Sub-grid representation of emission source clusters in regional air quality modeling

Abstract

A key aspect in modeling the fate of pollutants released into the atmosphere is the parameterization of sub-grid scale processes so as to quantify their effect on resolvable scale features. Averaging emissions of species over grid squares characteristic of present-day regional and global models generally results in underpredicting concentrations of these species. This also causes inaccurate predictions for secondary pollutant species derived through complex chemical mechanisms. An algorithm is developed to provide improved sub-grid representation of emissions. A two-dimensional finite element technique is used to discretize the horizontal domain. The three-dimensional transport equation is time-split into this two-dimensional component and a one-dimensional vertical component to represent the transport processes. Compared to the locally one-dimensional finite element method (LOD-FEM), this technique allows use of variable grid sizes in the discretization of the horizontal domain, thus providing for local mesh refinement and consequently improved sub-grid representation in the numerical solution. The advantages of the scheme are inherent: the computational burden in terms of storage requirements is not increased, and the need for developing the troublesome interface boundary condition of a nested-grid procedure is avoided. A simplified first-order chemistry module has been incorporated to test the scheme for both primary and secondary pollutant species. Several test cases are presented to verify the efficacy of the proposed algorithm. Results from test cases for a simplified scenario indicate that the procedure enables one to simulate a more realistic range of concentrations for both the reactant and product species. Furthermore, comparison with the analytical solutions indicates that the numerical scheme is capable of reproducing the analytical solution, both qualitatively as well as quantitatively, in a superior manner compared to a uniform mesh.