Spectroscopic Investigation Of The Surface Properties Of Typical Aqueous Organic Aerosol

Atmospheric aerosols are commonly composed by inorganic and organic mixtures and constitute an important fraction of air pollutants.Adsorption of volatile species onto aqueous aerosol surface results in aqueous aerosols coated by organic films.This structure is considered as inversed micelle,consisting of a hydrophilic core and a hydrophobic surface.The surface organization of highly ordered organic film may have significant impacts on the evaporation of water vapor,the droplet growth and the transport of atmospheric pollutants.Langmuir monolayer is a good model for investigating the surface properties of an organic film on the aqueous aerosol.Polarization modulation-infrared reflection absorption spectroscopy(PM-IRRAS)is a surface-sensitive technique used to in situ detection of multiple reactions on the organic monolayer.Low volatile organic compounds with long carbon chain observed in the atmosphere and marine surface were selected as proxies.The interaction between inorganics and organics,the interaction between different lipids and the photochemical oxidation of organic monolayers were investigated.(1)The presence of inorganics in the aerosol can affect the physiochemical properties of organic coatings,such as surface activity,solubility and surface structure.which in turn have effects on aerosol nucleation process.We focused on the effect of heavy metal ions(Ag+,Zn2+,Fe3+.Ca2+ and Al3+).Surface pressure-area(π-A)isotherms suggested that cations induce fatty acid monolayer expanded or condensed.IRRAS spectra show the new bands of carboxylate with different coordination types,which indicates that surface-bulk partitioning was also altered.The π-A isotherms show that Zn2+ and Fe3+ strongly interact with dipalmitoylphosphatidylcholine(DPPC)molecules and induce condensation of the monolayers in a concentration-dependent manner.Highly concentrated Zn2+ ions make the DPPC monolayer tightly packed.IRRAS spectra show that the Fe3+ ion binds to the phosphate group and causes dehydration of the carbonyl group to the formation of lipid-coated Fe particles.The interaction between cations and lipids gives rise to pollutants transfer into the atmosphere.(2)In addition to cations,anions also have impact on organic film structures and the formation of marine aerosols.The interactions of common anions(Br-,Cl-,NO3-,SO42-,CH3COO-and HCO3-)with DPPC molecules were investigated by π-A isotherm and IRRAS.The isotherm showed that different anions in the subphase induced the compressed or expanded characteristics of organic monolayers.The effect of different cations(Na+ and NH4+)and concentrations were also analyzed.The sequence of increasing areas for DPPC monolayer on the different solutions is in good agreement with Hofmeister series.IRRAS spectra showed that the alkyl chains of DPPC molecules were all-trans in the presence of SO42-.Phospholipids preferred to combine with SO42-relative to Br-,leading to the enrichment of organicGcoated sulfate salts in marine aerosols.(3)We further studied the interaction between different lipids at the air-water interface and established binary mixture,such as fatty acid and phthalate ester.The excess mixing area and Gibbs free energy based on π-A isotherms were used to analyze the miscibility and stability of mixed films.We focused on the interaction between fatty acid and phthalate ester,which is an important group of persistent organic pollutants in the atmosphere.Pure phthalate ester molecules cannot form a stable monolayer.Mixing with sea salts and fatty acids,the excess Gibbs free energy of mixed monolayer became lower,indicating the formation of a more stable monolayer.Among the ten components in the sea salts,Ca2+ ions play the main role in the compression of mixed films.In the mixture of fatty acid and alcohol,we compared the effect of headgroup,saturation and mixing ratio on the miscibility and stability of mixed films.The stearic acid/alcohol monolayer was less stable than the oleic acid/alcohol monolayer.The steric hindrance of the double bond in oleic acid is the reason for the loose packing of oleic acid monolayer.The negative value of the excess Gibbs free energy indicated the attraction between oleic acid and alcohol.The most stable mixed monolayer was obtained by equimolecular oleic acid and alcohol.Our results also highlight the effect of sea salts,which condensed the mixed monolayer and lowered the excess Gibbs free energy.(4)To better understand the heterogeneous reaction of organic coating on the aqueous aerosol surface,we have simulated the photosensitized reaction of an organic film in a Langmuir trough.Atmospheric brown carbon can absorb UV-visible light and transfer energy to low volatile organic compounds on the aerosol surface.Imidazole-2-carboxaldehyde,humic acid,atmospheric PM2.5 sample and secondary organic aerosol sample from chamber were used as photosensitizers.Stearic acid(SA),elaidic acid(EA),oleic acid(OA)and phospholipids(1,2-distearoyl-sn-glycero-3-phosphocholine(DSPC)and 1,2-dioleoylsn-glycero-3-phosphocholine(DOPC))with the same aliphatic length and different degree of saturation were chosen as film-forming species.The nature of the double bond(trans and cis)in unsaturated organic compounds have different effect on the organic film area.The OA monolayer possessing a cis double bond in the carbon chain was more expanded than the EA monolayer on the artificial seawater containing photosensitizer.The change in the relative area of DOPC monolayer indicated that DOPC did not react with photosensitizers under dark condition.The photochemical reaction initiated by the excited photosensitizer and molecular oxygen can generate more water-soluble DOPC hydroperoxidation,leading to an increase in the molecular area.However,DSPC did not yield any oxidized products under the same conditions.The changes of IRRAS spectra between light and dark experiments also support the mechanism of the photosensitizing reaction on unsaturated lipid monolayer.