Spectroscopic Study On Optical Properties And Absorption Components Of Secondary Organic Aerosol Of Monoaromatic Hydrocarbons

Benzene,toluene and other monoaromatic hydrocarbons emitted from anthropogenic sources such as vehicle exhaust are common volatile organic pollutants.The monoaromatic hydrocarbons emitted into the atmosphere mainly undergo photo-oxidation reactions with OH radicals to produce semi-volatile and non-volatile products,which form secondary organic aerosol（SOA）through nucleation or gas/particle partitioning.SOA are mainly fine particles(PM_2.5)with particle size less than 2.5μm.They can absorb and scatter sunlight and reduce atmospheric visibility.They are the culprit of haze weather and have gradually become an important factor affecting radiative forcing and disturbing regional climate change.With the intensification of climate change,the formation process and optical properties of monoaromatic SOA have become the hot research topic in the field of atmospheric chemistry.In this paper,the self-built smog chamber system is used to simulate the OH radical initiated atmospheric photooxidation reaction of monoaromatic hydrocarbons to produce SOA particles.The averaged mass absorption coefficient（）of monoaromatic SOA in the range of 200～600 nm is measured by UV-Vis spectrometer and total organic carbon analyzers.The fluorescence characteristics of SOA particles are detected by fluorescence spectrometer.Also,aerosol laser time-of-flight mass spectrometer（ALTOFMS）,Fourier transform infrared spectrometer（FTIR）and liquid chromatography-mass spectrometry（LC-MS）are used to measure the light absorbing components of monoaromatic SOA.The effects of nitrogen oxides and inorganic fine particles on the optical properties and light absorption components of monoaromatic SOA are investigated.The experimental results are as follows:The SOA particles of monoaromatic hydrocarbons have fluorescence characteristics,and the fluorescence spectrum peak isλ_ех/λ_em=270 nm/300 nm.While,thevalue of monoaromatic SOA particles in the range of 200～600 nm is between 13.67～70.24 cm~2/g.The online measurement results of ALTOFMS show that the laser desorption ionization mass spectrum of monoaromatic SOA particles contain the characteristic ion peak of phenolic compounds（m/z=93（C₆H₅O⁺））,benzene ring ion and its fragments（m/z=77（C₆H₅⁺）,m/z=65（C₅H₅⁺）and m/z=39（C₃H₃⁺））.There are obvious characteristic absorption peak of phenolic compounds in the UV-Vis spectrum of SOA collection solution;the stretching vibration peaks of-OH and C-O single bond of phenolic compounds are presented in the infrared spectrum,indicating that phenolic compounds are the main light absorbing component of monoaromatic SOA particles.The mass concentration,maximum fluorescence intensity andvalue of SOA particles produced by the reaction of monoaromatic hydrocarbons with different substituents with OH radical show the same trend of styrene>benzene>ethylbenzene>toluene>m-xylene>1,3,5-trimethylene.NO₂ is the common inorganic pollutant in the atmosphere.It can participate in the atmospheric chemical reaction of monoaromatic hydrocarbons and change the chemical components and optical properties of monoaromatic hydrocarbons SOA.The formation rate,mass concentration andvalue of ethylbenzene SOA particles increased gradually and then decreased with the increase of NO₂concentration.Based on the online detection of ALTOFMS and the validation of off-line electrospray ionization mass spectrometer and Fourier transform infrared spectrometer,nitrophenols,nitro-catechols and other nitro-aromatic compounds（NACs）produced by the nitrification of phenolic compounds were found to be the main components of SOA particles in the presence of NO₂.NACs have low volatility and can quickly form SOA particles through condensation or gas/particle partitioning,which significantly promote the formation of ethylbenzene SOA.The existences of electrophilic nitro group of NACs molecules enhance the light absorption capacity of ethylbenzene SOA particles.NACs have characteristic absorption peak at 318 nm in the UV-Vis spectrum and show strong light absorption properties.Inorganic fine particles have large specific surface area and reaction activity,which can affect the composition and optical properties of monoaromatic SOA.Na NO₃fine particles promote the formation of nitro-phenol products and increase thevalue of ethylbenzene SOA.Ca Cl₂fine particles are conducive to the condensation of weakly acidic phenolic compounds,so the light absorption capacity of SOA particles increases.On the contrary,Na₂SO₃fine particles can inhibit the condensation of phenolic compounds on its surface,so thevalue of SOA particles decreases.After the phenolic compounds coagulate on the surface of transition metal fine particles in d and ds regions,they can react with heavy metal ions and produce benzoquinone compounds via complexation and redox reaction,which reduce the content of phenolic compounds in SOA particles and thevalue.Fe³⁺ions have many empty orbits and have strong complexation and redox ability to diphenols.Fe Cl₃fine particles have a great impact on the optical properties of ethylbenzene SOA particles,so thevalue decreases greatly.The increase of Co Cl₂fine particle concentration and alkalinity is conducive to the complexation and redox reaction of Co²⁺ions with phenolic compounds to produce benzoquinone products,resulting in the gradual decrease ofvalue of ethylbenzene SOA particles and the weakening of light absorption ability.These experimental results are helpful to clarify the photochemical reaction mechanism of monoaromatic hydrocarbons under different atmospheric conditions,evaluate the radiation forcing effect of secondary organic aerosol and provide the scientific reference for the prevention and control of atmospheric haze pollution.