Molecular Dynamics Simulations On The Formation Mechanism Of Secondary Organic Aerosol

In recent years,frequent severe haze events,namely the fine particulate matter(PM_2.5)pollution,have been found to be mainly attributed to the formation of secondary aerosol.Secondary organic aerosol（SOA）is an important component of atmospheric fine particulate matter,and is formed by photochemical reaction of atmospheric volatile organic compounds（VOCs）and gas-particle transformation.SOA has a significant impact on global climate change,as well as the negative influence on air quality and human health.However,the comprehensive understanding of the formarion mechanism of SOA is still insufficient,due to the large variety of precursor species,plus the complex chemical and physical processes during the formation of SOA.Therefore,the influence of VOC on SOA formation urgently needs to be clarified,in order to provide a basis for the numerical study and the control of haze pollution.In this paper,molecular dynamics simulation method was used to study the interfacial processes between VOC and sulfuric acid,and the quantitative relationship（QR）between the structure and property of VOC and the formation of SOA was obtained.First of all,the FAC parameters that are widely used in laboratory studies were used for quantifying the SOA formation.And the selection of VOC was based on its sources and the magnitude of FAC so as to choose representative VOCs.Then,the selected VOCs were used to construct the VOC-sulfate two layer models,i.e.models containing VOC contaminated air box and sulfate box.Thus,we utilize the models to investigate the interfacial processes between various VOCs and sulfate.The results are as follows:（1）radial distribution function（RDF）was applied for the molecular dynamics simulations,and the RDF analyses confirmed that aromatics and olefins are the dominant VOCs precursors of SOA,which accords with the experimental researches.（2）Distances and intensities of molecular interactions between hydrogen atoms of VOC and sulfur atom of sulfate suggested that the aromatics,olefins and acetylene molecules have a stable vertical orientation on the surface and moreover some of them tend to be at one preferential position.Furthermore,mean square displacement（MSD）was used to study the diffusion of VOC molecules during the interfacial processes.MSD analyses revealed that VOCs with higher FAC generally diffuse slow.Whereas,VOCs with the lowest FAC such as ethane and acetylene,diffuse the fastest.The results suggested that the diffusion of VOCs may be negatively correlated with FAC,while VOCs that exhibit preferential orientation are generally the one with high FAC.（3）The correlation relationships between the total valid interactions Tg,diffusion coefficients D and FAC were discovered so as to build the quantitative relationship（QR）between FAC and the two variables as FAC=1.00 Tg-0.62 D.By analyzing the intermolecular interactions between VOC and sulfate,and the diffusivity of VOC,the correlation relationships between the total valid interactions Tg,diffusion coefficients D and FAC were discovered.When taking both the two variables into account,the R square（R²）of the QR is better than that of the QR built by either of the two variables.Thus,it can be considered that the total valid interactions of the two variables,Tg and D,can reflect the impact of the structures and properties of VOCs on the formation of SOA.By analyzing the two variables total valid interactions Tg and diffusion coefficients D,it is suggested that VOCs with polarity or existing hydrogen bonds with sulfate have strong induced dipole-permanent dipole interactions with sulfate.Moreover,conjugated bond and six-member ring structure may increase the stable contact between VOC molecules and sulfate interfaces and thus increases the valid dispersive interactions.At last,alkanes and non-aromatic species with lower molecular weight with lower FAC seems to be due to their high diffusion coefficient D less contributing to the formation of SOA,such as n-heptane,ethane and acetylene.In short,it is the strong induced dipole-permanent dipole interaction and valid dispersive interaction accumulation that determine the SOA formation capacity.（4）The QR,FAC=1.00 Tg-0.62 D,can help to explain how the different VOC have the different FAC.At last,the two variables total valid interactions Tg and diffusion coefficient D of dichloromethane in the aqueous VOC-sulfate model was used to predict the SOA formation capacity of dichloromethane and ethane.Based on the QR,the FAC of dichloromethane and ethane are predicted to be 3.43 and 8.11,respectively,which indicates that dichloromethane is a moderate or potent SOA precursor while ethane is a potent precursor.