Figure 1. The moles of condensed nitrate against temperature in a ternary system of RNC-sulfuric acid-nitric acid at fixed initial water vapor of 0.07848 mol (equivalent to 60% RH at 263.15 K and 101325 Pa in 1 m³). All curves have the same initial moles of HNO₃, H₂SO₄ and the RNC as 1.11151×10^−9­­, 2.03777×10⁻¹¹and 8.86895×10⁻⁸, respectively. The legend on the right indicates which RNC is present in each trace. (a) includes all RNCs in Group I (defined in Table 1) and MEA (as comparison) and (b) includes all RNCs in Group II and ammonia (as comparison).

To further elucidate the reason for such significant difference inT_c values among amines, the properties of several amines with distinctively different functional groups were compared (Table 1). Neither gaseous basicity (GB), aqueous basicity (K_b ) or the volatility of an RNC or its nitrate salt (p_sat and K_p , respectively) seems to directly determine the T_cof an RNC. For example, DMA and MEA have similar GB values yet distinctively different T_c . When compared with PZ, TMA have a similar K_b yet aT_c that is ~ 28 K lower. PZ has a much higher GB than MEA but their T_c values are comparable. AN and MEA have similar saturation vapor pressures at room temperature yet distinctive different T_c , ruling out any direct contribution on T_c from the volatility of the amine. The solid/gas dissociation constants of the nitrates of alkylamines are about 10 times lower than that of ammonium nitrate yet they exhibit comparable T_c , suggesting that saturation condensation of nitrate salts alone may not explain the difference in T_c for Group I and II RNCs. Furthermore, the molar mass of IBA is about 50% more than that of MEA, but their T_c values are essentially the same, suggesting that Van der Waals forces do not contribute significantly here. It appears that the electron affinity of the functional groups on the amines has little effect on theT_c . For example, the nitro group in AN-N and the methyl group in AN-M are electron withdrawing and electron donating groups, respectively. However, both RNCs showed little contribution to NA condensation at above 260 K.

One distinctive observation is that amines with only alkyl (e.g., MA) or aromatic (e.g., AN) substitutions showed much lowerT_c than those with hydroxyl (–OH) groups (e.g., MEA). Furthermore, IBA and PZ, showed T_c values comparable to that of MEA, with the former having a similar chemical structure as MEA, while the latter having a second amine group (–NH–) in the ring (Table 1). Since MEA and PZ do not share the same functional groups, their high T_c values could not be explained alone by the presence of –OH groups in the chemical structure.

Based on these observations, it is proposed that the ability of RNCs to form additional hydrogen bonds plays a critical role on how RNCs may facilitate the condensation of NA on nanoparticles. Generally, theT_cvalue will increase significantly when a RNC can form more hydrogen bonds. MEA, PZ and IBA, with one additional hydroxyl or amine group than monoamines, have T_c values of ~ 300 K. DAE, DGA and DIPA with two additional hydrogen bonds, showedT_c values as high as 323 K (Table 1), allowing NA to condense above room temperature.

Another factor that may affect the T_c is hydrophobicity. AN and AN-M both contain an aromatic ring which is more hydrophobic than a methyl group. The decreasing solubility of amines in water results in less NA condensed and dissolved in the particle, greatly diminishing the condensation of NA compared with MA. In the case of AN-O, the –OH attached to the aromatic ring can form additional hydrogen bonds, which seems to offset the negative effect of the non-polar aromatic ring and increase the T_c to 289K (+15 °C).

To evaluate the Kelvin curvature effect [Zhang et al, 2012] on the condensation of RNC and NA, the saturation vapor pressurep_sat of MEA was increased by factors of 10 and 10²; the corresponding T_cvalues decreased to 296 K and 288 K, respectively. This analysis suggests the significant impact of particle size on the condensation of NA in the presence of RNCs.

Although Figure 1 and Table 1 well demonstrate the enhancement of NA condensation by the presence of amines, ambient concentration of amines is normally much lower than 1900 pptv, nor would amines exist in the atmosphere without a relatively high concentration of ammonia. In reality, the ambient concentration of amines is usually 1–2 orders of magnitude lower than that of ammonia, although the amine:ammonia mol ratio could be higher near industries emitting amines, such as near a power-plant using CCS technology. As a result, a chemical system of ammonia, MEA, SA and NA with varying MEA:ammonia mol ratios was investigated. Figure 2a illustrates the change of theT_c value of an ASN system with varying amount of the ammonia replaced with MEA. The enhancement of the NA condensation by MEA is noticeable even at as low as 0.1 mol % (~ 2 pptv). It appears that as the mole fraction of MEA increases, theT_c increases following an excellent linear relationship (R² = 0.99 and Figure S2):

T_c =9.1615×[−log(MEA%)]+305.33 (1)

where MEA% is the MEA mole fraction in total RNCs (mol %).

In additional to the structures and concentrations of amines, amount of total water is another factor that may impact the condensation of NA. Our further investigation (Figure S3) suggested that increase in the total water vapor in the system leads to an increase inT_c of the system, hence facilitating the condensation of NA. However, the enhancement is not as strong as other effects such as additional hydrogen bonding or hydrophobicity. A similar trend was observed by Chee et al. [2019] when they studied the nanoparticle formation and growth in a DMA-NA system.

(a)