Theoretical Study On Nucleation Mechanism Of New Particles Formation Based On Methanesulfonic Acid In The Atmosphere

Atmospheric aerosols have become one of the important hot topics in the field of atmospheric optics,which is of great significance for assessing climate change and environmental health.New particle formation（NPF）is one of the main sources of atmospheric aerosols,and its formation mechanism is crucial for understanding the properties and effects of atmospheric aerosols.Therefore,in-depth study of the formation mechanism of NPF at the microscopic level has received extensive attention from the scientific community in recent years.Methanesulfonic acid（CH₃SO₃H,MSA）is an extremely important sulfur-based precursor molecule in atmospheric aerosols,and its nucleation reactions with water,small nitrogenous compounds and some low-volatile organic molecules have been extensively studied experimentally and theoretically in recent years.However,due to the challenges of the newly formed stable particles,such as complex structures,multiple interactions,diverse composition ratios,and undetectable nanoscale sizes,there are still many unknowns about the formation mechanism of new particles based on MSA.This thesis uses the method of combining quantum chemical calculation and molecular dynamics simulation,to investigate the nucleation process of stable clusters in MSA particles by small molecular nitrogen-containing compounds represented by ammonia（NH₃,A）and methylamine（CH₃NH₂,MA）,and carbonyl compounds represented by formaldehyde（HCHO,FA）,glyoxal（C₂H₂O₂,GL）and methylglyoxal（C₃H4O₂,MG）,respectively.The microscopic reaction mechanism of these two types of gas-phase molecules with MSA was elaborated,and the possible influence of related factors on their microscopic nucleation mechanism was evaluated.The main research contents are as follows:1.The nucleation mechanism of MSA,A and MA under acid-rich conditions and the influence of alkali were studied.Using quantum chemical calculations,the structure,energy,stability and intermolecular interaction types of nanoclusters formed by binary system（MSA）_2n-（MA）_n and the ternary system（MSA）_3n-（MA）_n-（A）_n（where n=1-4）were studied.The results showed that the lowest energy structure in each system has good thermodynamic and kinetic stability.In the most stable structures,all methylamines or ammonia are protonated,and form stable ion pairs with MSA,contributing to charge transfer and cluster stability.Under acid-rich conditions,MA and A have a synergistic effect on NPF,and the effect of A is more pronounced with the increase in cluster size.Excess MSA molecules not only improve the stability of the clusters,but also provide potential sites for further growth.This work reveals the microscopic reaction mechanism of mesylate-methylamine-ammonia particles under acid-rich conditions,demonstrating the importance of acid-rich conditions and acidic particles.2.The nucleation mechanism of mesylate with three aldehydes and the influence of water,ammonia and methylamine on the nucleation were studied.The structure,energy,type of intermolecular interactions,and formation stability of binary nanoclusters X-MSA/SA（X=FA/GL/MG）and the ternary nanoclusters X-MSA/SA-Y（X=FA/GL/MG,Y=W/A/MA）were studied using quantum chemical calculations.The results showed that the most stable configuration of the FA/GL/MG-MSA/SA binary cluster contains only one hydrogen bond,no proton transfer occurs between molecules,and the binary cluster is poorly thermodynamically and kinetically stable at room temperature.For the ternary clusters formed under the action of W/A/MA molecules,their stability was significantly improved.The main reason for this is that the acid here is able to protonate the carbonyl oxygen,thereby making the carbonyl carbon more electrophilic and easily attacked by nucleophilic molecules such as W,A and MA to generate stable hydration products or aminolysis products.In addition,with the increase in the acidity of acidic molecules,the activity of the aldehyde and the basicity of the nucleophilic molecules in the system,the product formed by the aldehyde participating in the reaction become more and more stable.