Study On The Influence Of Coating On Soot Particle Morphology And Its Mechanism

Soot particle is an important component of atmospheric aerosols.It has strong ability to absorb visible lights and thus heat the atmosphere.Researches have shown that soot particle,coming after CO2,is the second most important species caused global warming.However,unlike CO2,soot particles have much shorter residence time in the atmosphere,which means that if reduced the emission,the heating effect caused by soot would be reduced significantly in a short time.It's an effective way to mitigate the greenhouse effect.There are still a great deal of uncertainties about the radiative forcing caused by soot particles,mainly due to the diversity properties of soot particles and the complexity of the soot aging process in the atmosphere.During the aging process,as the coating of other substances,or cloud process,soot particles usually change from the relatively loose branching structure to compact structure.Studies have shown that restructuring of soot particles will enhance the absorbing and scattering ability of light.However,there is still no clear mechanism about the restructuring process.Systematically studies about soot restructuring mechanism can improve the understanding of the soot aging process in the atmosphere,and further improve the prediction accuracy of radiative forcing of atmospheric soot particles.In this study,an invert burner with natural gas as the fuel has been applied to produce stable soot particles.The Volatility Hygroscopic-Tandem Differential Mobility AnalyzerAerosol Particle Mass analyzer(VH-TDMA-APM)integrated system has been applied to produce mono-size distribution soot particles,and analyze the size and mass changing of soot particles before and after processing.Through the relations between the mass of coating substance and parameters,such as the growth factor of diameter Gfd,effective density ?eff,shape factor ?,and mass-mobility diameter Dfm,the restructuring extent can be quantitatively studied.In addition,the morphology of soot particles collected under different conditions has been characterized by electron microscopy.The relationship between properties of coating and the extent of restructuring of soot particles are verified through the parameters,such as convexity and roundness.The main findings of this study include:(1)Six polycyclic aromatic hydrocarbons(PAHs)have been selected to study the relationship between coating and restructuring extent of soot particles,including phenanthrene,anthracene,pyrene,triphenylene,fluoranthene and perylene.Results showed that PAHs with low melting point have stronger ability to promote the restructuring of soot particles relative to the high melting point PAHs.Based on the relationship between the melting points of PAHs and the size of molecular clusters,it suggests that PAHs with low melting point are more likely to exist on the surface of soot particles in the form of subcooled liquid at room temperature,which can provide stronger ability to promote restructuring.Based on the rule that the scattering cross section of soot will increase with the restructuring extent,the study excluded that the restructuring of wetted soot particles were promoted by air resistance or electrostatic force.It inferred that PAHs located nonuniformly on the surface of soot particles,when part of places had enough amount of PAHs,restructuring can be activated.(2)Nine kinds of liquid coating with different properties were selected to study the relationship between the amount of liquid coating on the surface of soot particle and the restructuring degree of soot particle.The results show that the saturated vapor pressure of liquid coating has a close relationship with the restructuring ability,while coatings with higher saturated vapor pressure have relatively stronger ability to promote the restructuring of soot particle.The results from the scanning electron microscopy experiments also show that restructuring degree of soot particle has low relativity with the surface tension,but has a high relativity with the saturated vapor pressure of the coating.According to Kelvin effect and the observed experiment results,two kinds of coating modes,pendular ring and uniformity,are proposed.The liquids with high saturated vapor pressure tend to adhere to the surface of soot particles in the form of pendular ring,which has high efficiency to promote restructuring,while the low saturated vapor pressure liquids tend to coating in a uniform way with a relatively lower efficiency to promote the restructuring of soot particles.(3)During the observing under environmental transmission electron microscopy,soot particles collected on the substrates performed less degree of restructuring compared with the soot particles without substrates.By further comparing the morphology changing of soot particles without substrates,with hydrophilic substrates,hydrophobic substrates and film substrates under the same treatment conditions,it is shown that the contact area of soot particles with the substrates can obviously restrict the restructuring of soot particles.Film substrates can relatively closer simulate soot particle restructuring without substrates.The experiments further compared the morphological changes of sodium chloride particles and sulfuric acid droplets on different substrates due to the change of environmental humidity.The results show that,compared with the hydrophobic substrate,sodium chloride solution droplets,or sulfuric acid droplets have a stronger wetting ability on the hydrophilic substrate,thus after the evaporation of water,sodium chloride particles and sulfuric acid droplets had more pronounced morphology changes compared with their initial state.