A Brief History of Applied Transport and Dispersion Models

A brief history is given of applied transport and dispersion (T&D) models, based on the section that I wrote on “Air Pollution Applications” in the AMS Monograph entitled “100 Years of Progress in the Atmosphere and Related Sciences”. Although air pollution was obviously bad prior to WW-I, the primary impetus for development of applied T&D models during and after WW-I was the widespread use of chemical weapons. The earliest applied models were analytical (e.g., Gaussian and K-theory) models used for surface boundary layer releases. This concern continued through WW-II, but, with the advent of nuclear weapons, new emphasis was placed on plume rise and dispersion of large thermal radiological explosions. Thus, the full troposphere and stratosphere had to be modeled. Deposition (wet and dry) was a main concern for many radiological substances. The next major step change was the Clean Air Act and amendments beginning in the 1970s, which initially focused on T&D models for industrial sources, such as fossil power plant stacks. The first applied models used by the EPA were analytical plume rise and Gaussian T&D models. Soon, computer codes were written to solve these equations and produce outputs at many spatial locations and every hour of the year. Regional pollution interests arose in the 1980s, initially fueled by the acid rain problem. EPA and others developed regional 3-D time-dependent Eulerian model systems to account for multiple sources and T&D and deposition over broad areas. Chemical mechanisms were included. This type of model is still in wide use, but, as computer speed and storage have increased, spatial domains have become larger, grid sizes have become smaller, time steps have been reduced, and more physical and chemical mechanisms are included. In the past few years, these regional air pollution models have become routinely linked with outputs of NWP models such as WRF.

At the same time that the use of regional models has grown, the puff, particle and plume T&D models for small scales and mesoscales have been improved. Several agencies and countries now have Lagrangian particle or puff models that are linked with an NWP model and are applied at all scales (mainly for no more than a few sources). In addition, great advances have been made in specialized applied T&D models for urban areas, complex terrain, and industrial chemicals with molecular weights significantly different from that of ambient air.

The brief history will include examples of T&D models used by various agencies and countries in each decade and category.