Sulfuric acid and
ammonia are believed to account for a large fraction
of new-particle formation in the atmosphere. However, it remains unclear
how small clusters grow to larger sizes, eventually ending up as stable
aerosol particles. Here we present the largest sulfuric acid–ammonia
clusters studied to date using quantum chemical methods by calculating
the binding free energies of (SA)
n
(A)
n
clusters, with n up to
20. Based on benchmark calculations, we apply the B97-3c//GFN1-xTB
level of theory to calculate the cluster structures and thermochemical
parameters. We find that the cluster structures drastically evolve
at larger sizes. We identify that an ammonium ion is fully coordinated
in the core of the cluster at n = 7, and at n = 13 we see the emergence of the first fully coordinated
bisulfate ion. We identify multiple ammonium and bisulfate ions that
are embedded in the core of the cluster structure at n = 19. The binding free energy per acid–base pair levels out
around n = 8–10, indicating that at a certain
point the thermochemistry of the clusters converges toward a constant
value.
Iodic acid (IA) has
recently been recognized as a key driver for
new particle formation (NPF) in marine atmospheres. However, the knowledge
of which atmospheric vapors can enhance IA-induced NPF remains limited.
The unique halogen bond (XB)-forming capacity of IA makes it difficult
to evaluate the enhancing potential (EP) of target compounds on IA-induced
NPF based on widely studied sulfuric acid systems. Herein, we employed
a three-step procedure to evaluate the EP of potential atmospheric
nucleation precursors on IA-induced NPF. First, we evaluated the EP
of 63 precursors by simulating the formation free energies (ΔG) of the IA-containing dimer clusters. Among all dimer
clusters, 44 contained XBs, demonstrating that XBs are frequently
formed. Based on the calculated ΔG values,
a quantitative structure–activity relationship model was developed
for evaluating the EP of other precursors. Second, amines and O/S-atom-containing
acids were found to have high EP, with diethylamine (DEA) yielding
the highest potential to enhance IA-induced nucleation by combining
both the calculated ΔG and atmospheric concentration
of considered 63 precursors. Finally, by studying larger (IA)1–3(DEA)1–3 clusters, we found that
the IA-DEA system with merely 0.1 ppt (2.5×106 cm–3) DEA yields comparable nucleation rates to that of
the IA–iodous acid system.
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