Acid–base clusters and stable salt formation are critical drivers of new particle formation events in the atmosphere. In this study, we explore salt heterodimer (a cluster of one acid and one base) stability as a function of gas-phase acidity, aqueous-phase acidity, heterodimer proton transference, vapor pressure, dipole moment and polarizability for salts comprised of sulfuric acid, methanesulfonic acid and nitric acid with nine bases. The best predictor of heterodimer stability was found to be gas-phase acidity. We then analyzed the relationship between heterodimer stability and
Atmospheric aerosol particles represent the largest uncertainty in our understanding of global climate through their participation in cloud formation and the absorption and scattering of radiation
Ammonia is the most abundant base in the atmosphere and its reaction with sulfuric acid has been well studied
Recently, computational efforts have focused on accurately representing the formation and growth of acid–base clusters
This large variety in systems studied has yielded insights into the factors that determine cluster formation and growth. Generally, the enhancing efficiency of the base on heterodimer stability and NPF is known to correlate with base strength, which has been attributed to a more favorable proton transfer and the formation of essentially nonvolatile ionic salts and has been shown to be generally true for the most abundant bases in the atmosphere: ammonia, methylamine, dimethylamine and trimethylamine
In this study, we aim to use these computational methods to identify what molecular properties predict heterodimer stability, or more specifically the Gibbs free energy of formation of the heterodimer (
In addition to these molecular properties, we further explore the relationship between heterodimer stability and NPF rate for SA salts. The goal of this work is to develop computationally efficient approaches for calculating NPF rate that can be applied to models that estimate the impacts of NPF on climate and air quality. We represent NPF rate as
Acid and base compounds in this study. Abbreviations are as follows: ammonia (AMM), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), trimethylamine N-oxide (TMAO), guanidine (GUA), monoethanolamine (MEA), putrescine (PUT) and piperazine (PZ), sulfuric acid (SA), methanesulfonic acid (MSA) and nitric acid (NA).
Two-component acid–base particle formation was studied by making systematic changes in temperature and concentration to understand the effects of simulation conditions and acid/base molecular properties on
Experimental and calculated properties examined in this study.
In order to simulate cluster formation and growth, one must calculate accurate structures and thermochemical properties of neutral SA–base clusters up to the cluster size of four SA and four base molecules (4SA4base). Thermochemistry of clusters containing AMM, DMA, GUA and TMAO were taken from our previous studies
In addition of a full data set for SA–base clusters, we studied heterodimers of NA and MSA with the nine above-mentioned bases. The same quantum chemical methods were used as in SA–base calculations. In order to detect whether proton transfer was occurring in the heterodimer, the Molden program
Theoretical methods allow us to perform particle formation simulations at conditions where particle formation rates are not experimentally measurable. This means that very low or high temperatures and vapor concentrations can be used to estimate
In the cluster formation process, the changes in enthalpy (
Calculated enthalpy (
The molecular structures of SA–base heterodimers are presented in Fig.
Heterodimers of SA with AMM, MA, DMA, TMA, TMAO, GUA, MEA, PUT and PZ, respectively.
Figure
Calculated GA vs. literature
In the gas phase, the strongest bases are, in decreasing order, PUT, TMAO and GUA, whereas in the aqueous phase the order is GUA, PUT and DMA. GUA is a very strong base both in gas and aqueous phases because its cationic form has six
Cyclic and acyclic configurations of protonated PUT. The lowest energy acyclic structure is 14.6 kcal mol
As PUT and PZ are diamines, they can accept two protons and form
Because heterodimer stability has been shown to be a good proxy for
Calculated
Over the observed
Figure
Interestingly, the NA–PZ salt is an anomaly in the cutoff for
Deprotonated
Figure
Base vapor pressure plotted against
The stabilities of a heterodimer and other small clusters are known to affect the ability of a cluster to grow to a large aerosol particle
Heterodimer stability (
Previously, theoretically calculated
Individual data points and trendlines from Fig.
Interestingly, as temperature increases, this lognormal relationship transitions to linear, with a larger spread of data points around the trendline. Practically, this implies that
With respect to the lognormal relationship between
These findings are notable in that
In order to combine simulated particle formation rates at different conditions for all acid–base systems, we calculated the heterodimer concentration, which is a function of
Heterodimer concentration plotted against
Figure
To test the robustness of our calculations, heterodimer concentrations of CLOUD experiments were calculated using Eq. (
Heterodimer concentration plotted against
All data calculated for this study are plotted in Fig.
Here, we attempt to reduce this uncertainty for nine salts of SA and further simplify the expression used to calculate
When applied to ammonia, a simple monotonic relationship between
As a contrast to the SA–AMM system, we also examined the behavior of the SA–GUA salt, a strong-acid and strong-base combination. Figure
All SA salts plotted with
When
Here,
Because each base has its own correlation between
Here, we have shown that heterodimer stability is largely predicted by the gas-phase acidity of the constituent acid and base across 27 acid–base pairs. In addition, we found that trends between heterodimer stability and physical properties such as volatility, dipole moment and polarizability did not hold for the wide variety of bases studied here, despite a trend existing for the smaller set of AMM, MA, DMA and TMA. We emphasize here the importance of studying a variety of bases with different structures and physical properties in order to make sure our understanding of salt NPF remains unbiased. We have also shown the relationship between
In addition, we have presented a facile way of predicting
The Supplement contains the following information on the code and data used in this study:
monomer structures and properties, methanesulfonic acid and nitric acid complexes, acidity measures, base dipole moment and polarizability, boundary conditions in particle formation simulations, simulated particle formation rates, hydrogen bonding in clusters and predictive expressions of
The supplement related to this article is available online at:
NM performed the quantum chemical calculations and cluster formation simulations and initiated this study's design and supervised the data analysis process. SC curated, analyzed, investigated and visualized cluster data. SC prepared the bulk of the manuscript with contributions from NM, JNS and KB. JNS and KB acquired funding and supervised the project. All authors have read and agreed to the published version of the manuscript.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank the CSC-IT Center for Science in Espoo, Finland, for computational resources.
This research has been supported by the Jenny ja Antti Wihurin Rahasto and the National Science Foundation (grant no. CHE-1710580).
This paper was edited by Ari Laaksonen and reviewed by two anonymous referees.