The information gleaned from source apportionment can be useful in considering potential

control strategies. However, a key consideration in weighing alternative strategies is which types

of emissions are cost effective and practical to control, a consideration not addressed by source

apportionment modeling.

Decoupled Direct Method (DDM): The DDM calculates the local, first-order sensitivity

coefficients of gas-phase pollutant concentrations to perturbations of model inputs, e.g. initial

conditions, boundary conditions, emissions. Theoretically, the DDM is applicable to the

calculation of higher order sensitivity coefficients as well, but these are much more numerous

than the first order coefficients and generally will be less useful (Environ, 2006a). DDM can also

be used to explore model sensitivities to perturbations in meteorological parameters and chemical

rate constants, although resource intensive and at present somewhat less useful. The decoupled

direct method is advantageous both in terms of computational efficiency and stability of the

numerical solution. DDM has been used as a sensitivity analysis tool in several studies. A single

model run using DDM can potentially provide the information equivalent to tens, if not hundreds

of brute force emissions reductions model runs. DDM has been incorporated into several models

including CAMx (Environ, 2006a) and CMAQ (Cohan, 2005).


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