AERMOD: A Dispersion Model for Industrial Source Applications. Part II: Model Performance against 17 Field Study Databases STEVEN G. PERRY,* ALAN J. CIMORELLI,  ROBERT J. PAINE, # ROGER W. BRODE, @ JEFFREY C. WEIL, & AKULA VENKATRAM,** ROBERT B. WILSON,  RUSSELL F. LEE, @@ AND WARREN D. PETERS && *Air Resources Laboratory, NOAA, and National Exposure Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina  U.S. Environmental Protection Agency Region 3, Philadelphia, Pennsylvania # ENSR International, Westford, Massachusetts @ MACTEC Federal Programs, Inc., Durham, North Carolina & Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado **College of Engineering, University of California, Riverside, Riverside, California  U.S. Environmental Protection Agency Region 10, Seattle, Washington @@ Charlotte, North Carolina && OAQPS, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina (Manuscript received 21 January 2004, in final form 26 October 2004) ABSTRACT The performance of the American Meteorological Society (AMS) and U.S. Environmental Protection Agency (EPA) Regulatory Model (AERMOD) Improvement Committee’s applied air dispersion model against 17 field study databases is described. AERMOD is a steady-state plume model with significant improvements over commonly applied regulatory models. The databases are characterized, and the per- formance measures are described. Emphasis is placed on statistics that demonstrate the model’s abilities to reproduce the upper end of the concentration distribution. This is most important for applied regulatory modeling. The field measurements are characterized by flat and complex terrain, urban and rural condi- tions, and elevated and surface releases with and without building wake effects. As is indicated by com- parisons of modeled and observed concentration distributions, with few exceptions AERMOD’s perfor- mance is superior to that of the other applied models tested. This is the second of two articles, with the first describing the model formulations. 1. Introduction In 1991, the U.S. Environmental Protection Agency (EPA) in conjunction with the American Meteoro- logical Society (AMS) formed the AMS and EPA Regulatory Model (AERMOD) Improvement Com- mittee (AERMIC) with the expressed purpose of in- corporating the current understanding of the planetary boundary layer (PBL) into a state-of-the-art applied dispersion model, AERMOD. AERMIC’s work clearly has benefited from the model development activities worldwide over the past few decades, especially in the parameterization of mean winds and PBL turbulence, dispersion in the CBL, the treatment of plume/terrain interactions, plume– building interactions, and urban dispersion. AERMOD (Cimorelli et al. 2003) is a steady-state plume model aimed at short-range (up to 50 km) dis- persion from stationary industrial-type sources—the same scenarios that are currently handled by the EPA’s Industrial Source Complex Short-Term model (ISCST3) (U.S. Environmental Protection Agency 1995). The meteorological conditions are assumed to be steady during the modeling period (typically 1 h) and horizontally homogeneous. Vertical variations in the PBL, however, are incorporated into the model’s pre- dictions. For flow in complex terrain AERMOD incor- porates the concept of a dividing streamline (Snyder et al. 1985). The model considers the influence of building wakes on plume rise and dispersion using the algo- rithms of the Plume Rise Model Enhancements (PRIME) model (Schulman et al. 2000). In urban areas, AERMOD accounts for the dispersive nature of the “convective like” boundary layer that forms during nighttime conditions by enhancing the turbulence re- sulting from urban heat flux (Oke 1978, 1982). This paper is the second of two describing the newly developed AERMOD modeling system. Cimorelli et al. (2005, hereinafter Part I) describe the model formula- Corresponding author address: Steven G. Perry, U.S. Environ- mental Protection Agency, MD-81, Research Triangle Park, NC 27711. E-mail: perry.steven@epa.gov 694 JOURNAL OF APPLIED METEOROLOGY VOLUME 44 © 2005 American Meteorological Society JAM2228