Abstract
Abstract Two methods for solving stiff systems of ordinary differential equations describing nonlinear chemical kinetics in air quality models, namely the hybrid and the quasi-steady state approximation (QSSA) schemes, are compared with respect to their accuracy and computational speed. Their implementation for parallel and vector processing computers is discussed. Tests are conducted using two different photochemical mechanisms. Also, a new test problem is developed to represent various degrees of stiffness encountered in urban and regional air quality simulations. It is concluded that the hybrid scheme is more accurate than the QSSA scheme. Different techniques are considered to eliminate conservation errors and make the solutions more accurate. As for the computational efficiency, the QSSA scheme is approximately two to four times faster than the hybrid scheme.
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