COMPARISON OF STIFF CHEMISTRY SOLVERS FOR AIR-QUALITY MODELING

Four fast solvers and their variations are compared in terms of the ac curacy of the solutions and computation time when they are used to sol ve a system of stiff ordinary differential equations describing the ca rbon bond IV mechanism in air quality modeling. The solvers are the Ur ban Airshed Model (UAM) solver, the quasi-steady-state assumption (QSS A), and the Hybrid solvers, each with an additional version employing the steady-state algorithm of the UAM solver, and the new implicit-exp licit hybrid (IEH) solver under two different sets of error tolerance. The solvers were run for one 6-min time step under 256 different init ial conditions for both daytime and nighttime. In terms of accuracy, t he IEH solvers are the most accurate, while the QSSA solver is the lea st accurate. In terms of computation time, QSSA and QSSA with UAM stea dy state are the fastest, while UAM is the slowest for daytime integra tion and widely variable for nighttime integration. The more tolerant version of IEH (IEH26) and the Hybrid solver coupled with UAM steady s tate are fast, but the former is more accurate under all tested condit ions, assumes only one steady-state species (O1D), conserves the nitro gen mass, and is applicable to other stiff chemical systems. According ly, IEH26 should be an excellent candidate as a fast and accurate chem istry solver in air quality modeling, combustion, and other reactive f low systems.