOD.  This paper evaluates that new theory against field observations and demonstrates that the updated theory provides better agreement with field observations than the current AERMOD. 
AERMOD
Because of these issues, a research study was initiated in late 2016 with an overall objective of improving the building downwash algorithms in PRIME.  Based on those measurements, new equations (PRIME2) were developed to better describe the turbulence increase and velocity deficit in building wakes for these structure types.  The PRIME2 building wake equations for turbulence intensity increase and velocity deficit were shown to agree better with wind tunnel observations than the current PRIME equations in AERMOD.  The new equations were documented in a journal article and were added to AERMOD's PRIME subroutine.  A new version of AERMOD was then compiled with the new enhanced turbulence and wind speed equations (PRIME2) for evaluation.  The key feature of the PRIME2 equations is that building wake enhanced turbulence decays rapidly back to ambient levels above the top of the building versus the current PRIME theory that has constant enhanced turbulence extending up to the height of the wake.  This paper provides details on the implementation of the PRIME2 code into AERMOD, PRIME plume rise enhancements, the field databases used to evaluate PRIME2, and the evaluation of PRIME2 against three field databases.  The paper shows that AERMOD with the PRIME2 building downwash equations and other enhancements provides the overall best agreement with field observations.Implications: While AERMOD/PRIME is supposed to provide accurate and unbiased estimates (within a factor of two), recent research has identified several problems with the current building downwash theory in AERMOD and comparisons against field observations have shown significant under and overpredictions.  This theory has been added to the PRIME building downwash algorithm in AERMOD and is currently included as an Alpha option in AERMOD. 
PRIME