BETR global – A geographically-explicit global-scale multimedia contaminant fate model

Abstract

We present two new software implementations of the BETR Global multimedia contaminant fate model. The model uses steady-state or non-steady-state mass-balance calculations to describe the fate and transport of persistent organic pollutants using a desktop computer. The global environment is described using a database of long-term average monthly conditions on a 15° × 15° grid. We demonstrate BETR Global by modeling the global sources, transport, and removal of decamethylcyclopentasiloxane (D5).

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.