Mechanistic Study Of Secondary Organic Aerosol(SOA) Formation From Methylglyoxal(MGLY)

There are large amounts of secondary organic aerosols(SOA)in the atmosphere,which are mainly produced from the photo-oxidation and ozonation of volatile organic compounds(VOCs).Methylglyoxal(MGLY)is a common VOCs in the atmosphere,belonging to carbonyl compounds,and can generate SOA through a variety of pathways,among which oligomerization reaction is an important channel.The formation of SOA could be influenced by a number of factors,which is very important to study their the effects on SOA formation.However,so far,the reaction mechanism of oligomers produced by MGLY photo-oxidation under different influencing factors is far from being clear.Therefore,in this study,we investigated the effects of radical,temperature,and pH on SOA formation of MGLY photooxidation using a bulk reactor,and analyzed the products through gas chromatography and liquid chromatography to clarify the reaction mechanism of SOA formation.The main results are as follows:The oligomerization reaction of MGLY under dark conditions was investigated by varying the temperatures and pH.At pH=5,the oligomerization of MGLY was 10.9%at 25℃ and could reach 30.1%when the temperature increased to 72℃;the oligomerization also reached 31.2%when the temperature was constant and pH=1.At the same time,the oligomerization rate was as high as 38.7%with pH=1 and a temperature of 72℃,when the extent of MGLY oligomerization was significantly enhanced.When pH=1,the oligomerization formed by hydration/acetal is acid-catalyzed in the aqueous phase to become the main oligomer,and when pH=5,the oligomerization of aldehyde alcohol condensation became the main mechanism,and oligomers on the β parent by increasing the C3H4O2 unit could be formed.The effects of different temperatures and pH on the formation of MGLY monomers and oligomers were investigated.With the increase of hydrogen peroxide(H2O2)concentration,the content of OH radicals produced in the solution increased,and thus the reaction rate was accelerated,resulting in the increment of oligomers production,which was consistent with the results of experimental simulations.At the same time,the lower pH conditions favored for oligomerization,and the same amount of oligomers could be formed 40 min earlier at lower pH conditions.Two oligomerization mechanisms were obtained:mechanism 1:precursor+acetaldehyde unit;mechanism 2:precursor+pyruvate unit.Under mechanism 1 we derived two pathways:m/z of 119+44n as well as 133+44n;under mechanism 2 we verified two pathways m/z of 89+ 88n and 133+88n.When acetaldehyde was used as the unit,a large number of low aromatic aliphatic compounds could be easily formed,while when pyruvate was used as the unit,highly aromatic aliphatic compounds could be easily formed.The effects of different temperatures and pH on the formation of MGLY monomers and oligomers were also investigated through the photo-oxidation experiments of MGLY with SO4radicals.The higher the concentration of sodium persulfate(Na2S2O8),the higher the concentration of SO4-radicals produced under photo-oxidation,and the faster the reaction proceeds,however,the content of MGLY oligomers decreased during this process,indicating that SO4-radicals did not promote the polymerization of MGLY.Under different pH conditions,the rates of MGLY monomers and oligomers formed in the solution were almost the same,indicating that pH had little effect on the rate of photo-oxidation,The reaction products were analyzed,and again two of the oligomerization mechanisms were derived:mechanism 1:precursor+acetaldehyde unit;mechanism 2:precursor+pyruvate unit.Under mechanism 1,we obtained two pathways:154+44n and 212+44n,in which saturated aromatic compounds were produced;under mechanism 2,we also verified two pathways:168+88n and 212+88n.Under SO4-radicals oxidation,mechanism 2 produced fewer oligomers,and more products underwent their own condensation and esterification reactions.By studying the effects of different factors on SOA production from photo-oxidation of MGLY and the proposed reaction mechanism,we can better explain the causes of SOA formation from atmospheric MGLY and present experimental data to support the contribution of SOA formation from MGLY and provide theoretical guidance for model development and SOA prevention and control.