3. Results and Discussion
As expected, our E-AIM simulation on ammonia and SA binary system without any NA showed the complete condensation of the sulfate into the solid phase as ammonium sulfate. The focus was then moved to a ternary RNC-SA-NA system, since NA is in general always in large excess than SA in the atmosphere. Figure 1a (black curve) shows that ammonia and NA, which were generally considered to be unstable in the particles in the atmosphere at room temperature, can thermodynamically condense into particle phase in the ternary system when the temperature is low enough (below 273 K). As the temperature decreases, more and more condensed ammonia and NA will exist in condensed phases, facilitating the growth of ammonium sulfate FNCs. This observation is consistent with the experimental findings by Wang et al. [2020], where they identified that ammonia and NA in such ternary system could only condense at or below 278 K, but above 263 K.
Our simulations revealed the different temperature-dependence of the condensation of NA in the presence of varying RNCs other than ammonia. The most commonly observed organic amines in the atmosphere are monoamines with methyl groups, MA, DMA and TMA (Table 1). It was clear that these monoamines may all contribute to the condensation of NA on particles at low temperature (Figure 1a). For example, TMA and TEA showed very similar effects on the NA condensation as ammonia. Considering the potential high concentrations of these alkylamines in some urban and coastal environment, it is possible that both ammonia and amines can contribute to the growth of FNCs under cold weather conditions by facilitating the condensation of available gaseous NA on the freshly formed particles.
To our surprise, alcohol amines, such as MEA, can condense with NA even at room temperature and higher (Figure 1a, blue dash line). Since ammonia is not likely to contribute to the NA condensation at these temperatures [Liu et al., 2018; Wang et al., 2020], our findings suggest that an environment polluted with MEA and NA may experience rapid new particle growth due to the condensation of NA facilitated by MEA. Further investigations on several other amines with additional –NH2 or –OH group(s) (labeled in red in Table 1) also showed significant condensation with NA at room temperature (Figure 1b).
To further quantitatively describe the influences of ammonia and different amines on the condensation of NA, the transitional temperature of NA condensation (Tc ), which is defined as the temperature at which the moles of total nitrate in the condensed phases equals to 5% of the initial moles of NA, was identified and summarized in Table 1, along with the estimated uncertainties (details in Supporting Information, Figure S1 and Table S1).
Common monoamines, including MA, DMA, TMA, EA, DEA and TEA, all showedTc values ranging from 263 K to 275 K, similar to that of ammonia. It implies the limited contribution of alkyl groups in enhancing NA condensation. Group I RNCs is hereby defined as those that will only condense with NA at or below 278 K. Other RNCs withTc values at or above 288 K are collectively referred as Group II RNCs in this study. Our results indicate that the existence of Group II amines ( e.g., MEA and PZ) in the atmosphere may greatly facilitate the condensation of NA at room temperature and enhance the new particle formation process by growing the critical clusters over the “valley of death” with excessive particle savaging.
(a)