Laboratory Study On The Formation Of Secondary Organic Aerosols From Typical Unsaturated Heterocyclic Compounds

In recent years,haze pollution in China is very serious,which has attracted wide attention owing to its influence and harm to climate,atmospheric environment and human health.The formation of atmospheric aerosols is complicated,which includes not only the direct emission of primary particles,but also the formation of new particles and secondary organic aerosol(SOA).Volatile organic compounds(VOCs)are important precursors of SOA in the atmosphere.The SOA formation in the atmosphere includes gas phase atmospheric reactions,gas-particle partitioning and condensation nucleation.There are many kinds of VOCs precursors and oxidants in the atmosphere,coupled with the complex and changeable atmospheric environment,the current mechanism of SOA formation remains to be improved.In recent years,studies have shown that unsaturated heterocyclic compounds that emitted from fossil fuels and biomass combustion are potential precursors of atmospheric brown carbon owing to their heteroatoms in molecular structure.However,the SOA formation potential,reaction mechanism and the effects of environmental factors on the photooxidation of unsaturated heterocyclic compounds are still unclear.It is necessary to carry out relevant laboratory simulations to improve the parameters of atmospheric model simulation so that we can better understand and accurately evaluate the complex atmospheric chemical reaction process.Therefore.the nucleation characteristics and SOA formation of unsaturated heterocyclic compounds were explored microscopically and macroscopically by means of theoretical calculation and laboratory simulation.The SOA formation potential of different VOCs precursors were compared.the effects of different environmental factors on SOA formation were studied,the formation mechanism of SOA was inferred and some of the physical and chemical properties of SOA were characterized.The details are as follows:(1)The bonding characteristics and nucleation potential of typical unsaturatedheterocyclic compounds furan,pyrrole,thiophene and furan derivatives were studied by theoretical calculation combined with low temperature matrix isolation FTIR and gas phase FTIR experiments.The results show that the heterocyclic compounds could interact with the hydrogen bond donor,where either the heteroatom or the ? system is the bonding site,thus forming X-H…Y(X=O/N,Y=O,S)or O-H…? hydrogen bonded structures.The measured OH-stretching transitions of the complexes in the IR spectra were assigned to the O-H…O bonded furan hydrogen boned complexes,and the N-H…O bonded pyrrole conformer.In addition,by comparing the bonding ability of different heterocyclic compounds,we found that the bonding ability of pyrrole is higher than that of furan and thiophene,while the hydrogen bonding complex formed by dihydrofuran and 2-methylfuran is more stable than furan hydrogen bonding complex.(2)The SOA formation potential of unsaturated heterocyclic compounds furan,pyrrole,thiophene and furfural were studied by chamber simulation experiments.Comparing the nucleation rate,growth rate,particle size distribution and SOA yield of different VOCs precursors,we found that the SOA formation potential of pyrrole was significantly higher than that of thiophene,while the SOA formation potential of furan was the lowest.The SOA yields of pyrrole,thiophene and furan in the photooxidation initiated by OH were 29.6± 3.0%,27.0 ± 2.0%and 0.8 ± 0.1%,respectively.In another set of experiments,we compared the SOA formation potential of furan,furfural and 3-methylfuraldehyde.It can be concluded that the aldehyde substitution on the furan ring increases the formation potential of SOA obviously.When there is another alkyl substitution on the furan ring,the enhancement of SOA formation is more obvious.(3)By studying the effects of different environmental factors on the photooxidation of SOA by 2,5-dimethylfuran(DMF),we found that SO2 not only promoted the formation of SOA,but also increased the amount of inorganic salts produced during the photooxidation.With the increase of DMF/OH concentration ratio,the formation rate of SOA increased gradually,and the corresponding amount of SOA also increased obviously.The addition of(NH4)2SO4 seed aerosol accelerated the formation rate of SOA and significantly shortened the time for the reaction to reach equilibrium.However,the existence of seed aerosol can weaken the amount of SOA produced by DMF photooxidation,and this inhibition increased with the increase of the surface area of seed aerosol.Additionally,the effect of light intensity on the formation of SOA promoted the formation of OH radicals,and correspondingly,enhanced the photooxidation of DMF.However,the enhancement of light intensity accelerated the aging of SOA,which led to the decrease of the mass concentration of SOA(4)We investigated the effects of NOx level and humidity on the formation of secondary organic aerosol(SOA)generated from the photooxidation of furan in the presence of NaCl seed particles.A significant difference was observed both in the variation of SOA mass concentration and SOA yield with the initial experimental conditions.Varying VOC/NOx ratios over the range 48.1 to 8.2 contributed to the effective formation of SOA in the presence of NaCl seed particles,with the SOA yield ranging from 0.04%to 1.01%,respectively.The increase of SOA yield with increasing NOx concentration was continuously observed at high NOx levels owing to a corresponding increase of the amount of low-volatility hydroxyl nitrates and dihydroxyl dinitrates that can partition into particle phase.In addition,varying RH from 5%to 88%increased the SOA yield from 1.01%to 5.03%.The enhanced SOA formation from humid conditions may result from the high OH concentration,rapid furan decay rate,enhanced carbonyl-rich products condensation,and the aqueous-phase reactions.Using hybrid quadrupole-orbitrap mass spectrometer equipped with electrospray ionization(HESI-Q Exactive-Orbitrap MS),three carbonyl-rich products and three kinds of organonitrates were identified in the collected SOA.Based on the MS analysis and FTIR spectra,the reaction mechanism of furan photooxidation was proposed.The morphology characteristics of SOA particles formed by photooxidation of furan were obtained by TEM analysis.(5)The effects of SO2 and NH3 on secondary aerosol formation in the photooxidation of 3-methylfuran(3-MF)were investigated to simulate the atmospheric reactions.Experimental results demonstrated that both SO2 and NH3 have significant promotion effect on 3-MF derived aerosol formation.The generated SOA mass concentrations with SO2 and NH3 addition were 1.2 and 1.3 times,respectively,which was larger than that without inorganic gases.However,the values of generated SOA mass concentration increased to 1.6 times in the presence of both SO2 and NH3 than that in the absence of any inorganic gases.Ammonium carboxylates generated from the reactions of NH3 and produced organic acids were observed as the main particulate N-containing organic compounds,which were assigned as important brown carbon constituents according to their distinct absorption spectra at the visible range.These products showed obvious absorption in the near ultraviolet region and visible light range,which may enhance the aerosol absorbance in some areas and enhance the climatic radiative forcing.In this paper,different research methods are combined to improve the physical and chemical parameters of hydrogen-bonded complexes formed by unsaturated heterocyclic compounds,and the reaction mechanism of SOA formation from furans photooxidation under different environmental conditions is proposed,which provides theoretical support for outfield observation and atmospheric chemical model simulation analysis.The present study is of great significance for better understanding of atmospheric combined pollution.