How non-equilibrium aerosol chemistry impacts particle acidity: the GMXe AERosol CHEMistry (GMXe–AERCHEM, v1.0) sub-submodel of MESSy

Abstract. Aqueous-phase chemical processes in clouds, fog, and deliquescent aerosols are known to alter atmospheric composition and acidity significantly. Traditionally, global and regional models predict aerosol composition by relying on thermodynamic equilibrium models and neglect non-equilibrium processes. Here, we present the AERosol CHEMistry (GMXe–AERCHEM, v1.0) sub-submodel developed for the Modular Earth Submodel System (MESSy) as an add-on to the thermodynamic equilibrium model (i.e., ISORROPIA-II) used by MESSy’s Global Modal-aerosol eXtension (GMXe) submodel. AERCHEM allows the representation of non-equilibrium aqueous-phase chemistry of varying complexity in deliquescent fine aerosols. We perform a global simulation for the year 2010 by using the available detailed kinetic model for the chemistry of inorganic and small oxygenated organics. We evaluate AERCHEM’s performance by comparing the simulated concentrations of sulfate, nitrate, ammonium, and chloride to in situ measurements of three monitoring networks. Overall, AERCHEM reproduces observed concentrations reasonably well. We find that especially in the USA, the consideration of non-equilibrium chemistry in deliquescent aerosols reduces the model bias for sulfate, nitrate, and ammonium, when compared to simulated concentrations by ISORROPIA-II. Over most continental regions, fine aerosol acidity simulated by AERCHEM is similar to the predictions by ISORROPIA-II but tends to simulate slightly lower aerosol acidity in most regions. The consideration of non-equilibrium chemistry in deliquescent aerosols leads to a significant higher aerosol acidity in the marine boundary layer, which is in line with observations and recent literature. AERCHEM allows investigating the global-scale impact of aerosol non-equilibrium chemistry on atmospheric composition. This will aid the exploration of key multiphase processes and improve the model predictions for oxidation capacity and aerosols in the troposphere.