4. Atmospheric Implications
Thermodynamic models are useful to evaluate how a chemical may condense on a particle since the feasibility of such process in the atmosphere is governed by the final physical and chemical equilibria of the system. To accurately estimate the impact of amine-assisted NA condensation during NPF, it would be ideal to use kinetic information on how fast amine and NA can condense on FNCs, which unfortunately is currently lacking for many of the amines in this study. However, several kinetic studies involving Density Function Theory (DFT) and Atmospheric Clusters Dynamic Code (ACDC) [Elm et al., 2020] have suggested that: (1) An 1:1 DMA-NA cluster appears to have a slightly lower kinetic energy barrier to form than that of ammonia and NA [Liu et al., 2018; Chee et al., 2019], suggesting that it will be faster to form DMA-NA clusters than the ammonium nitrate ones. Since recent laboratory experiments [Wang et al. 2020] demonstrated rapid ammonia and NA condensation at +5 °C, it is reasonable to assume that DMA can also quickly condense with NA at a similar temperature. (2) MEA and PZ may form FNCs at a kinetic rate that is comparable or faster than that of DMA with SA [Xie et al., 2017; Ma et al., 2019]. Theoretical studies showed that gas-phase NA will undergo proton transfer reaction with hydroxyl radicals ~1000 times faster than gaseous SA [Gonzalez and Anglada, 2010; Long et al., 2011], suggesting that the reactions between amines and NA (which will likely be a proton transfer reaction) will probably proceed no slower than those between amines and SA. Therefore, it is likely that MEA and PZ could condense with NA at a rate comparable to or faster than that of DMA with NA.
Our thermodynamic simulations suggest that there are two distinct groups of amines (Table 1): Group I with a low Tc of +5 °C or below and Group II with a Tc of +15 °C or above. Both groups may assist condensation of NA on nanoparticles under various ambient conditions and play an important role in the NPF, especially in polluted urban, agricultural and industrial areas.
At relatively low ambient temperatures (~ −10 °C to 0 °C), monoamines will likely condense with NA on particles, suggesting that the contribution of amines to NPF may be underestimated without the consideration of monoaminium nitrates. Considering the high ambient concentrations of monoamines, especially in some metropolitan centers, it is likely that monoamines will further contribute to NPF by facilitating the NA condensation at low temperature (e.g., during winter or near the top of the planetary boundary layers) in additional to their assistance in forming FNCs. Low-temperature amines and NA condensation could be an additional pathway to consider when evaluating the lifetime of monoamines in the atmosphere.
In warmer weather conditions, on the other hand, all Group II amines may facilitate the growth of FNCs by assisting in the NA condensation regardless of their ambient concentrations, because such NA condensation did not appear to be affected by gaseous ammonia concentration in our simulations. This could be particularly important in heavily industrialized areas with significant manufacturing and/or applications of these amines (e.g., a power plant using MEA based CCS technology). Our results indicated that it is possible to consider all Group II amines together with one total concentration, which can then be used in Equation 1 in order to estimate their impact on NPF under specific atmospheric conditions for field and modeling studies.
When the temperature in the atmosphere changes relatively quickly in a short period of time, the contribution of Group II RNCs to NA condensation becomes more significant. Furthermore, our simulation results suggest that even at very low concentration, the condensed MEA and NA may facilitate the formation of a thin layer of solution at the particle surface [Hsiao et al., 2016]. Such modification to particle surface may change critical properties of the particles, including the viscosity [Shiraiwa et al., 2011], surface tension [Ovadnevaite et al., 2017] and Kelvin curvature effect [Zhang et al., 2012], surface uptake and heterogenous reactions [Kolb et al., 2011; Rossignol et al., 2016], hygroscopicity and CCN potentials [Lavi et al., 2013].