2. Method
E-AIM employs thermodynamic state equations that consider phase and chemical equilibria of chemicals in solid, liquid and gas states under given initial conditions [Wexler and Clegg, 2002]. E-AIM solves these equations numerically and predicts all possible phases (gas, aqueous solution, organic solution, and solid) as well as the distributions of a chemical in each phase. We used E-AIM to evaluate the equilibrium state of a chemical system involving SA, NA, ammonia and amines under various conditions, including temperature, RH, and the initial moles of the chemicals. Detailed descriptions and justifications on the default parameters used in our simulations are included in the Supporting Information. Several chemicals in this study have multiple forms in condensed phases. For example, ammonia may exist in an aqueous solution as dissolved free ammonia (base form) and protonated ammonium cation (acid form). Since the primary interest of this study is the distribution of a chemical in gas and condensed phases, to simplify the analysis of the simulation results, the total moles of all forms of a chemical in all condensed phases were aggregated together. For example, the moles of nitrate as ammonium nitrate solid and aqueous nitrate anion were reported together as the moles of condensed nitrate,ncond. (NO3).
The typical temperature and RH ranges used in our simulations were 260 K to 330 K (–13 °C to 57 °C) and 10% to 90%, respectively, while the total pressure and volume were fixed at 101,325 Pa and 1 m3, respectively. The typical initial moles of an RNC, SA and NA were 1.11151×10−9, 2.03777×10−11 and 8.86895×10−8, respectively, corresponding to a mixing ratio of 1915 pptv RNC, 0.44 pptv SA and 24 pptv NA at 60 % RH, 263.15 K, and 101,325 Pa. These values were chosen to represent typical atmospheric conditions in polluted megacities as suggested by Wang et. al. [2020]. Cases where ammonia was partially replaced with amines were also explored with the initial moles of all RNCs fixed at 8.86895×10−8 and the mol ratio of amine and ammonia varied. The initial moles of HNO3, H2SO4 and RNCs were fixed when the temperature or RH input was changed to provide a consistent basis of comparison.