A model for tropospheric multiphase chemistry: application to one cloudy event during the CIME experiment

A multiphase box model for a remote environment of the troposphere has been developed with an explicit chemistry for both gas and aqueous phase. The m odel applied to a set of measurements performed by Voisin et al. (2000) dur ing the European CIME experiment for a cloud event on 13th December 1997 at the top of the Puy de De Dome (France). The results of the simulation are compared to the measurements in order to follow the evolution of the ambien t chemical composition as a function of the pH and of the varying water con tent. After verifying that the model retrieves the main features observed i n the behavior of species in the cloud droplets, a detailed analysis of the simulated chemical regime is performed. It essentially discusses the sourc es and sinks of radical in aqueous phase, the relative importance of the ox idation pathways of volatile organic compounds by the main radicals and the conversion of S(IV) into S(VI) which seems to be influenced by the presenc e of peroxonitric acid, HNO4, in aqueous phase in the environmental conditi ons that are considered with low H2O2, These numerical results are then com pared with the theoretical study from Herrmann et al, (2000), who proposed a slightly different mechanism, including C2 chemistry and transition metal chemistry whereas they neglect some reaction pathways, such as the one inv olving OHCH2O2 radical. This double confrontation between model results and both real experimental data and numerical results from Herrmann et al. (20 00) underlines limitations of such modeling approach that does not include any dynamical or microphysical coupling but also demonstrates its capabilit y to identify the main oxidants or reactants in aqueous phase in real envir onmental conditions more realistic than a purely theoretical approach. The originality of this study resides in the explicit and exhaustive ways the c hemical reactions are treated in aqueous phase and in a first attempt to co mpare such a detailed chemical scheme to real environmental conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.