| Sea salt aerosol(SSA)constitutes an important chemical link between the ocean and the atmosphere.SSA coated by surface-active organics commonly possesses inverse micelle structure with an aqueous core coated by a hydrophobic organic monolayer.The chemical composition and surface organization of organic films of SSAs have important influences on their physical,chemical and optical properties,which in turn have effects on many atmospheric processes.For instance,changes in packing order and stability of the organic coatings will alter their permeability toward water and trace gases,and thus have impact on aerosol growth,cloud condensation nuclei activity,light absorption and scattering,and heterogeneous reactions,etc.Through experimental simulation of organic films of SSAs and investigation of their surface properties with infrared spectral,the interactions between surfactant molecules and further interactions with aqueous ions and water-soluble organic compounds were studied at the molecular level,after which the influence of organic coatings on atmospheric processes of SSAs was discussed.Langmuir monolayers are monomolecular layers formed by the spontaneous diffusion of long-chain surface-active organics with low volatility at the air-water interface.With Langmuir trough,the changes in surface pressure(?)along with decreasing molecular area(A)of single and mixed component monolayers were obtained.The ?-A isotherms provide macroscopic information about the interfacial behavior of corresponding monolayers above an aqueous surface.In addition,the combination of IRRAS with Langmuir trough enables us to consider the surface properties of monolayers from microscopic viewpoints.Understanding the interfacial properties of monolayers will be helpful for the prediction of the fate and behaviors of surfactants in the real-world SSA.The interfacial characteristics of organic coated SSA are affected by both the composition of the saline core and species of film-forming molecules,thus the whole composition of the aerosol should be considered in the study of its interfacial processes.In this study,using low volatile organic compounds commonly observed in the marine aerosol surface as model molecules,and using pure water or a mixture of ten components of sea salts or solutions containing water-soluble organic compounds as the subphase,a series of SS A proxies were established based on monolayers at the air-aqueous interface.The main conclusions of this study are as follows:(1)The interfacial properties of organic compounds with different molecular structures can be investigated by examining their single component monolayers.From the distribution of ?-isotherms of fatty acid and fatty acid methyl ester monolayers,a chain-length dependence in packing order of the organics was observed,thus it can be speculated that the stability of the monolayers increases with increasing chain length(C14<C16<C18<C20).Comparing the ?-A isotherms and IRRAS spectra of the monolayers of seven C18 organics,the stability of the monolayers was found to be more easily affected by the hydrophobic chain structure than the hydrophilic head group.In addition,the sea salts that dissolved in the saline core exert significant impacts on physiochemical properties of organic coatings,such as their surface activity,solubility and surface structure,which in turn have effects on aerosol nucleation process.The interaction mechanisms between the sea salts and the organic monolayers were analysed based on the IRRAS spectra.It was revealed that the ordering of surface molecules and surface-bulk partitioning of fatty acid monolayers are sensitive to the inorganic ions in the subphase.(2)Considering the high chemical complexity of surface organic films of real SSA,we further studied the interactions between different lipids by establishing their binary monolayers at the air-water interface.Through determination of ?-A isotherms of mixed monolayers with different composition and mixing ratio,the excess molecular area of mixing(?Aex)and the excess Gibbs free energy of mixing(?GeX)were calculated,after which the thermodynamic stability and interactions between aerosol surface molecules were analysed.The monolayers of stearic acid/oleic acid(SA/OA)and stearic acid/elaidic acid(SA/EA)were used to clarify the effects of saturation degree and double bond configuration on the mixed monolayers.SA molecules formed condensed packed monolayers at the air-water interface.The measurements here indicated that SA/OA and SA/EA monolayers are more loosely packed,and thus likely to induce defects into the lipid monolayers.This obstructive effect on the molecular ordering is significantly remarkable for OA when the cis-double bond exists at the middle of chain.A trans-double bond in the alkyl chain of EA also produces the obstruction,but the degree is not as much as that caused by the cis-one.In addition,the individual effects of the sea salts on the isotherms of the mixed monolayers were examined,among which Ca2+was found to play a dominate role in film contraction,and Ca2+is also highly enriched in real SSAs.This is maybe due to the formation of stable bidentate chelating complex between Ca2+and COO groups.As an important class of surfactants present at the interface of sea surface microlayer(SML)and SSA,the effect of sterols on surface properties of SSA has not received much attention and yet to be fully elucidated.The use of sterols as one of the binary components in the mixed systems allows us to interpret the changes in interfacial properties induced by lipids with structures significantly diferent from that of fatty acids.Systems consisting of SA and sterols of different structures spread at the air-water interface were used to form mixed monolayers.The increases in lift-off area of SA monolayer indicated the expanding effect of sterols and further lowering effect on its stability.In addition,it was shown that the nature of the sterols plays a role in the interaction of SA,with SA/cholesterol exhibiting the strongest stability,followed by SA/stigmasterol,and SA/ergosterol being the weakest.The interactions between film components have been analyzed in terms of thermodynamic parameters including ?Aex and ?Gex,which revealed that repulsive forces between SA and sterol molecules prompted the occurrence of the expanding effect observed in the monolayer systems.The decrease in conformational order of SA monolayer upon addition of sterols can also be evidenced by the lower peak intensities of vas(CH2)and vs(CH2).The observed difference in the interactions of the sterols with SA is related to the specific structures of the sterol molecules.The repulsive interaction between the two components and bulkier hydrophobic portion of sterols were the reason for reduced molecular packing effectiveness at the interface.(3)Surface seawater is the largest sink and transport medium for perfluorocarboxylic acids.From laboratory sea spray simulations and field observations,perfluorocarboxylic acids have been found to be strongly accumulate in the SML and SSAs.Perfluorooctanoic acid(PFOA)is widely spread in the SML,and it is highly soluble in aqueous environments.With longer chain length and negligible solubility,perfluorotetradecanoic acid(PFTA)can form highly condensed monolayer at the air-water interface.In this study,we focused on the interactions of water soluble and insoluble perfluorinated pollutants with Langmuir monolayers of fatty acid and alcohol applied as simplifed models of SSAs.Unlike that of SA and stearyl alcohol(C180H),the isotherm of PFTA monolayer possessed a much larger lift-off area,which indicated that the alkyl chains of PFTA were more loosely packed.This is due to the replacement of hydrogen with fluorine atoms yields remarkable surface activity and weaker intermolecular interactions in PFTA.Expansions in the ?-A isotherms indicated that the monolayers were disordered by sea salts in the subphase.This destabilization effect was confirmed by IRRAS spectra.Furthermore,PFOA molecules dissolved in aqueous subphases were found to be incorporated into lipid monolayers via alterations in ?-A isotherms of S A and C180H.The introduction of perfluorocarboxylic acids will alter the surface properties and thus the atmospheric fate and behavior of SSAs.A consequence of perfluorocarboxylic acids partitioning to the air-water interface is the possibility of their atmospheric transport distribution,and deposition via aerosol particles.Meanwhile,our work highlights the importance of SSA for the global transport of other persistent,water soluble and surface-active substances. |
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