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%.