A comparison of water uptake by aerosols using two thermodynamic models

Abstract. A comprehensive comparison between two aerosol thermodynamic equilibrium models used in chemistry-climate simulations, EQUISOLV II and EQSAM3, is conducted for various relative humidities and chemical compositions. Our results show that the concentration of total particulate matter as well as the associated aerosol liquid water content predicted by these two models is comparable for all conditions, which is important for radiative forcing estimates. The normalized absolute difference in the concentration of total particulate matter is 6% on average for all 200 conditions studied, leading to a regression coefficient of about 0.8 for the water associated with the aerosol between these two models. Relatively large discrepancies occur, however, at high ammonium, low nitrate/sodium concentrations at low and medium relative humidities (RH<60–70%), which is analyzed and discussed in detail. In addition, the prediction of the partitioning of ammonium and nitrate is investigated under realistic atmospheric conditions. The data collected during the Mediterranean Intensive Oxidant Study (MINOS) campaign are simulated using both models. The results show that both models can reproduce the concentration of total particulate matter for 90% of the time within a factor of 2, while the predicted concentration of aerosol water by these two models is significantly correlated. The largest difference exists near RH's of 70–80% which is the RH range for the transition of mixed ammonium salts from the solid to the liquid phase.