1 Introduction

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.

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),and 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 emerged by the mid 1980's (e.g., see Briggs (1988) and

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.

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