Joint work with Greg. Yarwood.
The decoupled direct method (DDM) is being implemented in the Comprehensive Air Quality Model with Extensions (CAMx) to calculate sensitivities with respect to initial concentrations, boundary concentrations, and emissions. This model contains a hybrid solver for the chemical rate equations in which the species are divided into three categories: radicals, fast species and slow species. Separate algorithms are used for each category of species. Equations were derived for the DDM taking into account the different algorithms for different species and other details of the hybrid solver. The DDM was also applied to the Bott algorithm for solving the horizontal advection equation. While the advection equation itself is linear in the sensitivities, the Bott algorithm contains a nonlinear flux-limiting step. Consequently, appropriate sensitivity equations were derived corresponding to the Bott algorithm. Some additional modifications were made to CAMx to improve the efficiency in calculating sensitivities.