Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications
Elemental ratio measurements of organic compounds using aerosol mass spectrometry: characterization, improved calibration, and implications
复制标题
DOI：
10.5194/acp-15-253-2015
复制
发表时间：
2015-01-01
影响因子：
6.3
通讯作者：
Worsnop, D. R.
中科院分区：
文献类型：
--
作者：
Canagaratna, M. R.;Jimenez, J. L.;Worsnop, D. R.
有机气溶胶 (OA) 颗粒的元素组成为 OA 来源、化学演化和效应提供了有用的限制。 Aerodyne 高分辨率飞行时间气溶胶质谱仪 (HR-ToF-AMS) 广泛用于测量 OA 元素成分。本研究评估了原子氧与碳 (O : C)、氢与碳 (H : C) 和有机质量与有机碳 (OM : OC) 比率以及碳氧化态 ((OS) 超过 bar (C)) 的 AMS 测量结果，以获得大幅扩展的多功能氧化 OA 标准实验室数据集。对于扩展的标准数据集，Aiken等人引入的方法。 (2008) 使用实验测量的所有离子的离子强度来确定元素比率（此处称为“Aiken-Explicit”），再现了已知的 O:C 和 H:C 比率值，分别在 20%（相对误差的平均绝对值）和 12% 以内。更常用的方法使用经验估计的 H2O+ 和 CO+ 离子强度来避免这些离子的气相空气干扰（此处称为“Aiken-Ambient”），在已知值的 28% 和 14% 范围内重现多功能氧化物质的 O:C 和 H:C。然而，后一种方法的值系统性偏低，对于醇和简单二酸观察到较大的偏倚。对标准化合物高分辨率质谱中 H2O+、CO+ 和 CO2+ 片段的详细检查表明，Aiken-Ambient 方法低估了 CO C，尤其是许多氧化物质产生的 H2O+。 AMS-真空紫外 (VUV) 电离组合测量表明，这些离子是通过 AMS 蒸发器（通常在 600 摄氏度下操作）上脱水和脱羧产生的。据观察，在汽化器温度低至 200 摄氏度时，热分解是有效的。这些结果一起用于开发在空气中测量的 AMS 光谱的“改进环境”元素分析方法。改进的环境方法使用特定的离子片段作为标记来校正分子功能依赖性系统偏差，并在已知分子值的 28% (13%) 范围内重现各个氧化标准品的已知 O:C (H:C) 比率。对于氧化有机标准品的理论标准混合物，改进环境 O : C (H : C) 值的误差更小，这更能代表环境 OA 中存在的复杂物种混合物。对于环境 OA，改进的环境方法生成的 O : C (H : C) 值比之前发布的 Aiken-Ambient 值大 27% (11 %)；观察到 OM : OC 值相应增加了 9%。这些结果意味着环境 OA 的相对氧含量比之前估计的要高。然而，通过两种方法计算的环境 OA 的 (OS) over bar (C) 值非常吻合（平均相对差异为 0.06 (OS) over bar (C) 单位）。这表明，(OS) 超过 bar (C) 是比 O : C 更稳健的氧化度量，可能是因为 (OS) 超过 bar (C) 不受水合或脱水的影响，无论是在大气中还是在分析过程中。
Elemental compositions of organic aerosol (OA) particles provide useful constraints on OA sources, chemical evolution, and effects. The Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is widely used to measure OA elemental composition. This study evaluates AMS measurements of atomic oxygen-to-carbon (O : C), hydrogen-to-carbon (H : C), and organic mass-to-organic carbon (OM : OC) ratios, and of carbon oxidation state ((OS) over bar (C))for a vastly expanded laboratory data set of multifunctional oxidized OA standards. For the expanded standard data set, the method introduced by Aiken et al. (2008), which uses experimentally measured ion intensities at all ions to determine elemental ratios (referred to here as "Aiken-Explicit"), reproduces known O: C and H: C ratio values within 20% (average absolute value of relative errors) and 12%, respectively. The more commonly used method, which uses empirically estimated H2O+ and CO+ ion intensities to avoid gas phase air interferences at these ions (referred to here as "Aiken-Ambient"), reproduces O: C and H: C of multifunctional oxidized species within 28 and 14% of known values. The values from the latter method are systematically biased low, however, with larger biases observed for alcohols and simple diacids. A detailed examination of the H2O+, CO+, and CO2+ fragments in the high-resolution mass spectra of the standard compounds indicates that the Aiken-Ambient method underestimates the CO C and especially H2O+ produced from many oxidized species. Combined AMS-vacuum ultraviolet (VUV) ionization measurements indicate that these ions are produced by dehydration and decarboxylation on the AMS vaporizer (usually operated at 600 degrees C). Thermal decomposition is observed to be efficient at vaporizer temperatures down to 200 degrees C. These results are used together to develop an "Improved-Ambient" elemental analysis method for AMS spectra measured in air. The Improved-Ambient method uses specific ion fragments as markers to correct for molecular functionality-dependent systematic biases and reproduces known O : C (H : C) ratios of individual oxidized standards within 28% (13 %) of the known molecular values. The error in Improved-Ambient O : C (H : C) values is smaller for theoretical standard mixtures of the oxidized organic standards, which are more representative of the complex mix of species present in ambient OA. For ambient OA, the Improved-Ambient method produces O : C (H : C) values that are 27% (11 %) larger than previously published Aiken-Ambient values; a corresponding increase of 9% is observed for OM : OC values. These results imply that ambient OA has a higher relative oxygen content than previously estimated. The (OS) over bar (C) values calculated for ambient OA by the two methods agree well, however (average relative difference of 0.06 (OS) over bar (C) units). This indicates that (OS) over bar (C) is a more robust metric of oxidation than O : C, likely since (OS) over bar (C) is not affected by hydration or dehydration, either in the atmosphere or during analysis.