Distributions, Sources And Formation Of Dicarboxylic Acids In Atmospheric Aerosols Over The East China Sea

Low molecular weight dicarboxylic acids are one of identified important constituentsof organic aerosols in the atmosphere. These dicarboxylic acids have low vapourpressure and easily mainly partition in the particle phase. They could affecthygroscopicity of atmospheric aerosols and reduce surface tension of aerosol droplets.The effects could increase the efficiency of aerosols to be activated as cloudcondensation nuclei （CCN） and eventually affect cloud formation, global energybalance and regional/global climate. Considered the importance, much attention hasbeen paid to dicarboxylic acids in atmospheric aerosol particles.In this study, total suspended particles （TSP） samples and size-fractioned particlesamples were collected over the East China Sea during the periods from12May to6June and12to28August,2011, respectively. The concentrations and size distributionsof water-soluble dicarboxylic acids were measured to study their temporal and spatialdistribution characteristic, diurnal variation characteristics. The influence of air masstrajectories and meteorological conditions on the concentration level of dicarboxylicacids was investigated. The formation mechanisms of dicarboxylic acids werediscussed. The results were presented below:The concentrations of oxalic acid, malonic acid and succinic acid were26.0¹475.5ng·m^-3,0.1⁶1.4ng·m^-3and0.1¹32.4ng·m^-3;74.5⁴78.6ng·m^-3,0.6⁴4.6ng·m^-3and1.3⁶4.7ng·m^-3in spring and summer, respectively. Oxalic acid was themost abundant dicarboxylic acid, accounting for87%of the sum of the threedicarboxylic acids. The average concentration of total dicarboxylic acids in summerwas lower than that in spring. However, the lowest concentration of dicarboxylic acidswas observed in spring when the air mass was from the clear marine atmosphere. Nosignificant difference between daytime and nighttime samples was observed for theconcentrations of dicarboxylic acids in TSP samples. While the mass fraction ofdicarboxylic acids in nighttime samples was higher than in daytime samples for particle sizes＞2.1μm. The concentrations of dicarboxylic acids decreased withincreasing distance from the coastal.The spatial distributions of dicarboxylic acids in aerosols were associated with thesources and transport pathways of air mass and meteorological factors. In spring, highconcentrations of dicarboxylic acids were observed when the air mass passed overpolluted regions. The reverse was true when the air mass came from the clean marineatmosphere. It is interesting that the concentrations of dicarboxylic acids increased incloudy and fog days. In summer, the aerosol samples were mainly influenced by cleanmarine atmosphere and the concentrations of dicarboxylic acids had a negativecorrelation with ambient temperature.The concentrations of dicarboxylic acidsdecreased in rainy days in both spring and summer.Correlation analysis between dicarboxylic acids and major ions showed that,aqueous-phase oxidation of VOCs from natural/anthropogenic sources was proposed tobe the major formation pathway of dicarboxylic acids, while dicarboxylic acids emittedfrom vehicular exhaust/biomass burning and dicarboxylic acids in the gas phaseadsorbed onto alkaline coarse particles made little contribution over the East China Seain spring. In summer, aqueous-phase oxidation of volatile organic compounds seemedthe major formation pathway of dicarboxylic acids, while primary sources made littlecontribution. Aqueous secondary productions of oxalic acid via the oxidation ofglyoxylic acids and decays of higher diacids contributed to41%and59%in spring and75%and25%in summer, respectively.Size distributions of oxalic acid, malonic acid and succinic acid in aerosols were alsostudied over the East China Sea. In spring, for oxalic acid and succinic acid, bimodaldistributions were observed throughout the cruise campaign. Malonic acid was moreabundant in droplet mode off the coasts, but it showed bimodal distribution over theremote ocean. The size distribution difference between coast and remote ocean samplessuggested a different formation pathway, or a combination of different formation routes.In off coast aerosol samples, majority of dicarboxylic acids in droplet mode may becontrolled by the photochemical oxidation of their precursor emissions from biomassburning, followed by long-rang atmospheric transport. A small amount of dicarboxylic acids presented on coarse particles may be associated with marine origins. In remoteocean aerosol samples, dicarboxylic acids in droplet mode and coarse particles may beexplained by the photochemical oxidation of organic matter and heterogeneousreactions of unsaturated fatty acid which emitted from the ocean, respectively. Insummer, bimodal distributions were observed for dicarboxylic acids. Aqueous-phaseoxidation of organic matter and the heterogeneous reactions of unsaturated fatty acidwhich emitted from the ocean were likely sources of dicarboxylic acids in fine andcoarse particles diacids, respectively.