Figure 2. NA condensation and hygroscopicity of a chemical system of RNCs-sulfuric acid-nitric acid. (a) The moles of condensed nitrate against temperature with varying mol fraction of MEA at fixed initial water vapor of 0.07848 mol (equivalent to 60% RH at 263.15 K and 101,325 Pa in 1 m3). (b) the mole ratio of water and sulfate in the condensed phases at different RH with varying amine:ammonia mole ratios at 298K. The reference curve (red solid line) only has ammonium sulfate [Chan and Chan, 2012]. All curves have the same total moles of RNCs as 8.86895×10−8 and the same initial moles of HNO3 and H2SO4 as 1.11151×10−9­­ and 2.03777×10−11, respectively. The percentage in the legends refers to the initial mole fraction (mol %) of the amine(s) in the total RNCs.
In addition toTc , deliquescence relative humidity (DRH) may also play an important role in the nucleation and growth of particles. The RH at a transition point, at which the water starts to condense on the particles, is referred as DRH. Generally, dry ammonium sulfate particles have a DRH ~79%. While ammonium nitrate may uptake water vapor at much lower temperature, the condensation of ammonia and NA is likely hindered at room temperature [Wang et. al., 2020]. Figure 2b shows the water uptake by the condensed phases at 298 K in an ASN system with additional amines (DMA and MEA representing Group I and II RNCs, respectively). The chemical compositions of the RNCs and varying amine:ammonia mol ratios relevant to the atmosphere were investigated. In all cases, the sulfate all condensed as ammonium sulfate; therefore, the mol ratio of aqueous water to sulfate is directly proportional to the level of water uptake by the condensed phases. Since ammonia and NA do not condense at 298 K in the system of ammonia-SA-NA, the DRH of the ASN system without amines is expected to be 79 % (the DRH of ammonium sulfate) [Chan and Chan, 2012]. Adding 0.01 % or 0.1 % mole fraction of DMA to the system only lowered the DRH by ~1 %. However, the presence of only 0.01 mol % MEA can lower the DRH of the condensed phases to ~71 % (Figure 2b, insert).
In contrast to the limited effects of DMA on particle DRH, MEA plays a significant role in lowering the DRH at high concentrations. For example, 0.1 mol % MEA can lower the DRH to 30 %, helping the particles absorb water in a relatively dry environment. These observations are consistent with the hygroscopic behavior of aminium cations in aqueous solutions [Qiu and Zhang, 2012; Clegg et al., 2013; Rovelli et al., 2017]. As shown in the Figure 2b insert, the system with 0.01 % MEA and 0.1 % DMA shows somewhat enhanced water uptake by the condensed phase than the cases with only 0.01 % MEA or 0.1 % DMA; however, the effect is limited and the presence of 0.01 % MEA was the main contributor to the lowered DRH. It seems that the effects of amines on the hygroscopicity of particles formed in the RNCs-SA-NA system are additive and dominated by Group II RNCs.