| The key stage of aerosol nucleation is the new particles formation(NPF).In this stage,acid molecules often play a very important role,they are generally considered to be the most critical precursors in the process of nucleation.In addition,alkaline molecules are ubiquitous in the atmosphere,such as ammonia(A),methylamine(MA)and dimethylamine(DMA)et.al,all of them can effectively promote the atmospheric nucleation process dominated by acid.Although sulfuric acid(H2SO4,SA)as one of the most important acid molecule in the atmospheric nucleation,it alone is not enough to explain the explosive NPF events observed in the stations and laboratories.It is difficult to reproduce the condition of the actual atmospheric nucleation in the smoke chamber and the lag of some key online detection methods,which limited together to explain the micro-mechanism of NPF in the china-haze from the experimental point of view.Obviously,other organic acids also play an important role in the NPF under the real atmospheric conditions.At present,the results from a large number of stations and experiments have confirmed that methanesulfonic acid(MSA,CH3SO3H)participated in the NPF of the atmosphere,but the micro-mechanism of the nucleation involving in methanesulfonic acid and basic precursors,the influence of relative humidity(RH)and other nucleation precursors on the micro-mechanism are not clear.In this paper,the micro-mechanism of MSA and several important alkaline nucleation species was studied systematically by combining the quantum chemical(QM)calculation with the Atmospheric Cluster Dynamics Code(ACDC)simulation under different atmospheric conditions,and the effects of RH on the nucleation mechanism were evaluated.The main research contents are as follows:(1)The optimization and energy of electronic structure of the global minimum configuration of trimer obtained by global search were investigated by density functional theory(DFT)method.Then the type and size of the noncovalent interaction in trimer were analyzed by AIM and NCI methods,and the influence of synergistic action on the stable trimer formation was evaluated.The results show that the six-or eight-cyclic hydrogen bonds are a typical characteristic of the stable trimers.The red shift of the H-OS vibration of MSA is larger than that of H-N of the basic donor molecule in trimer.In the MSA-X-Y(X,Y=A,M,D)trimer,proton transfer from MSA to X is exothermal and barrierless due to the synergy between X and Y molecules in MSA-X-Y trimers compared with MSA-X dimers.In addition,low temperature and high pressure significantly facilitate the formation of trimers,and the total evaporation rate of the trimers decreased as the alkalinities of X and Y increased.This work provides a new viewpoint on the microscopic mechanism of the formation of initial clusters involving in MSA at the atomic-molecular level,namely is synergistic effect has a significant effect on the stability of trimers in the typacial atmosphere.It is further confirmed that synergy plays an important role in atmospheric NPF events.(2)The formation mechanism of methanesulfonic acid(MSA)and ammonia(A)clusters is investigated using density functional theory(DFT)and Atmospheric Cluster Dynamic Code(ACDC)under the typical atmospheric conditions.The results suggest that hydrogen bonding and electrostatic interactions induced by proton transfer could provide the primary driving force that forms these clusters.Under anhydrous conditions,the concentrations of MSA and A have significant effects on the formation rate of MSA-A clusters,and the formation of the initial(MSA)2 dimer is a rate-determining step of cluster growth.Unlike the anhydrous nucleation process,the formation of the initial dimer(MSA)(A)dimer is a rate-determining step due to hydration in the whole nucleation process under atmospheric conditions with the high relative humidity(RH),and the formation rate increases significantly with RH and reached up to a factor of 105 at RH=100%.It also shows that the effect of the formation of the MSA-A clusters on the NPF cannot be ignored in the atmosphere over the ocean and coastal areas.In general,the effective nucleation events are often difficult to occur due to the high evaporation and poor stability of the MSA-A clusters under the typical atmospheric conditions.However,the contribution of the MSA-A system to the NPF cannot be underestimated in the heavily polluted weather with the high concentration precursors and high atmospheric RH.In addition,the formation of MSA-based ternary clusters through the participation of other species such as SA may be more important in promoting effective nucleation of MSA-A system in the coastal atmosphere.(3)Using,the nucleation mechanism of MSA-MA system under different atmospheric conditions was studied.The formation mechanism of MSA-MA system and the effect of hydration on the formation mechanism of clusters consisting of methanesulfonic acid(MSA)and methylamine(MA)are investigated by molecular mechanics(MM),semi-empirical(SE),density functional theory(DFT)and Atmospheric Cluster Dynamics Code(ACDC)methods under different atmospheric conditions.The results showed that the electrostatic interactions caused by hydrogen bonding and proton transfer are still the main driving forces for the cluster formation.The hydration can promote proton transfer significantly from the MSA to the MA molecules and form a typical acid-base ion pair structure.Although different concentrations of precursors have a significant influence on the formation rate of MSA-MA clusters,the effective nucleation event is still difficult to happen under the typical atmospheric conditions.Hydration can significantly increase the formation rate of the MSA-MA system and change the growth path of the initial clusters.The formation of(MSA)2 and(MSA)(MA)dimers is the rate-determining step of the anhydrous and hydrous systems,respectively.The formation rate increases with the relative humidity(RH)and reached up to a factor of 2700 at RH=40%.It also shows that the contribution of MSA-MA clusters to the NPF cannot be ignored under the atmospheric conditions with high relative humidity.In addition,the results show that the contribution of the the MSA-MA system is less than that of the SA-MA system to the NPF under the typical atmospheric conditions,unless the concentration of the precursors are very high([MSA]>6× 107 molecules·cm-3 and[MA]>1 ppt or[MSA]>1×106 molecules·cm(-3) and[MA]>100 ppt)or the involvement of other species(such as sulfuric acid). |
PDF Full Text Request