Modeling the Binding Free Energy of Large Atmospheric Sulfuric Acid–Ammonia Clusters

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.