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Fugacity approach

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ilkecelik's picture
Fugacity approach

Hello Usetox team,

I have a question regarding fugacity approach in Usetox. Rosenbaum et al. say Usetox is improved based on Mackay's fugacity approach. This model quantitates the mass transfer by advection, phase exchanges or first order decay, etc.  I believe Usetox considers only the first order decays among the multimedia. Is that correct?

USEtox Team
USEtox Team's picture
Mass balance modeling in USEtox

USEtox builds on a mass-based multimedia fate model, not on fugacity, and it is not stated in any of the official USEtox publications that USEtox "is improved based on Mackay's fugacity approach".

More specifically, USEtox uses chemical mass transfers among compartments and irreversible removals or transformations (biodegradation, photolysis, hydrolysis, etc). In this context, "first order” means that the rate of a transfer or transformation is directly proportional (by a fixed proportionality constant) to the inventory mass of a chemical. "Second order" means that the rate of a transfer or transformation depends both on the inventory mass of a chemical and the concentration (or intensity) of some other factor - microbes, photons, ozone or OH radicals, etc.

Most of the USEtox mass transfers by diffusion or advection are so strongly dependent on the chemical inventory mass as to be considered almost truly first order. Hence, these transfers are forced to be first order by defining a default value or seeking input for any potentially second order transfer factor - wind flow, river flow, rainfall, boundary layer thickness, etc. For transformation processes that have a tendency to be more strongly second order, first order is forced by fixing the second order factor to some default value. So even though the authors of USEtox are fully aware of second oder processes and include second order processes in the USEtox model, default assumptions are introduced to make the mass transfer and transformation processes appear to be first order in the matrix calculations. That is a default value is used for photon flux during daytime hours, microbe levels in soils, sediments and surface waters, and levels of ozone and OH radicals in the atmosphere. This is consistent across processes as implemented in USEtox and as further described in the official USEtox publications.