AERMOD Tech Guide

Gaussian Plume Air Dispersion Model

1.1 Background

In 1991, the American Meteorological Society (AMS) and the U.S. Environmental Protection Agency (EPA) initiated a formal collaboration with the designed goal of introducing current planetary boundary layer (PBL) concepts into regulatory dispersion models. A working group (AMS/EPA Regulatory Model Improvement Committee, AERMIC) comprised of AMS and EPA scientists was formed for this collaborative effort. The authors of this document are all members of AERMIC.

In most air quality applications, one is concerned with dispersion in the PBL, the turbulent air layer next to the earth's surface that is controlled by the surface heating and friction and the overlying stratification. The PBL typically ranges from a few hundred meters in depth at night to 1 - 2 km during the day. Major developments in understanding the PBL began in the 1970's through numerical modeling, field observations, and laboratory simulations; see Wyngaard (1988) for a summary. For the convective boundary layer (CBL), a milestone was Deardorff's (1972) numerical simulations which revealed the CBL's vertical structure and important turbulence scales. Major insights into dispersion followed from laboratory experiments, numerical simulations, and field observations (e.g., see Briggs, 1988; Lamb, 1982; Weil, 1988a for reviews). For the stable boundary layer (SBL), advancements occurred more slowly. However, a sound theoretical/experimental framework for surface layer dispersion and approaches for elevated sources existed by the mid 1980's (e.g., see Briggs, 1988; Venkatram, 1988).

During the mid 1980's, researchers began to apply this information to simple dispersion models for applications. This consisted of eddy-diffusion techniques for surface releases, statistical theory and PBL scaling for dispersion parameter estimation, a new probability density function (PDF) approach for the CBL, simple techniques for obtaining meteorological variables (e.g., surface heat flux) needed for turbulence parameterizations, etc. Much of this work was reviewed and promoted in workshops (Weil, 1985), revised texts (Pasquill and Smith, 1983), and in short courses and monographs (Nieuwstadt and van Dop, 1982; Venkatram and Wyngaard, 1988). By the mid 1980's, new applied dispersion models based on this technology had been developed including PPSP (Weil and Brower, 1984), OML (Berkowicz et al., 1986), HPDM (Hanna and Paine, 1989), TUPOS (Turner et al., 1986), CTDMPLUS (Perry et al., 1989); later, ADMS developed in the United Kingdom (see Carruthers et al., 1992) was added as well as SCIPUFF (Sykes et al., 1996). AERMIC members were involved in the development of three of these models - PPSP, CTDMPLUS and HPDM.

By the mid-to-late 1980's, a substantial scientific base on the PBL and new dispersion pproaches existed for revamping regulatory dispersion models, but this did not occur. In a review of existing or proposed regulatory models developed prior to 1984, Smith (1984) reported that the techniques were many years behind the state-of-the-art and yielded predictions that did not agree.7 well with observations. Similar findings were reported by Hayes and Moore (1986), who summarized 15 model evaluation studies. The need for a comprehensive overhaul of EPA's basic regulatory models was clearly recognized. This need including a summary of background information and recommendations was the focus of an AMS/EPA Workshop on Updating Applied Diffusion Models held 24-27 January 1984 in Clearwater, Florida (see Weil (1985) and other review papers in the November 1985 issue of the Journal of Climate and Applied Meteorology).

In February 1991, the U.S. EPA in conjunction with the AMS held a workshop for state and EPA regional meteorologists on the parameterization of PBL turbulence and state-of-the-art dispersion modeling. One of the outcomes of the workshop was the formation of AERMIC. As noted above, the expressed purpose of the AERMIC activity was to build upon the earlier model developments and to provide a state-of-the-art dispersion model for regulatory applications. The early efforts of the AERMIC group are described by Weil (1992). In going through the design process and in considering the nature of present regulatory models, AERMIC’s goal expanded from its early form. In addition to improved parameterization of PBL turbulence, other problems such as plume interaction with terrain, surface releases, and urban dispersion were recognized as needing attention.

The new model developed by AERMIC is aimed at short-range dispersion from stationary industrial sources, the same scenario currently handled by the EPA Industrial Source Complex (ISC) Model, ISC3 (U.S. EPA, 1995). This work clearly has benefitted from the model development activities of the1980's especially in the parameterization of mean winds and PBL turbulence, dispersion in the CBL, and the treatment of plume/terrain interactions. Techniques used in the new model for PBL parameterizations and CBL dispersion are similar to those used in earlier models. Turbulence characterization in the CBL adopts "convective scaling" as suggested by Deardorff (1972) and included in most of the models mentioned above - PPSP, OML, HPDM, etc. Algorithms used in these earlier models were considered along with variants and improvements to them. In addition, the developers of OML met with AERMIC to discuss their experiences. Thus, much credit for the AERMIC model development is to be given to the pioneering efforts of the 1980s.