上海城区硝基芳香族化合物的化学组成及特征分析

Chemical Composition and Characterization of Nitroaromatic Compounds in Urban Areas of Shanghai

为研究硝基芳香族化合物(NACs)在上海城区大气中的浓度水平、组成特征以及与气态前体物关系,采用大流量采样器采集了2020~2021年上海城区大气细颗粒物PM_(2.5)冬季39个和夏季46个样品,利用超高效液相色谱-静电场轨道阱高分辨质谱,对其中12个NACs物质进行了定量分析,结合后向轨迹、前体物及气象要素等相关性分析,并对其中含CHON类物质进行分子组成和分类分析.结果表明,冬季ρ(NACs)平均值(17.1 ng·m^(-3))是夏季(5.7 ng·m^(-3))的3倍,冬季气团主要来自于北方,夏季气团则主要来自于较清洁的东南部海洋.冬季NACs组成中含量最丰富的物种为4-硝基苯酚,夏季则为4-硝基苯酚(清洁天)和4-羟基-3-硝基苯甲酸(污染天),冬、夏两季均表现出昼高夜低的特点.单环和多环芳烃化合物识别和表征的芳环等值数(Xc)以及O/C和H/C值等特征的定性分析结果表明,上海城区冬、夏两季PM_(2.5)中CHON类化合物主要是芳香族化合物,且在PM_(2.5)污染天检测到的CHON类化合物的数量和丰度均高于清洁天.对比冬、夏清洁天和污染天的分析结果发现,相对丰度前10的CHON类化合物中80%的O/N≥3,并且RDBE值在5~8之间,表明这些化合物可能存在单硝基或者二硝基取代的苯环结构.其与气态前体物相关性分析结果表明,夏季人为源排放的VOCs(苯和甲苯等)氧化反应生成是NACs的主要来源,冬季则受生物质燃烧排放和人为源VOCs的NOx氧化二次生成共同影响.

The compositional characteristics,concentration of nitroaromatic compounds(NACs)in PM_(2.5) in urban Shanghai,and their correlation with gaseous precursors were investigated.A total of 39 winter and 46 summer PM_(2.5) samples from 2020 to 2021 were collected using a high-flow sampler and analyzed via ultra-performance liquid chromatography coupled with ESI-Orbitrap high-resolution mass spectrometry(UPLC-Orbitrap-HRMS).Quantitative analysis was performed on 12 NACs compounds,combined with backward trajectory meteorological elements,molecular composition,and classification analysis of CHON substances.The results showed that a total of 12 NACs had an average concentration in winter of 17.1 ng·m^(-3),which was three times higher than that in summer(5.7 ng·m^(-3)),mainly due to air masses in winter coming primarily from the northern part of China with more biomass burning,whereas more air masses in summer came from the cleaner southeastern ocean.4-Nitrophenol was the most abundant species of NACs in winter,whereas 4-nitrophenol(clean days)and 4-hydroxy-3-nitrobenzoic acid(polluted days)were the most abundant species in summer.Qualitative analysis based on features such as aromatic ring equivalence number(Xc),O/C,and H/C values for the identification and characterization of monocyclic and polycyclic aromatic compounds showed that CHON compounds were mainly aromatic compounds in winter and summer in urban Shanghai.The number and abundance of CHON compounds detected on PM_(2.5) polluted days were 2 and 1.5 times higher(winter)and 2.5 and 2 times higher(summer)than that on clean days,respectively.Comparing the analysis results of clean and polluted days in winter and summer,it was found that 80% of the CHON compounds with a relative abundance in the top 10 had O/N≥3 and RDBE values between 5 and 8.The results suggest that these highly abundant CHON analogs may have had mononitro-or dinitro-substituted benzene rings.Correlation analysis between gaseous precursors and NACs indicated that oxidative reactive formation of VOCs(benzene,toluene,etc.)from anthropogenic emissions was the main source of NACs in summer.By contrast,it was influenced by a combination of biomass combustion emissions and secondary formation of oxidative NOx from anthropogenic VOCs in winter.