Do Persistent Organic Pollutants Reach a Thermodynamic Equilibrium in the Global Environment?

Abstract

Equilibrium partitioning between different environmental media is one of the main driving forces that govern the environmental fate of organic chemicals. In the global environment, equilibrium partitioning is in competition with long-range transport, advective phase transfer processes such as wet deposition, and degradation. Here we investigate under what conditions equilibrium partitioning is strong enough to control the global distribution of organic chemicals. We use a global multimedia mass-balance model to calculate the Globally Balanced State (GBS) of organic chemicals. The GBS is the state where equilibrium partitioning is in balance with long-range transport; it represents the maximum influence of thermodynamic driving forces on the global distribution of a chemical. Next, we compare the GBS with the Temporal Remote State, which represents the long-term distribution of a chemical in the global environment when the chemical's distribution is influenced by all transport and degradation processes in combination. This comparison allows us to identify the chemical properties required for a substance to reach the GBS as a stable global distribution. We find that thermodynamically controlled distributions are rare and do not occur for most Persistent Organic Pollutants. They are only found for highly volatile and persistent substances, such as chlorofluorocarbons. Furthermore, we find that the thermodynamic cold-trap effect (i.e., accumulation of pollutants at the poles because of reduced vapor pressure at low temperatures) is often strongly attenuated by atmospheric and oceanic long-range transport.