Abstract
This paper analyses the cyclic behaviour of chemicals between different
environmental media using multimedia models. The importance of such
cyclic transport is defined in terms of the Feedback Fraction ? the
fraction of an emission that comes back to the medium of release after
transfer to other media. This fraction is calculated analytically and
using matrix techniques. Neglecting feedback, hence ignoring the cyclic
nature of the intermedia transport, the predicted media concentrations
for 317 chemicals in a 4-compartment, steady-state, closed-system
multimedia model were never underestimated by more than a factor 4 in
comparison to the exact multimedia model solution. When comparing
impacts between compounds, this is negligible relative to a variation
of up to 1x1012 between the estimated chemical concentrations and
therefore, feedback can often be ignored for organic chemicals. This
analysis of the importance of the Feedback Fraction, in conjunction
with resultant criteria for when cyclic exchanges between media can be
significant, facilitates a more transparent understanding of how
multimedia models distribute mass into the environment and to what
extent the media can be independently modelled. Depending on their
properties, compounds that do not require analysis using an exact
multimedia solution can either be evaluated using a single media model
or using two or more environmental media models linked by first order
intermedia mass transfer.
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