Development and Application of a New Model for the Atmospheric Transport and Surface Exchange of Semi-Volatile Organics Using the CMAQ Model Framework
PCBs and PCDD/Fs are toxic, persistent pollutants that can bioaccumulate in the food chain and become serious health hazards. Although the manufacture of materials such as PCBs has been banned in most parts of the world for some time, they are still found in significant concentrations in the environment. In view of this, it has become important to understand not only their sources, but also the mechanisms responsible for their transport in the environment. Since they are semi-volatile, it is necessary to consider their transport by atmospheric particulate matter as well as in the gas phase.
We have developed a capability to simulate the atmospheric behaviour of PCBs and PCDD/Fs within the framework of the CMAQ modelling system. To describe transport on particulate matter, we have added two gas/particle partitioning models — the Junge-Pankow adsorption model and the KOA absorption model to the basic CMAQ system. We have also included gas phase chemistry of these semi-volatile organic materials as well as their atmosphere/water surface exchange processes.
Using this modified model system, we have conducted simulations of the atomspheric behaviour of these materials for the years 2000 and 2002 on a domain covering most of North America. Validation studies show that both partitioning models give reasonable results when compared with available measurements of deposition rates and air concentrations. The simulations confirm that long range transport occurs by both gas phase and heterogeneous mechanisms. This causes these toxic materials to be deposited in pristine regions far from emission sources. In cases where they have entered the water table, large water bodies such as the Great Lakes can also become net sources.