CLEPS: A new protocol for cloud aqueous phase oxidation of VOC mechanisms

Abstract

Abstract. Organic compounds of both anthropogenic and natural origin are ubiquitous in the multiphasic atmospheric medium. Their transformation in the atmosphere affects air quality and the global climate. Modelling provides a useful tool to investigate the chemistry of organic compounds in the tropospheric multiphase system. While several comprehensive explicit mechanisms exist in the gas phase, explicit mechanisms are much more limited in the aqueous phase. Recently, new empirical methods have been developed to estimate HO• reaction rates in the aqueous phase: structure-activity relationships (SARs) provide global rate constants and branching ratios for HO• abstraction from and addition to atmospheric organic compounds. Based on these SARs, a new detailed aqueous-phase mechanism, named the cloud explicit physico-chemical scheme (CLEPS), to describe the oxidation of hydrosoluble organic compounds resulting from isoprene oxidation is proposed. In this paper, a protocol based on reviewed experimental data and evaluated prediction methods is described in detail. The current version of the mechanism includes approximately 850 aqueous reactions and 465 equilibria. Inorganic reactivity is described for 67 chemical species (e.g., transition metal ions, HxOy, sulphur species, nitrogen species, and chlorine). For organic compounds, 87 chemical species are considered in the mechanism, corresponding to 657 chemical forms that are individually followed (e.g., hydrated forms, anionic forms). This new aqueous-phase mechanism is coupled with the detailed gas phase mechanism MCM v3.3.1 through mass transfer parameterization for the exchange between the gas phase and aqueous phase. The GROMHE SAR enables the evaluation of the Henry's law constants for undocumented organic compounds. The resulting multiphase mechanism is implemented in a model based on the Dynamically Simple Model for Atmospheric Chemical Complexity (DSMACC) using the Kinetic PreProcessor (KPP). This model allows simulation of the time evolution of the concentrations of each individual chemical species in addition to detailed time-resolved flux analyses. The variable photolysis in both phases is calculated using the TUV 4.5 radiative transfer model. To evaluate our chemical mechanism, an idealized cloud event with fixed microphysical cloud parameters is simulated. The simulation is performed for a low-NOx situation. The results indicate the formation of oxidized mono- and diacids in the aqueous phase, as well as a significant influence on the gas phase chemistry and composition. For this particular simulation, the aqueous phase mechanism is responsible for the efficient fragmentation and functionalization of organic compounds. This new cloud chemistry model allows for the analysis of individual aqueous sub systems and can be used to analyze the results from cloud chamber experiments and field campaigns.