Study On The Photosensitization Mechanism Of Fatty Alcohols Induced By Brown Carbons

Fatty alcohols are photochemically inert lipids substances in the area of actinic radiation,and they are common surfactants in the environment.Such surface-active molecules are preferentially wrapped on the aerosol surface or adsorbed on the air-water interface,thereby affecting the physical,chemical,and optical properties of aerosols and air-water interfaces.Therefore,it is necessary to study the process of fatty alcohol chemical conversion(especially environmental photochemistry)to understand the influence of surfactants on atmospheric chemistry and climate.Brown carbon(BrC)is a carbonaceous aerosol that can absorb ultraviolet and visible radiation in the atmosphere,can heat the atmosphere,and has a positive effect on atmospheric radiative forcing.BrC is an important kind of photosensitive material in the environment.It can be used as a photosensitizer to promote the formation and growth of aerosol particles;or it can initiate the photochemical reaction of non-photoactive lipids,and its products can increase the complexity of environmental molecules.Obviously BrC plays a very important role in the aerosol photochemistry.However,there are few reports on its photosensitization reaction and mechanism with fatty alcoholsIn this study,BrC samples were obtained by liquid-phase reaction of methylglyoxal(MG)with ammonium sulfate(AS)or glycine(Gly),and 1-octanol was selected as a proxy of the environmental surfactant(fatty alcohols).UV-vis spectrometer,three-dimensional fluorescence spectrometer(EEM),attenuated total reflection-Fourier transform infrared spectrometer(ATR-FTIR)and gas chromatography-mass spectrometry(GC-MS)were used to study the effect of photolysis on the optical properties of BrC,as well as the photochemical reaction process of BrC and fatty alcohols.The main results are as follows:(1)Preparation of BrC and characterization of optical properties:The mixture of MG and AS or Gly was evaporated to prepare a BrC sample in the experiment.The absorption and emission spectra of MG-AS/Gly BrC were collected by using UV-vis and EEM spectrometers.The basic optical characteristics of the BrC chromophore were learned.The results show that MG-AS/Gly BrC have the maximum absorption peaks in the ultraviolet range,and a wide absorption tail that extends into the visible range,in accordance with the typical absorption spectra of atmospheric BrC.with the spectral characteristics of typical atmospheric BrC.(2)The photolysis kinetics process of BrC:The photodegradation experiments with different light times were set.The photolysis rates of MG-AS BrC and MG-Gly BrC at 280 nm under different conditions were measured at 0.2 min-1 and 0.02 min-1,respectively.The results suggested that the atmospheric lifetime of MG-AS/Gly BrC is very short(about 25～40 min),and it is easy to be photobleached.The results of EEM spectra showed that the fluorescence characteristics of secondary source BrC depended on the structure of chromophore,and MG-AS/Gly BrC had a good correlation with terrestrial source of fulvic acid.(3)Photosensitization mechanism of octanol:In the photochemical reaction of MG-AS/Gly BrC and octanol,a characteristic peak of the carbonyl group of the product carboxylic acid was observed around 1710 cm-1 in the ATR-FTIR spectrum;At the same time,GC-MS detected many functionalized and unsaturated VOCs such as octanoic acid,octenoic acid,octanal and many other functional and unsaturated VOCs products The above phenomenon was not observed in the dark or without BrC.The results show that BrC is excited to the triplet state under light conditions,and a series of free radical reactions are initiated after H-abstraction from octanol to form alkyl radicals.The photoproducts can increase the emission of non-biogenic VOCs in the atmosphere,which may affect the oxidative capacity of the atmosphere and lead to the formation of new particles,further affecting the climate,air quality and human health.