广州地化所发现珠江三角洲大气硫酸酯形成机制具有特异性

  

  蒎烯和异戊二烯是全球排放量最大的自然源二次有机气溶胶(BSOA)前体物,其与人为源排放的污染物(如氮氧化物和二氧化硫等)反应,生成大量有机硫酸酯(OSs)类化合物。尤其在人为源排放高的城市群地区,这一过程会显著促进BSOA的生成,加剧大气细粒子污染。当前,针对OSs形成机制的研究多通过实验室模拟开展。缺乏重污染地区的观测研究,无法验证高强度人为源排放背景下大气OSs的生成机制。 

  有机地球化学国家重点实验室王新明研究组的何全甫博士,通过在珠江三角洲的观测并结合MCM计算,发现在区内严重大气污染背景下,含硝基的蒎烯有机硫酸酯(pNOSs)主要通过日间的羟基自由基反应途径(OH-pathway)生成,而此前认为对pNOSs生成有显著促进作用的夜间硝基自由基反应途径(NO3-pathway)贡献极低;化学动力学计算的结果表明,原本认为实际大气中无法进行的酯化反应,在珠江三角洲强粒子酸性条件下,对于OSs生成有重要贡献;受区内氮氧化物大量排放的影响,异戊二烯环氧二醇中间体(IEPOX)形成受到抑制,导致异戊二烯OSs浓度极低。这些发现显著区别与已有的观测结果,更新了学术界对重污染地区大气OSs生成机制的认识。相关研究成果近日发表于Environmental Science & Technology (2014, 48, 9236-9245) 

  Abstract 

  Biogenic organosulfates (OSs) are important markers of secondary organic aerosol (SOA) formation involving cross reactions of biogenic precursors (terpenoids) with anthropogenic pollutants. Till now, there is rare information about biogenic OSs in the air of highly polluted areas. In this study, fine particle (PM2.5) samples were separately collected in daytime and nighttime from summer to fall 2010 at a site in the central of Pearl River Delta (PRD), South China. Pinene-derived nitrooxy-organosulfates (pNOSs) and isoprene-derived OSs (iOSs) were quantified using a liquid chromatograph (LC) coupled with a tandem mass spectrometer (MS/MS) operated in the negative electrospray ionization (ESI) mode. The pNOSs with MW 295 exhibited higher levels in fall (151 ± 86.9 ng m-3) than summer (52.4 ± 34.0 ng m-3), probably owing to the elevated levels of NOx and sulfate in fall when air masses mainly passed through city clusters in the PRD and biomass burning was enhanced. In contrast to observations elsewhere where higher levels occurred at nighttime, pNOSs levels in the PRD were higher during daytime in both seasons, indicating that the pNOS formation was likely driven by photochemistry over the PRD. This conclusion is supported by several lines of evidence: the specific pNOSwhich could be formed through both daytime photochemistry and nighttimeNO3 chemistry exhibited no day-night variation in abundance relative to other pNOS isomers; the production of the hydroxynitrate that is the key precursor for this specific pNOS was found to be significant through photochemistry but negligible through NO3 chemistry based on the mechanisms in the Master Chemical Mechanism (MCM). For iOSs, 2-methyltetrol sulfate ester which could be formed from isoprene-derived epoxydiols (IEPOX) under low-NOx conditions showed low concentrations (below detection limit - 2.09 ng m-3), largely due to the depression of IEPOX formation by the high NOx levels over the PRD. 

  

  有机地球化学国家重点实验室&广州地化所科技处 供稿 

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