Elucidating the Limiting Steps in Sulfuric Acid–Base New Particle Formation

The
molecular interactions between sulfuric acid (sa) and methylamine
(ma) are investigated using computational methods. The molecular structures
and vibrational frequencies of (sa)a(ma)b clusters, with a, b ≤ 4, were obtained with the ωB97X-D functional
using a 6-31++G­(d,p) basis set. The single-point energies were corrected
using the domain-based local pair natural orbital coupled cluster
methodDLPNO–CCSD­(T)with an aug-cc-pVTZ basis
set. The calculated Gibbs free energies (ΔG) of the clusters are used to calculate the evaporation
rates of the (sa)a(ma)b cluster system and compare them to the corresponding ammonia
clusters. With the atmospheric cluster dynamics code (ACDC), the new
particle formation rates of the (sa)a(ma)b clusters were simulated and compared to
the (sa)a(dma)b cluster system. It is found that methylamine is not capable of explaining
observed new particle formation event in the ambient atmosphere. Looking
into the calculated Gibbs free energy profiles it is found that the
limiting steps in forming stable (sa)3–4(base)3–4 clusters depend strongly on the formation of the
(sa)1(base)1 and (sa)2(base)2 clusters. These findings further support that compounds with
high basicity are required to form the very initial cluster nucleus,
which serves as the basis for forming new particles in the atmosphere.