Text S4. Activity considerations for amines
It is possible to carry out thermodynamic simulations with or without considering the activities of the chemicals in the aqueous solution, including the ammonium and aminium cations, the nitrate and sulfate anions and the dissolved amines. The scenario where activities of all chemical species are considered to be one is referred as the Raoult’s law method, which is likely applicable at high relative humidity due to the much-diluted ion concentrations. However, at a higher temperature or lower relative humidity, water condensation on the particle may be limited and the formed aqueous solution may have a very high ionic strength.8 Therefore, it is important to evaluate the uncertainties caused by the methods used for activity estimations in our simulations.
The activities of the anions involved in our simulations were relatively well studied.1 Ge et al.2 showed that it is reasonable to assume the activities of the aminium cations to be the same as that of ammonium cation. As highly polar molecules, the activities of amines dissolved in an aqueous solution may be described using the UNIFAC method.9
Our Tc values reported in Table 1 were based on simulations using the UNIFAC method as default to estimate the activities of the amines. Ge et al.2 showed that the estimation of activities of amines using UNIFAC method will generally lead to satisfactory results yet the dataset for comparison was far from comprehensive. Therefore, additional simulations were carried out using the Raoult’s law method for all the amines. One example comparing the results between Raoult’s law and UNIFAC methods is shown in Figure S2(d). The relative difference in Tc using the two activity estimation methods for amines in our study was generally below 2.5%.