under a Creative Commons License. Atmospheric Chemistry and Physics

Abstract. Modeling atmospheric aerosol and cloud micro-physics is rather complex, even if chemical and thermody-namical equilibrium is assumed. We show, however, that the thermodynamics can be considerably simplified by re-formulating equilibrium to consistently include water, and transform laboratory-based concepts to atmospheric condi-tions. We generalize the thermodynamic principles that ex-plain hydration and osmosis – merely based on solute solu-bilities – to explicitly account for the water mass consumed by hydration. As a result, in chemical and thermodynamical equilibrium the relative humidity (RH) suffices to determine the saturation molality, including solute and solvent activities (and activity coefficients), since the water content is fixed by RH for a given aerosol concentration and type. As a conse-quence, gas/liquid/solid aerosol equilibrium partitioning ca