Molecular-Level Understanding of Synergistic Effects in Sulfuric Acid–Amine–Ammonia Mixed Clusters

The abundance and basicity of a stabilizing
base have shown to
be key factors in sulfuric acid driven atmospheric new-particle formation.
However, since experiments indicate that a low concentration of ammonia
enhances particle formation from sulfuric acid and dimethylamine,
which is a stronger base, there must be additional factors affecting
the particle formation efficiency. Using quantum chemistry, we provide
a molecular-level explanation for the synergistic effects in sulfuric
acid–dimethylamine–ammonia cluster formation. Because
of the capability of ammonia to form more intermolecular interactions
than dimethylamine, it can act as a bridge-former in sulfuric acid–dimethylamine
clusters. In many cluster compositions, ammonia is more likely to
be protonated than dimethylamine, although it is a weaker base. By
nanoparticle formation rate simulations, we show that due to the synergistic
effects, ammonia can increase the particle formation rate by up to
5 orders of magnitude compared to the two-component sulfuric acid–amine
system.