Laboratory Simulation Research Of The Formation Of Aqueous Secondary Organic Aerosol

Secondary organic aerosol(SOA),accounting for 20%-80%of organic compounds in particulate matters,has a significant contribution to organic aerosol and plays an important role in visibility and climate.According to the increasing researches,the formation of SOA through aqueous-phase has a large proportion in the SOA.However,aqueous formation and reaction mechanisms of SOA are still poorly understood.In this study,the light-absorbing aqueous SOA was investigated with carbonyl compounds and AS/amines.At first,HA and MA were chosen as precursors to form aqueous SOA in the dark condition.The reaction products were measured by a UV-vis spectrophotometer and liquid chromatography-mass spectrometer-ion trap-time of flight(LCMS-IT-TOF).The results showed that all reactions fit the first-order kinetics and the reactions were accelerated with the increase of reactants'concentrations and pH.The main reaction mechanisms were aldolization of HA itself and nucleophilic attack of MA to HA,followed by dehydration.The main products were imines which made the solution from colorless to deep brown,which is likely responsible for about 1%of light absorption by atmospheric brown carbon.Then,reactions of GAld and AS/amines(MA and Gly)were observed for their ability to form SOA in cloud water and aqueous aerosol-The reaction products were monitored by a UV-vis spectrophotometer and the results showed that the product absorbance followed an order of Gly>MA>AS in the reactions of GAld and AS/amines.The liquid chromatography coupled with time-of-flight and ion trap mass spectrometry(LCMS-IT-TOF)spectra and Density Functional Theory(DFT)calculations suggested the reaction mechanisms involving aldol condensation of GAld itself,nucleophilic attack of AS/amines on GAld or its polymers,and more complex species production from the mutual reactions of the Schiff imine formed in above reactions.The reaction products of GAld and AS/amines were measured by a thermal-optical transmittance(TOT)carbon analyzer,which revealed that the aqueous SOA mass of the reaction products followed the same order as optical absorption in the UV-vis spectra.The products formed in the atmospheric aerosol particles were likely responsible for approximate 10%of light absorption by atmospheric BrC and could change the optical properties of the seed aerosols over their lifetime.Last.we investigated the differences of reactions of carbonyl compounds(GAld,HA,GX,MG.FAld and AAld)and AS/amines under light condition and dark condition.The UV-vis spectra results demonstrated that the other carbonyl compounds except for FAld reacted with Gly/MA/AS in 12 h in both dark and light conditions.The reaction rate followed the order of MA>Gly>AS in these reactions excepted for the reactions of AAld with Gly/MA/AS in both the dark and light conditions,which might be attributed to the highest pH of MA caused by its strong alkalinity'.In the general,reactions products in the light condition had higher absorption peaks in the UV-vis spectra than that in the dark condition except for the reactions of HA/GX/AAld and MA.In addition,the reaction rate constants were calculated and all reactions were fit for first-order reaction dynamics.The reaction products were measured by LCMS-IT-TOF and the results demonstrated that carbonyl compounds would undergo photodecomposition,which made the reaction mechanisms possible difference between light condition and dark condition.