| The 4-ethylguaiacol(EG)was selected as precursor.By changing the light sources(simulated solar light and ultraviolet light source)and the concentration of precursor,the process of aqueous-phase oxidation to secondary organic aerosol(aqSOA)under different free radicals(3C*and·OH)was investigated.With the help of quenching experiments and experiments under different saturated gas(air,oxygen and nitrogen saturation)conditions,we explored the contribution of reactive oxygen species(ROS)to aqSOA formation during EG aqueous-phase oxidation under the action of 3C*.A series of instruments including ultraviolet and visible spectrophotometry(UV-vis),high performance liquid chromatography(HPLC),high performance liquid chromatography-evaporative light-scattering detector(HPLC-ELSD)?ion chromatography(IC),total organic carbon analyzer(TOC),soot particle-aerosol mass spectrometry(SP-AMS),gas chromatography-mass spectrometry(GC-MS),liquid chromatograph-mass spectrometer-mass spectrometer(HPLC-MS-MS)and electronic paramagnetic resonance spectrometer(Micro EPR)were utilized to explore the reaction kinetics,light-absorption,oxidation characteristics,aqSOA mass yield and product composition.At last,we speculated the formation mechanism of aqSOA under free radicals.The main conclusions were as follows:1.EG aqueous-phase oxidation study under 3C*Under the conditions of three kinds of saturated gases(N2,O2,air),the degradation reaction rate of EG from high to low followed:air>N2>O2,the corresponding first-order reaction rate constants were 2.69×10-5 s-1,1.43×10-5 s-1 and 0.79×10-5 s-1,respectively.The calculated lifetime of the EG is about 10.5 h under air saturation condition.The contribution of ROS in the degradation of EG aqueous-phase was calculated by quenching experiments,reducing as 3C*>1O2(?O2·-)>·OH in turn.The high steady state concentration of 1O2in the solution after 1 h of illumination was measured by Micro EPR(about 3×10-8 M),which further proved that 1O2 played a major role in degradation.UV-vis analysis showed that the absorbance of high concentration(3 mM)and low concentration(0.3 mM)of EG degradation products increased with illumination time at wavelength 250 nm under simulated solar light conditions,while the absorbance of products increased at the first 1 h photolysis then decreased.SP-AMS measured that the yield rate of aqSOA also reached the highest at 1h under ultraviolet light(65.7%),and then decreases continuously.The oxygen-carbon ratio(O/C),carbon oxidation states(OSc)and f44 of the products under simulated solar light increased with illumination time,and the low-concentration of EG system(OSc=-0.8?0.3)was significantly higher than the high-concentration system(OSc=-0.9?-0.5),indicating that more high-oxidative aqSOA were formed under low-concentration systems.GC-MS and HPLC-MS/MS results showed that the products under simulated sunlight are abundant,including the functionalized product 4-ethyl-6-methoxybenzene-1,2,3-triol(C9H12O4),5-ethyl-3-methoxybenzene-1,2-diol(C9H12O3)and dimer(C18H22O4),which confirming that the reaction underwent polymerization and functionalization.EG removal by 3C*-induced oxidation might be attributed to direct reactions by 3C*and 1O2,chemical reaction with ROS and oligomerization via H-abstraction.By contrast,the products under ultraviolet light were very few and the degradation was more thorough.2.EG aqueous-phase oxidation study under·OHWe compared aqueous-phase oxidation reaction under·OH at different initial concentrations(high,medium and low concentrations were 3 mM,0.3 mM and 0.03 mM,respectively)under simulated solar irradiation,and found the EG removal rate was as follows:0.3 mM>0.03 mM>3 mM.As the photo-oxidation reaction progressed,the light absorption at250 nm increases continuously while that at 280 nm decreases continuously,indicating that the precursor was degraded into products with light-absorbing character.The increasing absorbance in the region of 300?400 nm and HULIS concentration during reaction indicated that the aqueous-phase reaction formed brown carbon(Br C).During the reaction process,the p H value of the solution kept decreasing and the production of organic acids(such as formic and oxalic acid)kept increasing,indicating that aqueous-phase reaction was an important source of organic acids.The mass yield of aqSOA under SP-AMS analysis in low concentration system increased first and then decreased with the increase of reaction time,while the yield in medium and high concentration system increased first and remained unchanged(?80%).The O/C and OSc of medium and low concentration systems increased with illumination time,while at the initial stage of reaction(0?5 h),the O/C and OSc increased with reaction time then remained nearly unchanged after 5 h.GC-MS detected EG-OH functionalized products such as oxidizing ketones,ring-opening compounds and dimers et al,as a result,we speculated that aqueous-phase oxidation of EG under·OH was dominated by functional group to generate oxidizing aqSOA,and the polymerization was auxiliary to generate dimers,while the ring-opening reaction of fragmentation was not obvious.Precursor EG selected in this paper is one of the biomass burning emissions.Under the action of 3C*and OH radicals formed aqSOA with strong light-absorption and hygroscopicity,which would have an important impact on the climate,environment and human health.The research data in this paper are also helpful to further understand the atmospheric aging process of biomass combustion flue gas,to provide key parameters for the atmospheric pollution model,and provide important foundation for clarifying formation mechanism of atmospheric complex pollution. |
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