| Polar organic matter in the atmospheric particles would have very importantimpact on air quality, climate and human health due to its strong hygroscopicity. Polarorganic compounds in aerosols have many sources, among which secondary organicaerosols (SOA) and biomass burning are the two most important sources. In recentyears, haze occurred frequently in the Pearl River Delta, Yangtze River Delta andother metropolitan areas. Haze has become a new disastrous weather in many cities.Large quantities of different kinds of pollutants emitted into the atmospheresimultaneously led to the complex interactions between the pollutants, and wouldform a complex atmospheric pollution and haze. It was found that the formation ofhaze is closely related with the high atmospheric oxidative capacity and the formationof polar organic compounds (especially the secondary organic aerosols, SOA). Inaddition, studies have shown that heterogeneous reactions might play a central role inthe transformation of tropospheric gaseous pollutants and in the formation ofsecondary aerosols. Analysis of the chemical composition, size distribution andsources of the polar organic compounds could provide scientific basis for makingpollution-control policies and studying the formation mechanism of air pollution andurban haze.In this study, a MOUDI (Micro Orifice Uniform Deposit Impactor) sampler wasused to collect seasonal atmospheric particle samples in Shanghai from2012to2013.The concentrations, size distributions and seasonal variations of polar organics andcarbonaceous aerosols in the collected particles were analyzed with an optical carbonanalyzer and a GC/MS. By this method, we studied the sources, formation ways of thetypical polar organic compounds, and their possible impact on the formation of haze.The following conclusions were reached:(1) The concentrations of carbonaceous aerosols in autumn and winter werehigher than spring and summer. The good correlation (R2=0.68) between organiccarbon (OC) and elemental carbon (EC) indicated that they were from similar sources.The size distribution of both OC and EC concentrations showed bimodal distributions,with the highest peak at the0.32~0.56μm size range and the second peak at 3.2~5.6μm. About68~83%OC and52~86%EC were in the fine particles (<1.8μm).The high OC/EC ratio implied that SOA has a significant contribution to organicaerosols in Shanghai. The changes of OC/EC ratio in different size segmentssuggested that the formation of the OC in aerosols in Shanghai was throughgas-particle partitioning.(2) The concentration of isoprene oxidation products (31.8~229.5ng/m3) was thehighest in the detected SOA tracers, followed by the α-pinene oxidation products(1.4~31.5ng/m3) and β-caryophyllene acid (the tracer of β-caryophyllene oxidationproducts,0.09~2.52ng/m3) and the concentration secondary organic tracer of toluene(0.39~5.27ng/m3). Terephthalic acid, syringic acid, malic acid and2-hydroxyglutaricacid were also quntified, with concentrations of9.1~69.4ng/m3,0~0.42ng/m3,14.1~297.4ng/m3and0.3~5.1ng/m3, respectively.(3)2-methyltetraols accounted for40~80%of isoprene photo-oxidation products.Similar bimodal size distributions were found for2-methyltetraols and2-methylglyceric acid, with peaks at0.32~1.0μm and, suggesting that heterogeneousreactions and liquid phase reactions were the main formation mechanisms of them.The peak at3.2~5.6μm suggested that part of the2-methyltetraols might be from soils.Unimodal size distribution was found for the C5-alkenetriols, with the peakconcentration in the range of0.32~1.0μm. This indicated that C5-alkenetriols wereprimarily generated by the heterogeneous reactions.(4) No obvious peak was found in the size distribution of cis-pinonic acid, anintermediate product of the photo-oxidation of α-pinene, suggested that the detectedparticulate cis-pinonic acid was from the absorption of gaseous product on thesampling film and the collected particles. While unimodal size distributions, with themaximum concentration in0.32~0.56μm, were found for the three detected oxidationproducts of α-pinene,3-hydroxyglutaric acid,3-hydroxy-4,4-dimethylglutaric acidand3-methyl-1,2,3-butanetricarboxylic acid. These three acids should be mainly fromthe atmospheric heterogeneous reactions.(5) Both β-caryophyllene acid and2,3-dihydroxy-4-oxopentanoic acid showedunimodal size distribution with the maximum concentration in0.32~0.56μm. Almost all of them was concentrated in the fine particles. This indicated that atmosphericheterogeneous reaction was the main formation way of them. The contribution ofparticles in the size segment of0.56~1.0μm for2,3-dihydroxy-4-oxopentanoic acidwas clearly higher than that for β-caryophyllene acid, suggesting that cloud/fogprocess (liquid phase reactions) contributed more to the toluene oxidation products.Atmospheric humidity had thus a significant impact on the secondary conversion oftoluene.(6) Size distributions of terephthalic acid, malic acid and2-hydroxyglutaric acidwere in good agreement with other SOA tracers, which suggested that they might bemainly derived from the heterogeneous reactions and liquid phase reactions of certaingaseous precursors. Significant concentrations of terephthalic acid were found in thesize segments of0.56~1.0μm and1.0~1.8μm, indicating that liquid phase reactionswere very important for the formation of terephthalic acid. Relative humidity shouldhave a great impact on the formation of terephthalic acid.(7) Ten saccharide compounds were quantified in the atmospheric particles inShanghai, including3anhydrosugars (levoglucosan, mannosan, galactosan),4monosaccharides (2fructose and2glucose) and3sugar alcohols (arabitol, sorbitoland mannitol), among which levoglucosan from biomass burning was the dominantspecies. The concentrations of levoglucosan in spring, summer, autumn and winterwere41.8ng/m3,23.1ng/m3,189.3ng/m3and157.5ng/m3respectively. Levoglucosanshowed unimodal size distribution in all seasons, with peak concentrations in the sizerange of0.32~1.0μm.(8) The concentrations of particulate monosaccharides and sugar alcohols inwinter were obviously lower than other seasons in Shanghai, in accordance with theweak activities of microbial in the soil under low temperature. Obvious differences inthe size distributions of monosaccharides and sugar alcohols in different seasons werefound. Glucose and fructose in spring and summer were mainly in coarse particles(>1.8μm), indicating that they were mainly from re-suspended soils. While more thanhalf of the glucose and fructose were found in fine particles, suggesting that biomassburning was an important source of them besides soil re-suspension. A large part of the fructose was in the size range of greater than10μm in spring and summer, andcould be from dust, microbial activity plant pollen. In fall and winter, fructose showedunimodal size distribution and mainly in fine particles, suggesting that biomassburning was the main source of fructose. Arabitol was mainly in fine particles exceptthat in spring, indicating that biomass burning was the main source of it. Sorbitol wasmainly in coarse particles, and should be mainly from re-suspended soils. |
