Dear all,
Focusing on freshwater eco-toxicity, the effect factor in USEtox is expressed as the change in PAF per change of concentration. This I understand, but what I do not understand is why the volume (m³) is incorporated into the CTU? I do not understand the physical meaning of CTU, a product of PAF, volume and time. What does this represent? How can you go in one step from a concentration related impact (PAF) to an mass related impact (CTU)? I expected that the concentration has to be used.
Wkr,
Dorian Heyndrickx
The characterization factor for aquatic ecotoxicity impacts (ecotoxicity potential) is expressed in comparative toxic units (CTU) that represents an estimate of the potentially affected fraction of species (PAF) integrated over time and volume, per unit mass of a chemical emitted. Hence, the unit is mass-based and is [CTU per kg emitted] = [PAF × m³ × day per kg emitted].
Please find further details in Henderson et al. (2011).
But how is this integration of PAF over time and volume executed?
When you look at the unit of the ecotoxicity effect factor, it is PAF × m³ per kg emitted, i.e. in the effect factor it is integrated over the water volume considered. This is done by taking the average of the log-values of the species-specific eco-toxicity data given in mg/L (sheet: Substance data), converting it to mg/m³ at the normal scale by dividing it by 1000 (sheet: Ecotox effect, cell G10) and inverting this value in cell G12. The effect factor is then given in PAF × m³ per kg emitted and multiplied by the fate factor given in days and the exposure factor which is dimensionless. This finally leads to the unit of the ecotoxicity characterization factor given in PAF × m³ × day per kg emitted.
So, do I understand it correctly that the actual concentration of the emission is not accounted for? So you multiply PAF * m³/kg with the unit kg total emission?
And not with kg/m³ (to account for the effect of the actual emission at the spot)?
Correct, the characterization factors from USEtox can simply be multiplied with the emission amount (either per time unit or absolute) to arrive at impacts.