Optical And Mass Spectrum Study On The Benzene Aqueous Secondary Organic Aerosol

Secondary organic aerosol?SOA?particles are an important part of atmospheric fine particles(PM_2.5).They can scatter and absorb solar radiation,affect atmospheric visibility,and endanger human health.Benzene,toluene and other aromatic hydrocarbon emitted by anthropogenic sources such as vehicle exhaust and solvent use are the main precursors of SOA.Benzene is the simplest aromatic compound,after discharged to the atmosphere;it reacts with OH radicals to produce semi-volatile and non-volatile products.These products form SOA particles via nucleation or gas/particle partitioning.In addition,benzene can dissolve in raindrops,clouds,liquid aerosol and other aqueous phase,and undergo a series of chemical reactions to produce SOA particles after evaporation of water.The contribution of aqueous-phase processes to SOA is equivalent to that of gas-phase processes,and it has become a hotspot in current atmospheric chemistry research.The aqueous-phase reaction device was set up to start the photooxidation of benzene in solution with OH radicals generated by hydrogen peroxide under ultraviolet light in this study.The TSI 9302 atomizer was used to atomize the reaction solution and generates SOA particles through the diffusion dryer.The particle size and chemical components of SOA are characterized by on-line and off-line mass spectrometry and spectroscopic instruments.The effects of heavy metal ions and reductive nitrogen compounds on the aqueous benzene are investigated.The following experimental results are obtained:The mass spectrum of SOA particles produced by OH radical initiated photooxidation of benzene in aqueous phase was measured on-line by the aerosol laser time-of-flight mass spectrometer,which contained the fragmentation peaks of benzene ring?mass-to-charge ratio m/z=39?C₃H₃⁺?and m/z=65?C₅H₅⁺??,aldehyde?m/z=29?CHO⁺??m/z=57?CHOCO⁺??and carboxylic acid?m/z=44?COO^-??.The off-line measurement by UV-Vis absorption spectrometer and fluorescence spectrometer shows that the aqueous benzene SOA has absorption at 275 nm and with fluorescence characteristics.The infrared spectrum measured by Fourier transform attenuated total reflection infrared spectrometer contains stretching vibration peaks of C=C,C-H of benzene ring,and the absorption peaks of C-O,O-C-O and O-H.Also,its electrospray ionization mass spectra contains ion peaks with m/z up to 915.These spectral information indicate that phenolics,aldehydes,carboxylic acids,aromatic ethers,and high molecular weight products are the main components of aqueous benzene SOA.The presence of heavy metal ions such as iron and copper ions in the aqueous phase can affect the chemical composition and absorptivity of the aqueous benzene SOA.Heavy metal ions can catalyze hydrogen peroxide to generate more OH radicals,resulting in increased content of phenolics,aldehydes and other high molecular weight compounds formed by the polymerization of phenolic products.The intensity of the absorption peak in the UV and IR spectrum increased,and the ion peak with m/z up to 1175 appeared in the electrospray ionization mass spectra of aqueous benzene SOA with heavy metal ions.With the increase of product content,the particle size of aqueous benzene SOA and the mass absorption coefficient that characterizes the absorption capacity increase.The order of promoting the absorption capacity of aqueous benzene SOA by heavy metal ions is iron ion>copper ion>ferrous ion>manganese ion>nickel ion.When reducing nitrogen compounds such as ammonia,ammonium sulfate,methylamine,and aminoacetic acid are present in the aqueous photooxidation system of benzene,the ultraviolet absorption peak of aqueous benzene SOA is red-shifted,and the fluorescence efficiency is increased.The results of on-line measurement by the aerosol laser time-of-flight mass spectrometer show that the mass spectrum of aqueous benzene SOA contains the fragmentation peaks of imidazoles?m/z=67?C₃H₃N₂⁺??m/z=40?C₂H₂N⁺??m/z=28?CH₂N⁺??,which are produced by the reaction of reductive nitrogen compound with glyoxal and other aldehydes.Also,the fragmentation peaks of nitrophenol?m/z=30?NO⁺??m/z=46?NO₂⁺??m/z=93?C₆H₅O⁺??were found in the mass spectra of aqueous benzene SOA with sodium nitrite.Phenolic compounds reaction with NO₂ radicals,which were generated by UV irradiation of aqueous nitrate to generate nitrophenol products.The imidazole and nitrophenol products have strong light absorption capacity.The reducing nitrogen compound increases the mass absorption coefficient of aqueous benzene SOA,and the promotion sequence is sodium nitrite>ammonia>aminoacetic acid>ammonium sulfate>methylamine.These works further improved the study of the formation mechanism and optical characteristics of aqueous aromatic SOA particles.