Application of α -QSS to the numerical integration of kinetic equations in tropospheric photochemistry

A major task in many applications of atmospheric chemistry transport problems is the numerical integration of stiff systems of Ordinary Differential Equations (ODEs) describing the chemical transformations. A faster solver that is easier to couple to the other physics in the problem is still needed. The integration method, α-QSS, corresponding to the solver CHEMEQ2 aims at meeting the demands of a process-split, reacting-flow simulation. However, this integrator has yet to be applied to the numerical integration of kinetic equations in troposphere chemistry. A zero-dimensional (box) model is developed to test how well CHEMEQ2 works on the troposphere chemistry equations. This paper presents the testing results as well as accuracy and efficiency evaluations . The reference chemical mechanisms herein used are RACM and its secondary reduced scheme ReLACS. The box model is forced and initialized by the DRY scenarios of Protocol Ver.2 developed by EUROTRAC. The comparison with standard VODE method shows that the single-point a-QSS approach is fast and moderately accurate as well as easy in coupling to reacting flow simulation models, which makes CHEMEQ2 one of the best candidates for on-line three-dimensional atmospheric chemistry transport modelling studies.

The testing results also show that serious discrepancies emerge between RACM and RADM2 when these mechanisms are used to simulate the chemistry with intensive anthropogenic NO emission described by URBAN and URBAN/BIO scenarios.

Keywords: Atmospheric chemistry; Chemical mechanism; Numerical ODEs solver; α -QSS method.