Formulating chemical fugacity for general circulation models

Abstract

Fugacity is identical to the partial pressure of an ideal gas in air, however, for real gases fugacity is much more accurate than partial pressure in chemical equilibrium calculations. Since the first use of environmental chemical fugacity models in the late 1970’s they have been utilised extensively in the study of the local and global partitioning and distribution of chemicals in the environment. Global climate models are becoming an increasingly important tool in understanding the effects of climate change. Climate models are now being applied to understanding the distribution of persistent organic pollutants (POPs). POPs are a class of toxic chemicals that can bioconcentrate, bioaccumulate and biomagnify. Chemical fugacity is being used to calculate the inter-phase fluxes, however, the intra-phase fluxes are still being calculated using chemical concentration. Here we formulate equations that are suitable for the solution of intra-phase chemical fluxes in general circulation models using a chemical fugacity approach. Using a relaxation experiment in a one-dimensional (in the vertical) ocean, we examine how the the fugacity approach compares to the standard approach of using chemical concentration. We find that there are two major differences. Firstly, in the equilibrium calculation the standard method homogenises the concentration in the water column, while the fugacity approach has a depth dependent concentration with a uniform fugacity. Secondly, the timescale of equilibration is different, even though the turbulent diffusivity is the same in both simulations. These results suggest that climate models that model POPs should use a fugacity approach for the calculation of intra-phase fluxes, and not only inter-phase fluxes.