Diamines Can Initiate New Particle Formation in the Atmosphere

Recent
experimental evidence suggests that diamines can enhance
atmospheric new particle formation more efficiently compared to monoamines
such as dimethylamine. Here we investigate the molecular interactions
between sulfuric acid (sa) and the diamine putrescine (put) using
computational methods. The molecular structure of up to four sulfuric
acid molecules and up to four putrescine molecules were obtained at
the ωB97X-D/6-31++G­(d,p) level of theory. We utilized a domain
local pair natural orbital coupled cluster method (DLPNO-CCSD­(T)/aug-cc-pVTZ)
to obtain highly accurate binding energies of the clusters. We find
that the (sa)1–4(put)1–4 clusters
show more ionic character than clusters consisting of sulfuric acid
and dimethylamine (dma) by readily forming several sulfate ions in
the cluster. To estimate the stability of the clusters, we calculate
the evaporation rates and compare them to ESI-APi-TOF measurements.
Using the atmospheric cluster dynamics code (ACDC), we simulate and
compare the new particle formation rates between the (sa)1–4(put)1–4 and (sa)1–4(dma)1–4 cluster systems. We find that putrescine significantly
enhances the formation of new particles compared to dimethylamine.
Our findings suggest that a large range of amines with different basicity
is capable of explaining various regions of the observed new particle
formation events. These results indicate that diamines, or related
compounds with high basicity, might be important species in forming
the initial cluster with sulfuric acid and subsequently more abundant
amines with lower basicity can assist in the new particle formation
process by attaching to the sulfuric acid–diamine nucleus.