Environmental Engineering Reference
In-Depth Information
4. Discussion
The choice to use only two bins or sections to represent the PM size distribution
was made in order to reduce the number of tracers being modelled, since nine
chemical components must be considered for each additional bin. By contrast, in
the AURAMS model we have used ten bins to represent the same PM size range
(Gong et al., 2006). However, a recent model intercomparison of regional PM
models suggested that CHRONOS performance for PM
2.5
was comparable to other
models in spite of its “bulk” treatment of PM
2.5
(McKeen et al., 2007).
Numerical experiments with GEM-MACH15, on the other hand, have shown
considerable sensitivity to the treatment of inter-bin mass transfer, which CHRONOS
neglects. The choice of dry deposition velocity and sedimentation velocity, which
are both size dependent, is also complicated by the consideration of only two PM
size bins. We account for such size dependence in GEM-MACH15 by subdividing
each bin into sub-bins in order to determine inter-bin mass transfer and dry-
deposition and sedimentation velocities.
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