A Comparison of Numerical Solvers for the Equations of
     Tropospheric Chemistry

     Rita Lorenzini, Linda Passoni-ENEA, Via Martiri di Montesole 4,
     Bologna, Italy

   The integration of ordinary differential equations systems (ODES) is
of a significant concern to tropospheric and stratospheric chemistry models.
The solution of these ODES requires a large computational effort because of
their stiff nature; in a three-dimensional photochemical model the sollution
of the ODES requires at least 70% of the total CPU time.
   Several numerical integrational techniques exist which attemt to provide
accurate and computationally efficient sollutions to chemical ODES.
   In proposed work we present a comparison of some of these techniques in
terms of sollution accuracy and required computational time.
   We have compared the Hybrid Solver method(Young e Boris, 1977), the QSSA
method (Hesstvedt et al.,1978) and the Chemical Solver for Ordinary Differential
Equation CHEMSODE (Jaro, 1996).
   The accuracy is evaluated by comparing the results of every method with
the solutions obtained by the Livermore Solver for Ordinary Differential
Equations (LSODE).
   The chemical scheme used is the SAPRC 90 scheme of the CALGRID model.
   The comparison has been made varing the parameters of the error tolerances,
and taking into account the treade-off between the solution accuracy and the
computational efficiemcy.