Study On The Atmospheric Liquid Phase Oxidation Mechanism Of Tetraethyl Phenol

This work presents a systematic investigation on the effects of initial concentration,lamp sources?Xe and Hg lamp?and radicals?·OH and ³C^*?on the formation of aqueous-phase secondary organic aerosol?aqSOA?from photooxidation of 4-ethylphenol?referred as EPhOH hereafter?,emitted from wood burning.Oxidative and aging properties of aqSOA composition were determined by soot particle-aerosol mass spectrometry?SP-AMS?.We monitor the molecular level composition using gas chromatography mass spectrometer?GC/MS?,high performance liquid chromatography/electrospray ionization mass spectrometry?HPLC-ESI/MS?and ion chromatography?IC?.UV/Vis and HPLS with Evaporated light scattering?ELSD?was used to characterize the formation of light absorbing products such as humus-like substance?HULIS?.Moreover,the yield of aqSOA was calculated,and aqSOA formation mechanism under different conditions was estimated based on the products.The main conclusions are as follows:1.Liquid photochemical reaction of low concentration EPhOH under simulated sunlight?Xe lamp?The addition of oxidizing free radicals can accelerate the liquid phase oxidation of low concentration EPhOH?0.3 mM?.The degradation rate constant?k?of EPhOH under three systems?Xe,Xe+³C*,Xe+·OH?followed:k_Xe+3C*(0.07661 h^-1)>k_Xe+·OH(0.04103 h^-1)>k_Xe(0.02907 h^-1).The molar absorption coefficient?MAE?at 280 nm and 365 nm of the formed product increases with increasing reaction time,and the HULIS mass also increases with the increase of the illumination time.The products determined contains four organic acids such as formic acid?0⁵ ppm?,acetic acid?0⁰.2 ppm?,malic acid?0³ ppm?and malonic acid?0².5 ppm?,among these,formic acid content was the largest.The oxidative characteristics?oxygen-carbon ratio O/C?of aqSOA of three systems obtained by SP-AMS varied between 0.45⁰.58?Xe?,0.49⁰.59?Xe+·OH?and 0.33⁰.62?Xe+³C^*?,respectively.GC/MS and HPLC/MS confirmed that the oxidation products were4-methylphenol?C₇H₈O?,4-ethylcatechol(C₈H₁₀O₂),dimer(C₁₆H₁₈O₂),hydroxylated dimer(C₁₆H₁₈O₃?C₁₆H₁₈O₃ and C₁₈H₂₀O₃),etc.Similarly,SP-AMS also detected the corresponding fragment ions?m/z 108,138,256,284?.AqSOA yield was in the range of 113.9-116.5%under three systems.It is speculated that the formation mechanism of aqSOA of low concentration of EPhOH was:competition between functionalization and oligomerlization,resulting in aqSOA oxidation and absorbance characteristics change with reaction time.2.Liquid photochemical reaction of high concentration EPhOH under simulated sunlightThe photolysis rate constant of EPhOH?3 mM?in the three systems was lower than that of low EPhOH?0.3 mM?.Accordingly,concentration of four organic acids in aqSOA was higher than that in low concentration.The O/C ratio at the same illumination time was lower than that of low concentration,indicating that the high concentration EPhOH degradated more slower,as a result,less oxidative aqSOA formed.AqSOA components via SP-AMS showed that the concentrations of dimer(C₁₆H₁₈O₂⁺)and hydroxylated dimer(C₁₆H₁₈O₃⁺,C₁₆H₁₈O₄⁺)were significantly higher than that at low concentration,suggesting that the polymerization reaction is more likely to occur at a high concentration,but the aqSOA yield is similar to that at a low concentration?110.5¹13.8%?.At last,we inferred that the formation mechanism of aqSOA fro high concentration of EPhOH was as follows:polymerization dominated leading to dimers/hydroxylated dimers,and functionalization as a supplement to form oxidative products.3.Liquid-phase photochemical reaction of EPhOH under ultraviolet light?Hg lamp?irradiationThe liquid-phase oxidation rate of EPhOH?0.3 mM?under Hg lamp was greatly improved compared with that under Xe lamp.The reaction rate constant followed:k _Hg+·OH(0.6393 h^-1)>k_Xe+·OH(0.04103 h^-1).A large amount of HULIS was produced in the initial stage of the reaction,and its concentration reached a maximum value?60.13 mg/L?at 1 h,and then decreased rapidly to 5.79 mg/L for 23 h.The amount of aqSOA was determined by SP-AMS(added a certain amount of SO₄^2-for calibration,and the aqSOA mass was expressed by Org/SO₄^2-ratio).It was found that Org/SO₄^2-increased gradually between 0³ h,and decreased after 3h and retain stable at 23 h.The corresponding aqSOA O/C reached 1.2 at 3 h and then gradually decreased to 0.2 at 3 h.GC/MS showed the production of ring-opening intermediates such as toluene?C₇H₈?,benzoic acid(C₁₀H₁₂O₂),4-methyl 3-hexanone(C₇H₁₄O)and 2,3-butanediol(C₄H₁₀O₂).Combined with the above aqSOA production,oxidation,absorbance and other characteristics,aqSOA forming mechanism under ultraviolet light irradiation was:functionalization dominated in the initial stage of the reaction,and fragmentation dominated at the later stage.