Characterization Of A New Aerosol Simulation Chamber And Application To Secondary Organic Aerosol Formation Of Methylglyoxal

Secondary organic aerosol(SOA)can be formed through the oxidation of volatile organic compounds(VOCs)in the atmosphere.Methylglyoxal(MGLY)is an important SO A precursor,which can be formed through a variety of pathways,while the formation of SOA could be influenced by a variety of factors.The effects of different factors on SOA formation from MGLY photooxidation and the mechanism of oligomer formation are not clear at present.Therefore,in this study,we investigated the effects of three factors including initial MGLY concentration([MGLY]0),seed acidity and relative humidity(RH),on SOA formed from MGLY photooxidation using a smog chamber.Gas chromatography mass spectrometry(GCMS)and liquid chromatography mass spectrometry(LCMS)were used to investigate MGLY monomers as well as oligomers in the particle phase,and other oligomers that might exist in the particle phase.Based on Based on these results a possible mechanism for SOA formation was proposed.The main results are as follows:A smog chamber with a reactor volume and specific surface area of 18 m3 and 2.17 m-1,respectively,was designed,equipped with a complete set of control system and analytical and testing instruments.60 black-light lamps with a central wavelength of 354 nm on two symmetrical walls in the chamber to provide light source.Based on steady-state light luminous flux method,the effective light intensity(JNO2)was calculated to be JNO2=(4.53±0.02)×104 s1 in the smog chamber;The wall loss rate constants for gaseous pollutants(O3,NO2 and NO,etc.)and seed particles were kO3=(4.15±0.67)×10-6 s-1,kNO2=(6.56±1.41)×10-6 s-1,kNO=(1.55±0.06)×104-6 s-1 and kparticle=(4.38±0.39)×10-5 s-1,respectively,which were at the same level with other smog chambers,implying the smog chamber we built is reliable.The effects of different factors([MGLY]0,RH and seed acidity)on SOA formation from MGLY photooxidation:the SOA formation was ascribed to uptake of MGLY on seeds under dark condition,while was ascribed to the reaction of MGLY with OH radicals under light illumination.The experimental results showed that the greater the[MGLY]0,the greater the MGLY uptake;the greater the seed acidity,the greater the MGLY uptake;and the higher the RH conditions,the greater the MGLY uptake.The results of orthogonal analysis showed that the order of strength of the three factors on MGLY uptake was RH>[MGLY]0>seed acidity.Different factors on the effect of MGLY photooxidation under light illumination:the amount of SOA increased significantly with increasing[MGLY]0 and more SOA was formed under high RH conditions.In the experiments on the effect on SOA formation of seed acidity were very complicated,under lower RH(11%)the amount and growth rate of SO A from MGLY photooxidation first increased and then decreased with increasing seed acidity,while under higher RH(50%)the amount and growth rate of SOA increased with increasing acidity.The effect of seed acidity might fit a Gaussian distribution which is inconsistent with the existing experimental and model results of SOA formation from MGLY hydration reaction.Therefore,it is concluded that there exist other ways to form SOA besides the MGLY hydration reaction,and the formation of low volatile organic compounds formed from the reaction of MGLY with OH radicals and their further reactions should not be ignored.The effects of[MGLY]0,RH and seed acidity on the amount of MGLY monomers and oligomers in the particle phase were investigated.The concentrations of MGLY monomers and oligomers in the particle phase increased with increasing[MGLY]0.However,the effects of seed acidity and RH on MGLY monomers and oligomers are not consistent with the existing findings,which the hydration reaction of MGLY was the main pathway,indicating that the formation of oligomers during MGLY photooxidation no longer depends on hydration reactions.It is further confirmed that the reactions of MGLY with OH radicals,esterification reactions and aldol reactions could not be neglected during the formation of SOA from MGLY photooxidation.LCMS analysis showed that MGLY oligomers were formed mainly by the reaction of MGLY with OH radicals,and the intermediate products reacted through esterification or aldol reactions to form macromolecular organic compounds which could further react with acetaldehyde or pyruvate as unit to form oligomers.The investigation of the effects of different factors on SOA formation from MGLY photooxidation and the proposed mechanism of oligomers formation could better help understand the SOA formation from MGLY in the actual atmosphere,provide experimental data support for the contribution of the photooxidation process of MGLY to the formation of SOA,and provide some theoretical guidance for the development of the model and air pollution control.