Formation And Transformation Of Carbonyl Products And Their Contributions To SOA During M-xylene Photooxidation

Modern industrial processes and solvent use,etc.generate large amounts of benzene homologues.China,as a modern industrial powerhouse,is one of the most important sources of benzene homologues emission.Benzene homologues are generally highly volatile and toxic,affecting human health and air quality.Moreover,they are representative precursors of secondary organic aerosol（SOA）,have received lots of attention.It is generally accepted that atmospheric·OH-initiated photooxidation of benzene homologues form different carbonyl products（including monocarbonyl and dicarbonyl,etc.）,and these carbonyls as active intermediates will further react to form SOA,which will have adverse effects on air quality,global climate and human health.However,the transformation and contribution mechanism of monocarbonyl and dicarbonyl products in benzene homologues-SOA process rarely be investegated.Therefore,in this paper,m-xylene was used as SOA precursor to study aforementioned issues.Variations of number concentration,mass concentration and particle size distribution were investigated by Scanning Mobility Particle Sizer（SMPS）,while derivatization and solid-phase microextraction（SPME）combined with gas-phase mass spectrometry（GC-MS）was used to figure out composition and concentration changes of carbonyl products during m-xylene photooxidation.Furthermore,we focused on the photoreaction/hydroxyl reaction of substantial monocarbonyl and dicarbonyl products（formaldehyde and glyoxal,methylglyoxal）,and discovered their potential connections,transformation processes,and contribution role to formation of SOA.1.By detection of number and mass concentration of SOA at different reaction time（45,60and 120 min）in m-xylene photooxidation,it was found that the SOA number concentration remained increasing within 120 min(2.74±0.15×10⁴to 7.00±0.27×10⁴particles cm^-3)while mass concentration reached a maximum at 60 min.The value at 60 min(113.5±3.5μg m^-3)was4.4 and 1.2 times higher than that at 45(25.7±2.2μg m^-3)and 120 min(86.0±1.3μg m^-3).It means that 60 and 120 min are respectively the maximum for mass concentration and number concentration of SOA.And this may be related to varing percentage of different particle size ranges at corresponding time.It was found that percentages of 14.1-30.0 nm and 31.1-63.8 nm increased from 82.1%to 93.8%and decreased from 17.3%to 5.1%from 45 to 120 min,while66.1-151.2 nm remained almost unchanged（from 0.6%to 1.1%）.Meanwhile,the qualitative and quantitative analysis of the carbonyl products at at different reaction time（5,20,45,60 and 120 min）firstly confirmed that the products were formaldehyde,acetaldehyde,butyraldehyde,pentanal,hexanal,heptanal,methylcyclopentanone,m-tolualdehyde,glyoxal and methylglyoxal.Methylglyoxal,glyoxal,formaldehyde and acetaldehyde were found to be the main carbonyl products.Among them,the production of methylglyoxal was the first at 5 min,followed by formaldehyde and acetaldehyde.Concentration of glyoxal exceeded methylglyoxal at 20 min,and was still higher than that of formaldehyde,methylglyoxal and acetaldehyde at 45 and 60 min.However,when lasted till 120min,formaldehyde had the highest concentration,followed by methylglyoxal and acetaldehyde,while glyoxal was undetected.The vairable concentration ranking of these four products may be related to their different formation pathways.Next,we classified carbonyl products into monocarbonyl and dicarbonyl,it was discovered that RC of monocarbonyl and dicarbonyl to SOA from 45 to 120 min accounted for 38.9%-70.8%and 61.1%-29.2%,demonstrating that the main contributing carbonyl products in formation of SOA coverted from dicarbony to monocarbonyl.It can be attributed to 75%reduction in concentration of dicarbonyl during45-120 min while concentration of monocarbonyl had a slight fluctuation.Further by adding 140 ppb glyoxal or formaldehyde at 45 min into m-xylene photooxidation,it was ascertained that addition of the product could be reflected on formation of SOA.The mass concentration of SOA decreased to one-fifth of the original m-xylene reaction at60 min after additional formaldehyde participated in,which was related to the accumulation of glyoxal,indicating a hiding correlation between dicarbongyl and monocarbonyl（glyoxal and formaldehyde）.2.Through glyoxal/formaldehyde hydroxyl reaction and mixed in low concentration,it was found that SOA mass concentration measured in glyoxal reaction was 1.7～2.5 times higher than that of formaldehyde in corresponding concentration under the same conditions（amount of·OH,reaction time,etc.）,which shows that glyoxal can produce higher SOA than that of formaldehyde.And formaldehyde is the product of glyoxal hydroxyl reaction,reflecting that there has a conversion pathway between glyoxal and formaldehyde.When adding 140 ppb formaldehyde in glyoxal hydroxyl reaction,therefore,formaldehyde inhibited decomposition of glyoxal to produce formaldehyde,led to accumulation of glyoxal,so glyoxal consumption,formaldehyde production and SOA mass concentration during the mixed reaction are lower than single glyoxal reaction.Glyoxal photoreaction in high concentration not only confirmed that part of glyoxal can be decomposed to produce formaldehyde under the illumination,but also glyoxal reaction is is likely to be reversible,i.e.formaldehyde can be polymerized to produce glyoxal.Moreover,the addition of·OH facilitated the forward reaction,and produced more formaldehyde.According to SOA mass concentration in the glyoxal photoreaction/hydroxyl reaction,it was found that mass concentration measured in two reactions increased with the extension of time,however the hydroxyl reaction created more formation of SOA.In methylglyoxal photoreaction and hydroxyl reaction,formaldehyde and glyoxal were detected.Mass concentration of SOA produced by hydroxyl reaction was higher than that of the photoreaction.In the mixed glyoxal and methylglyoxal hydroxyl reaction experiments,SOA mass concentration produced within 10 min was significantly higher than that of the single glyoxal or methylglyoxal hydroxyl reaction and the mixed photoreaction.But the mass concentration at 60 min was lower than that of aforementioned three reactions.It’s quite likely that amount of·OH is not sufficient to support increament of SOA mass concentration as quick as before.