天津市大气细颗粒物中脂类化合物的单体稳定碳同位素研究

Organic matter is one of the most abundant species in atmospheric fine particles (PM2.5)，which has important implications for visibility and global climate change. Among the types of atmospheric organic matter, alkyl lipids such as n-alkanes, fatty acids and n-alcohols are excellent tracers for investigating the origin and fate of atmospheric aerosols. Stable carbon isotopic compositions of lipids have been successfully applied to investigate the origins and source apportionments of organic aerosol in pristine environments, while little is known in polluted areas. In this project, PM2.5 samples in summer and winter will be investigated at an urban site in Tianjin based on organic molecular distributions and compound-specific stable carbon isotopic compositions of lipid compounds. Based on the analysis of concentration and stable carbon isotope ratio of lipids, efforts are made to identify the differences of source signals in haze/non-haze days. Combined with positive matrix factorization and Bayesian models, contribution of fossil sources and biogenic sources to lipid compounds in PM2.5 are further assessed at a comprehensive way. In general, the objective of this project is to broaden our knowledge of the atmospheric transport processes in heavily polluted areas, which provides useful scientific evidence to protect the atmospheric environment and improve the air quality in urban region.

有机物是大气气溶胶细颗粒物（PM2.5）中含量最为丰富的物种之一，对大气能见度和全球气候变化都有重要影响。正构烷烃、脂肪酸及正构脂肪醇三种脂类化合物是气溶胶中有机物的重要组分，有机单体稳定碳同位素在揭示这些脂类分子的来源、归宿和大气化学行为等方面具有显著优势。目前，我国针对大气气溶胶中脂类化合物的有机单体碳同位素组成研究还非常少。本申请拟通过天津市夏季和冬季大气细颗粒物的采样，将大气环境化学和同位素地球化学手段相结合，在获取脂类化合物浓度水平和有机单体稳定碳同位素的基础上，结合排放源样品的同位素组成，正交矩阵因子法和贝叶斯同位素模型相互结合校验，进而明确灰霾生消过程中脂类分子碳同位素的组成特征，深入理解并量化不同季节的灰霾生消过程中化石源和生物源对细颗粒物中脂类分子的相对贡献。本申请所获成果有助于增进理解重霾区有机物的来源、消亡和大气传输过程，从而为区域大气环境保护和改善提供科学依据。

煤炭燃烧、生物质燃烧及烹饪等人为排放源可贡献大量的有机污染物。其中正构烷烃、脂肪酸含量占可检测有机物的25%–50%。它们的来源广泛，Ｃ-Ｃ连接的烷基化结构特征使得化学性质稳定，能很好地被保存下来并记载了原始排放源的信息。本申请通过对北京和天津市大气总悬浮颗粒物和细颗粒物的采样，以及华北地区主要排放源（生物质燃烧和煤炭燃烧）的采样，在定量检测有机气溶胶生物标记物的基础上，运用有机单体稳定碳同位素和放射性碳同位素辨析灰霾生消过程中生物标记物的同位素信号的差异性，达到深入理解化石源和生物源对颗粒物中生物标记物的相对贡献的目的。研究结果表明：夏季的生物质燃烧显著影响了生物标记物的稳定同位素组成（负偏信号特征）；冬季民用煤燃烧主要影响正构烷烃的稳定碳（正偏）和氢同位素（负偏）组成；在秋冬季节，西北方向空气团的输送，使有机气溶胶中C24:0脂肪酸相对亏损13C，而西南方向的空气团则相反（富集13C）；化石源标记物C23正构烷烃在冬季的浓度是其它季节的13倍且明显富集13C，主要是不利的气象因素和大量的人为源排放所导致。夏季主要通过贡献D同位素较重的水源改变植物蜡中D同位素，从而改变大气生物标记物的D同位素组成。利用稳定性碳和放射性碳（13C-14C）技术，评估了北京、天津城区正构烷烃、正构脂肪酸的来源。研究结果初步表明：由于长距离输送，蒙古等富含黄土沉积的区域可能搬运了大量的沉积数年至百年的生物标记物 (D14C: -200‰ to -470 ‰)。本申请所获成果可为边缘海区域生物标记物的来源提供参考和来源依据，有助于增进理解重霾区有机质的大气物理化学过程，并为区域大气环境保护和改善提供科学依据。