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)_<i>a</i>(ma)_<i>b</i> clusters, with <i>a</i>, <i>b</i> ≤ 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 (Δ<i><i>G</i></i>) of the clusters are used to calculate the evaporation rates of the (sa)_<i>a</i>(ma)_<i>b</i> 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)_<i>a</i>(ma)_<i>b</i> clusters were simulated and compared to the (sa)_<i>a</i>(dma)_<i>b</i> 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