Theoretical Study On The Mechanism Of Atmospheric Methanesulfonic Acid-driven Nucleation Enhanced By Amines

Atmospheric particles can exert significant effects on visibility and human health,and they are closely linked to climate change.Secondary particles from new particle formation（NPF）processes contribute over half of total number concentration of particles in the atmosphere.Gas-phase sulfuric acid（SA）originated from SO₂ oxidation is suggested to be the key precursor driving NPF,however,the nucleation rates observed in fields can not be explained solely by SA-based nucleation.Recently,a study reported that atmospheric methanesulfonic acid（MSA）-driven nucleation will be of significance to atmospheric particle formation with stringent regulation of SO₂ emission implemented globally.As an important organic base,amines may enhance MSA-driven nucleation via acid-base reaction.Nevertheless,other than several studied alkylamines（methylamine（MA）,dimethylamine（DMA）and trimethylamine（TMA））,the enhancing potential of the remaining large number of amine species is still unclear,precluding further estimation of the contribution to particle formation from MSA.From a scientific perspective,unveiling the molecular mechanism of MSA-driven nucleation enhanced by representative amines（considering their atmospheric concentrations,structural features,etc.）is in favor of the identification of new amine precursors with strong enhancing effect on MSA-driven nucleation.Combining quantum chemistry calculation and atmospheric cluster dynamic code（ACDC）simulation,this thesis has performed an investigation on the mechanism and kinetics of MSA-driven nucleation enhanced by several amines and identified atmospheric amine species with high enhancing potential.The obtained findings are as follows:（1）Clarifying the molecular factors leading to the lower enhancing potential of DMA on MSA-driven nucleation than that of MA:Evidence from experiments suggested that DMA,which is the strongest enhancing agent in SA-driven nucleation,has a lower enhancing potential to enhance MSA-driven nucleation than MA.This indicates that the enhancing potential of amines on MSA-driven nucleation is affected by the exact molecular structures.By comparing DMA-MSA and MA-MSA nucleation systems,it is found that-CH₃ groups of DMA present significant steric hindrance effect in nucleating with MSA with the increase of cluster size.Both the steric hindrance effect of-CH₃ groups,and the weak hydrogen bond（H-bond）forming capacity of DMA preclude further growth of DMA clusters.This study clarifies that steric hindrance effect is another important structural factor in addition to H-bond forming capacity which affect the enhancing potential of an amine on MSA-driven nucleation.The enhancing potential of amines are therefore jointly determined by gas-phase basicity,H-bond forming capacity and steric hindrance.（2）Identifying amine species with high enhancing potential on MSA-driven nucleation:Based on the existing conclusions in（1）,we found that compared to MA,which is the known strongest enhancing agent in MSA-driven nucleation,monoethanolamine（MEA）among detected amine species in the atmosphere has a higher gas-phase basicity,stronger H-bond forming capacity（with a-OH group）,and also a weak steric hindrance effect.In addition,MEA has significant sources of emission.By comparing MEA-MSA and MA-MSA nucleation systems,it is found that MEA can enhance MSA-driven nucleation significantly at ppt level and has a higher enhancing potential than MA.This study elucidates the significant role of active groups of amines in H-bond formation other than the amino group,and suggests that MEA should be the amine species with the strongest enhancing potential on MSA-driven nucleation.In conclusion,on the basis of the investigation on the mechanism and kinetics of MSA-driven nucleation enhanced by MA,DMA and MEA,we proposed a new important structural factor determining the enhancing potential of amines,and identified MEA as the possible strongest enhancing agents in amines.The results of this thesis are of significance for the in-depth understanding of MSA-driven nucleation and particle formation in the atmosphere,and also provide theoretical basis for macroscopic atmospheric pollution prevention and control.