Study On The Formation Of Nitrous Acid And Nitric Acid At The Gas-Liquid Interface And The Participation Of Organics In The Nucleation Mechanism

Aerosols are the main cause of current air pollution,which play an important role in global radiation balance and cloud formation,and in turn affect global climate change and adversely affect human health.New particle formation(NPF)is the main source of atmospheric aerosol particles.Generally,NPF includes two steps:the formation of a critical nucleus and its subsequent rapid growth.At present,a large number of theories about aerosol nucleation have been proposed,such as H2SO4-H2O binary nucleation theory,H2SO4-H2O-NH3 ternary nucleation theory,ion-induced nucleation theory,and nucleation theory involving iodine oxide.In the above theories,sulfuric acid(SA)is considered to be the main contributor to NPF.Meanwhile,many field measurements show that the aerosol samples contain a considerable concentration of organic components.For example,alkaline substances(methylamine,dimethylamine,amides etc.),organosulfates can effectively increase NPF,and organosulfates can account for 4%-30%of the total atmospheric fine particulate matter(PM2.5).However,there is still a lack of comprehensive understanding of the mechanism of organics participating in NPF,especially at the molecular level.Liquid water(fog droplets,cloud droplets and aerosol water)is ubiquitous in the atmosphere,having an important influence on the formation of secondary organic aerosols(SOA)in the atmosphere.The gaseous precursors or their gaseous primary oxidation products react in the atmospheric aqueous phase to produce low-volatility and high-oxidizing organic compounds(such as organic nitrogen compounds and organic sulfides,etc.).After the evaporation of water,the products remain in the particle phase,forming the aqueous phase secondary organic aerosol(aqSOA).Studies have shown that aqSOA is an important source of SOA in areas with high humidity and high pollution.In addition,the liquid water in the atmosphere can also affect the atmospheric tropospheric chemical process.This is because these droplets have unique surface properties,which have an important influence on the atmospheric reaction process.When the reaction occurs on the surface of a droplet or at the interface between liquid water and a hydrophobic medium,the asymmetry of the interface,the existence of free hydroxyl groups of water,and the unique properties of the interface water often led to unexpected solvation effects or even change the original reaction mechanism.At present,there are still many controversies on the reaction mechanism of the gas-liquid interface.In this dissertation,density functional theory(DFT)and ab initio molecular dynamics(AIMD)were used to study the reaction mechanism of chlorine(Cl)radicals and nitrogen oxides in the gas phase and the gas-liquid interface and the nucleation mechanism of organic matter(2-methyltetrol sulfate ester,propionamide)participating in NPF.The main contents and conclusions of the study include the following four aspects:1.Study on the reaction mechanism of Cl radicals and NO in gas phase and gasliquid interfaceStudying the potential sources of atmospheric nitrous acid(HONO)is one of the important challenges of current atmospheric chemistry research.In this study,BornOppenheimer molecular dynamics(BOMD)and metadynamics simulation methods were used to study the mechanism of the formation of HONO for the reaction of Cl radicals and NO in the presence of one or two water molecules((Cl)(NO)(H2O)n(n=12)),as well as on the surface of the droplet.Metadynamics simulation results show that there is a free energy barrier of 0.95 kcal mol-1 for the reaction of Cl radical and NO to form HONO in the presence of one water molecule,while the reaction of Cl radical and NO form HONO in the presence of two water molecules is barrierless.In the dihydrate system((Cl)(NO)(H2O)2),only one water molecule directly participates in the reaction,and the other water molecule acts as a "solvent" molecule.The reaction mechanism of the Cl radical and NO on the surface of the droplet is similar to that of the dihydrate system,and it also follows the single-water molecular mechanism.In order to estimate the atmospheric importance of the reaction between Cl radicals and NO,the reaction rate of the monohydrate system((Cl)(NO)(H2O)1)was calculated.The results show that under the conditions of NO concentration of 8.56×1012 molecule cm-3,Cl radical concentration of 8.00×106 molecule cm-3,and water molecule concentration of 5.18×1017 molecule cm-3,the monohydrate system could account for 52.9%of the unknown HONO production rate(Punknown)at site 1 and 70.6%of Punknown at site 2 in East China Sea.This study identified the importance of the reaction system of NO,Cl and water molecules in the formation of HONO in the marine boundary layer region.2.Study on the reaction mechanism of Cl radicals and NO2 in gas phase and gasliquid interfaceNitric acid(HNO3)is one of the main air pollutants and has an important impact on air pollution.However,the formation of HNO3 in the marine boundary layer region is still poorly understood.In this study,Born-Oppenheimer molecular dynamics(BOMD)was used to study the mechanism of the formation of HNO3 for the reaction of Cl radicals and NO2 in the presence of one to three water molecules((Cl)(NO2)(H2O)n(n=1-3)),as well as on the surface of the droplet.The thermodynamic integration was used to calculate the relative free energy variation along the reaction coordinates for the(Cl)(NO2)(H2O)1(monohydrate)system.The result reveals that the reaction of Cl radical and NO2 form HNO3 in the presence of one water molecules is barrierless,and is exergonic 26.5 kcal mol-1.This suggests the formation of HNO3 from the monohydrate system is thermodynamically favorable.For all the studied systems,the formation of HNO3 can be directly observed during the BOMD simulation,and only one water molecule directly participates in the reaction.The loop structures,which were expected to be ubiquitous on the air-water interface,were not observed.Compared with the reaction of Cl radical and hydrated NO,the reaction of Cl radical and hydrated NO2 is barrierless in the presence of one water molecule.This study identified the importance of the reaction system of NO2,Cl and water molecules in the formation of HNO3 in the marine boundary layer region.3.Study on the nucleation mechanism initiated by 2-methyltetrol sulfate2-Methyltetraol sulfate(MTS)is present in aerosol samples all over the world.In this study,quantum chemistry calculation,kinetic calculation,and molecular dynamic simulation(MD)methods were used to study the contribution of MTS and aerosol nucleation precursors(sulfuric acid(SA),water(W),ammonia(N),methylamine(MA)),dimethylamine(DMA),trimethylamine(TMA))to the new particle formation(NPF).The results suggest that the molecular interactions in the studied clusters are mainly hydrogen bond interactions and electrostatic interactions.The formation free energies of all clusters are negative,indicating that clusters can form spontaneously under standard atmospheric conditions.The stability of the cluster is estimated based on the total evaporation rate of the cluster.(MTS)(SA)and(MTS)(W)are the most stable and unstable clusters,respectively.Then,the MD simulation was used to investigate the contribution of the MTS-SA system to NPF in a larger time and space scale.The results show that MTS can form larger clusters by self-aggregation or absorption of S A molecules.The diameter of the formed cluster is larger than the critical cluster diameter(?1 nm),which indicates that MTS can initiate NPF alone or together with SA.4.Study on the formation mechanism of new particles involving propionamide and sulfuric acidPropionamide(PA)is an important pollutant that is emitted into the atmosphere through many ways.It is widely present in the atmosphere and may participate in the formation of new particles(NPF).In this study,the formation mechanism of(PA)m(SA)n(m=0-3,n=0-3)clusters was studied by using computational chemistry and kinetic calculation methods,and the enhancement of the H2SO4(SA)-based NPF by PA was evaluated.Studies have shown that the formation free energies of all clusters are negative,indicating that clusters can form spontaneously under standard atmospheric conditions.Further.the=O group in PA plays an important role in the clusters with more PA than SA,and the basicity of bases exert a greater influence with increasing number of SA.PA can enhance the SA-based NPF at the parts per billion(ppb)level,which is typical for concentrations of C3-amides in,for example,urban Shanghai(China).Monomer evaporation is the main degradation pathway of(PA)m(SA)n clusters,which is different from the degradation pathway of SA-DMA system.The formation rate of PA-containing clusters is comparable to the rate coefficients for PA oxidation by hydroxyl(OH)radicals,indicating that participating in the SA-based NPF is a crucial sink for PA.