The tropospheric degradation of isoprene: an updated module for the regional atmospheric chemistry mechanism

A highly condensed reaction scheme for the tropospheric oxidation of isoprene is presented. This mechanism was implemented into the regional atmospheric chemistry mechanism (RACM), which is an established chemical module for regional air quality modelling but contains an isoprene chemistry which is no longer state-of-the-art. The reaction scheme developed here is based on the recently published Mainz isoprene mechanism (MIM) that has been constructed for application in global chemistry transport models. The MIM code was reduced to a size suitable for use in regional atmospheric chemistry models. Redundant reactions were identified and removed from the reaction scheme by means of sensitivity analyses. The revised mechanism was successfully tested against the results of smog chamber experiments carried out in the European photoreactor EUPHORE. A model intercomparison between both the original and the updated RACM mechanism was performed for a number of well-defined scenarios employing conditions ranging from very clean to highly polluted air masses. The calculations revealed large deviations in the concentration–time profiles for key species of the isoprene degradation, particularly under “low-NOx” conditions. The new isoprene chemistry requires only a few additional reactants (7) and chemical reactions (7) and, therefore, offers the possibility for the successful application of the revised reaction scheme in chemistry-transport models (CTM) without an excessive increase in computational efforts.