Short communicationBETR global – A geographically-explicit global-scale multimedia contaminant fate model
Introduction
Multimedia mass-balance models are well-established as scientific and decision-support tools for understanding the behavior of chemical pollutants in the environment (MacLeod et al., 2010), especially persistent organic pollutants (Wania and Mackay, 1999). Global-scale multimedia mass-balance models have been particularly important in establishing the link between chemical emissions in industrialized regions and their presence in the Arctic. Global models that describe the environment as a set of latitudinal zones, and are spatially discretized only along a north–south axis have been successfully applied to study transport of persistent chemicals to the Arctic (Scheringer and Wania, 2003). In 2005, the BETR Global multimedia contaminant fate model was introduced (MacLeod et al., 2005a). It has a similar structure as the latitudinal zone models, but describes the global environment on a 15° × 15° grid.
The 2005 version of BETR Global was used to model the global fate and transport of polychlorinated biphenyls (PCBs) (MacLeod et al., 2005a), and the transport and deposition of a set of persistent organic substances to the North American Great Lakes (MacLeod et al., 2005b). A modified version of the model capable of simultaneously modeling the fate and transport of a weak acid and its conjugate base was developed to study the global mass balance and transport of perfluorinated acids (Armitage et al., 2009a, Armitage et al., 2009b). The model was recently used to analyze the global mass budget of PCBs and the effects of a climate change scenario on their steady-state global distribution (Lamon et al., 2009). Most recently, as part of the work of the Task Force on Hemispheric Transport of Air Pollutants (TF HTAP), BETR Global has been applied to quantify inter-continental source–receptor relationships for persistent organic pollutants in support of the Convention on Long-range Transboundary Air Pollution (CLRTAP) (Gusev et al., 2010).
Several incremental modifications have been made to the BETR Global model since 2005, including changes to the model algorithms and updates of the model’s database of environmental characteristics. Among the most important modifications of the model since 2005 are; 1) the introduction of seasonally-variable hydroxyl radical fields in the atmosphere, 2) algorithms to account for the intermittent nature of contaminant scavenging by rainfall in both steady-state and dynamic calculations, and 3) improvements in the efficiency of solving the system of mass-balance equations.
Our purpose here is to introduce two updated software implementations of the BETR Global model, BETR Global V.2.0, and BETR-Research that are being made publically available for direct download. BETR Global 2.0 is coded in Visual Basic for Applications (VBA) as an add-on for Microsoft Excel, and was developed from the original code-base of the 2005 version. BETR-Research is a re-implementation of the model in Python which is more computationally efficient, and more easily modifiable, but which lacks the graphical user-interface of the VBA version. During the development of BETR-Research a thorough code-review was performed on both BETR implementations. We illustrate the new version of the model using a case study of the global fate and transport of decamethylcyclopentasiloxane (D5), which can be replicated using the VBA version of BETR Global 2.0 by following the tutorial on the BETR Global website (http://sites.google.com/site/betrglobal/home). BETR-Research can be downloaded from http://betrs.sourceforge.net.
Section snippets
Model software and formulation
The BETR modeling framework allows geographically-explicit contaminant fate models to be assembled by linking a set of regional fugacity-based multimedia mass-balance models together. The fugacity concept is described in detail by Mackay (2001), and the general structure of models based on the BETR framework is described by MacLeod et al. (2001). The BETR Global parameterization of the framework describes the global environment on a 15° × 15° grid as a set of 288 multimedia regions linked by
Environmental database
The BETR Global model includes a database of information about the global environment used in model calculations. The environmental parameters in the database have been selected to represent long-term average conditions. Some environmental parameters are fixed values derived from GIS databases, such as the area of each region, and the fractions of the surface covered by soil, vegetation, and water. Other parameters, such as temperature, atmospheric hydroxyl radical concentrations, and air- and
Range of applicability
Like all models that use two-point upwind differencing to approximate the derivative of concentrations in space, BETR Global overestimates transport of chemicals due to numerical diffusion (Warren et al., 2009). The extent of overestimation is largest when steep concentration gradients exist at spatial scales smaller than the model regions. This can occur, for example, shortly after emission from a point source into an uncontaminated environment, or for substances with half-lives in air and
Illustrative application to decamethylcyclopentasiloxane (D5)
As an illustration of the BETR Global model, we applied the model to describe the fate and transport of decamethylcyclopentasiloxane (D5, CAS#: 541-02-6) in the global environment. This case study is very similar to a recent study using the Danish Eulerian Hemispheric Model (DEHM) (McLachlan et al., 2010). Here we provide only a summary of the input data and model results for the D5 case study. A detailed tutorial that guides users through these calculations using BETR Global 2.0 is available (//sites.google.com/site/betrglobal/home
Conclusions
BETR Global 2.0 and BETR-Research are now available for public download. BETR Global 2.0 has a graphical user-interface, while BETR-Research is more computationally efficient and the code is more amenable to modification. Both versions of the model can be applied to analyze the global-scale fate and transport of persistent organic pollutants at a process-level, and to evaluate global mass budgets based on emission estimates and physico-chemical properties of chemicals.
Acknowledgments
Development of BETR Global has been supported by research grants from the Natural Sciences and Engineering Research Council (Canada), Region 5 of the United State Environmental Protection Agency, The Swiss Federal Office for the Environment (FOEN) Air Quality Division, and the ArcRisk European Community 7th Framework Programme Project (http://www.arcrisk.eu). This work was also supported by the Director, Office of Science, Office of Biological and Environmental Research, Climate and
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Present address: Department of Applied Environmental Science, Stockholm University, SE-106 91 Stockholm, Sweden.