COMPARISON OF STIFF CHEMISTRY SOLVERS FOR AIR-QUALITY MODELING

Four fast solvers and their variations are compared in terms of the accuracy of the solutions and computation time when they are used to solve a system of stiff ordinary differential equations describing the carbon bond IV mechanism in air quality modeling. The solvers are the Urban Airshed Model (UAM) solver, the quasi-steady-state assumption (QSSA), and the Hybrid solvers, each with an additional version employing the steady-state algorithm of the UAM solver, and the new implicit-explicit hybrid (IEH) solver under two different sets of error tolerance. The solvers were run for one 6-min time step under 256 different initial conditions for both daytime and nighttime. In terms of accuracy, the IEH solvers are the most accurate, while the QSSA solver is the least accurate. In terms of computation time, QSSA and QSSA with UAM steady state are the fastest, while UAM is the slowest for daytime integration and widely variable for nighttime integration. The more tolerant version of IEH (IEH26) and the Hybrid solver coupled with UAM steady state are fast, but the former is more accurate under all tested conditions, assumes only one steady-state species (O1D), conserves the nitrogen mass, and is applicable to other stiff chemical systems. Accordingly, IEH26 should be an excellent candidate as a fast and accurate chemistry solver in air quality modeling, combustion, and other reactive flow systems.