ISSA (iterative screening and structure analysis)—a new reduction method and its application to the tropospheric cloud chemical mechanism RACM/CAPRAM2.4

An automated reduction method ISSA (iterative screening and structure analysis) has been developed. It is aimed at the analysis of complex atmospheric chemical multiphase mechanisms and produces reduced mechanisms for specifiable application purposes. Cyclic and non-cyclic reactions identified by a structure analysis are separately evaluated. The normalized valuation coefficients are calculated in a box model framework by using time-averaged reaction rates. Starting with a set of target species, important reactions and species are selected together in an iteration procedure. So, only one threshold value fixed for all box model scenarios is necessary. For every scenario a specific reduced mechanism is obtained. The sum of reactions and species included in the specific reduced mechanisms generates then the ISSA-reduced mechanism. All reactants in the reduced mechanism are included in the verification procedure where the concentrations simulated with the full and the reduced mechanism are compared. The maximum relative deviation of daily maxima was found to be a suitable deviation measure for atmospheric trace species concentrations.An application of the ISSA method to the large cloud chemical mechanism RACM/CAPRAM2.4 resulted in reduction rates of 55% for reactions (46% gas phase, 60% liquid phase), 23% for species, and 23% for phase transfers. The deviation between full and reduced mechanism averaged over all scenarios and reactants was 2.5%. The liquid-phase part of this application was compared with a condensed version of the CAPARAM2.4 mechanism developed simultaneously with the full version. It was found that these two reduced versions of CAPRAM2.4 differ significantly. Whereas the condensed version achieves good verification results only for the target species, the ISSA-reduced version reproduce very well the complete full mechanism results and should be useful for future large-scale models, which will include both detailed microphysics and complex (reduced) multiphase chemistry.