Study On The Air-water Interface Reactions Of S/N-containing Compounds And Criegee Intermediates And Their Effects On Aerosol Formation

Aerosols affect the Earth's-atmosphere system in several manners,such as reducing atmospheric visibility,altering cloud formation and directly or indirectly affecting climate and human health.The secondary formation from the reaction and aggregation of gaseous molecules is one of the important sources of atmospheric aerosols.Exploring the mechanism of aerosol formation is important to prevent the explosive growth of fine particles.Studies have shown that new particle formation(NPF)contributes at least 50%of the aerosol particles.Sulfuric acid is an important driver species in NPF,which can be stabilized by hydrogen bonding interactions or acid-base reactions.In addition,organic compounds are also important components of the tropospheric particles.Among them,alkaline organic compounds,such as methylamine and dimethylamine,can effectively increase NPF.Nevertheless,atmospheric organic amines and NH3 concentrations are also insufficient to explain the observed rates of new particle formation and growth.Liquid-phase chemistry may be another important pathway for secondary organic aerosol(SOA)formation in the atmosphere.The air-water interface is prevalent in the atmosphere,which plays a pivotal role in tropospheric chemistry.Water droplets or aerosol particles can absorb and dissolve small molecules in the atmosphere,and a series of chemical reactions can occur at the droplet interface.The low volatile compounds generated from the air-water interface in turn promote the formation of aerosols.The influence of the air-water interface on chemical mechanisms and the comparison with the known reaction mechanism in the gas phase can provide a more comprehensive and in-depth understanding of the aerosol formation process.In this dissertation,density functional theory(DFT)and ab initio molecular dynamics(AIMD)were used to explore the SO3 hydration catalyzed by amines,the interaction between sulfuric acid and amides,and the atmospheric reactions of Criegee Intermediates(CIs).At the same time,the difference of reaction mechanism between the gas phase and air-water interface was compared.The main contents and conclusions of the dissertation include the following four aspects:1.Study on the hydration mechanism of SO3 catalyzed by aminesThe H2SO4 in the atmosphere mainly stems from the oxidation and hydration of sulfur oxides,and the hydration of SO3 is the final step in the formation of H2SO4.The direct hydration of SO3 in the atmosphere needs to overcome a high barrier and the reaction requires the participation of catalysts(such as HO2,nitric acid,formic acid,NH3).In this section,AIMD simulations were employed to explore the key bond length and structure evolution of SO3 hydration in the presence of a series of organic amines,and the differences of reaction mechanisms between the gas phase and air-water interface were compared.In the presence of organic amines(such as methylamine and dimethylamine),the SO3 hydration reactions in the gas phase are nearly barrierless processes.The catalytic effect is proportional to the basicity of the gas phase organic amines:the stronger the basicity,the faster the reaction.Furthermore,the product H2SO4 directly forms clusters with amines via acid-base reaction.At the air-water interface,the ring structures formed by amines,SO3 and interface water molecules promote the transfer of protons.The ammonium cation(-RNH3+)and the bisulfate anion(HSO4-)are stable with the help of hydrogen bond interaction.This cluster formation mechanism is different from the direct aggregation of sulfuric acid and organic amines,and it is an alternative pathway for SO3 to participate in nucleation.2.Intermolecular interactions between a series of amides and sulfuric acidAmides are a special class of organic amines in the atmosphere with both a carbonyl and an amino group in their molecular structure.Through the interaction with sulfuric acid,amides may participate in the formation of atmospheric particles.In this section,the interaction of H2SO4 with formamide,methylformamide,dimethylformamide,acetamide,methylacetamide,and dimethylacetamide was studied by DFT calculations.Structural optimization and free energy value calculations were carried out at the M06-2X/6-311++G(3df,3pd)level.In the binary cluster structures,the amide can stabilize the H2SO4 molecule via three hydrogen bonding interactions,and its binding capacity is stronger than NH3 but weaker than MA.Among them,the trans-configuration amides are easier to combine with H2SO4than cis-configuration.In addition,water molecules have a weak enhancement capability on NPF.It can be inferred from the evaporation rate that the molecular clusters of formamide and H2SO4 cannot directly participate in the formation of critical nucleus3.Reaction of CIs with methanesulfonic acid at the air-water interfaceThe atmospheric reaction of Criegee intermediate can affect the concentration of OH radicals,the oxidation of sulfur oxides,and the formation of aerosols.To date,the research on interface behavior of CIs is still limited.In this section,the reaction mechanisms between CIs and methylsulfonic acid(MSA)at the air-water interface were studied using the Born-Oppenheimer molecular dynamics(BOMD)simulation method,and the mechanisms were compared with the reactions in the gas phase.The addition reaction between CIs and MSA in the gas phase is a barrierless process which follows a ring-structure mechanism.Moreover,rate constant calculation indicates that the reaction between CIs and MSA is the main acid removal pathway.At the liquid surface,the interaction between CIs and MSA is more complex than that of the gas phase,and the reaction involves three main pathways:1)direct addition reaction,2)H20-mediated hydroperoxide formation,and 3)MSA-mediated Criegee hydration.These reaction pathways follow a ring-structure or stepwise mechanism and are completed in a short time(ps).The results of this work expand our understanding of CIs atmospheric behaviors in heavily polluted cities and marine areas4.Effect of MEA on Criegee intermediate reactionsCIs can react with a variety of functional groups,such as-OH,-COOH or-NH2 group.In previous studies,only one kind of functional group was considered,and the research on the interaction between multifunctional groups was lacking.In this section,DFT calculations and AIMD simulations were carried out to investigate the effect of NH2 and OH groups in monoethanolamine(MEA)molecules on the CIs reaction.The results showed that among the four evaluated MEA configurations,two functional groups in the g'Gg' and tGg' configurations,-NH2 and-OH,have the synergistic effect on the C2 CIs.According to rate calculation,it was found that the reaction between CIs and NH2 groups in MEA is prior to that with OH groups.Moreover,OH groups promote the reaction activity between CIs and NH2 groups in MEA,while the presence of NH2 groups weakens the reactions of CIs and OH groups of MEA.At the air-water interface,CIs react with NH2 groups of MEA molecules directly or are mediated by water molecules,resulting in additional product formation.Due to the high energy barrier,the reaction of CIs with OH groups of MEA was not observed during the simulation time.