A new aerosol flow reactor to study secondary organic aerosol
A new aerosol flow reactor to study secondary organic aerosol
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DOI：
10.5194/amt-12-4519-2019
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发表时间：
2019-08
影响因子：
3.8
通讯作者：
K. Pereira;G. Rovelli;Y. Song;Alfred W. Mayhew;J. Reid;J. Hamilton
中科院分区：
文献类型：
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作者：
K. Pereira;G. Rovelli;Y. Song;Alfred W. Mayhew;J. Reid;J. Hamilton
抽象的。气体-颗粒平衡分配是用于描述二次有机气溶胶（SOA）的增长和损失的基本概念。然而，最近的文献表明，气体-颗粒分配可能受到动力学限制，由于气溶胶的物理状态（例如玻璃状、粘性）而防止气溶胶相挥发。粘性气溶胶内扩散常数的实验测量是有限的，并且不能代表 SOA 中观察到的复杂化学成分（即多组分混合物）。由于需要解决有关颗粒的物理状态和化学成分对气体-颗粒分配的影响的基本问题，我们提出了新建的 0.3 m3 连续流反应器 (CFR) 的设计和操作，该反应器可用作深入了解 SOA 的成分和物理状态的工具。 CFR 用于在不同的实验条件（即相对湿度、VOC 和 VOC/NOx 比率）下通过 α-蒎烯、柠檬烯、β-石竹烯和甲苯的光氧化生成 SOA。每个实验最多收集 102 毫克 SOA 质量，允许使用高精度的成分和单颗粒分析技术，由于分析需要大量有机气溶胶质量，这些技术通常无法实现。使用一套离线分析技术来确定生成的 SOA 的化学成分和物理状态，包括衰减全反射红外光谱；碳、氢、氮、硫 (CHNS) 元素分析； 1H和1H-13C核磁共振波谱(NMR)；超高效液相色谱超高分辨率质谱（UHRMS）；高效液相色谱离子阱质谱（HPLC-ITMS）；和电动天平（EDB）。生成的 SOA 样品的氧碳 (O∕C) 和氢碳 (H∕C) 比率（使用 CHNS 元素分析仪测定）与文献值吻合良好，并且与特征 Van Krevelen 图轨迹一致，观察到的斜率为 -0.41。在单独的重复实验中形成的两个 SOA 样品的元素组成显示出出色的再现性，观察到 SOA 样品的 O∕C 和 H∕C 比率在分析仪器的误差范围内（仪器精度相对于参考标准为 ±0.15%）。由于能够使用高精度 CHNS 元素分析仪来确定 SOA 样品的元素组成，因此我们能够使用 UHRMS（一种常用技术）评估报告的 SOA 元素组成的准确性。在所有研究的实验中，使用 UHRMS 为每个 SOA 样品获得的 SOA O/C 比率低于从 CHNS 分析仪（更准确和非选择性技术）获得的 O/C 比率。 ΔO/C 比率的平均差异范围为 19% 至 45%，具体取决于 SOA 前体和形成条件。 α-蒎烯 SOA 标准品是使用半制备型 HPLC-ITMS 与自动馏分收集器耦合，然后进行 1 H NMR 光谱，从收集的 SOA 质量中生成的。使用该方法对 α-蒎烯 SOA 进行了高达 35.8±1.6 %（校准图斜率不确定性的传播误差）的量化；与大多数以前的研究相比有相当大的进步。从收集的 SOA 样品中产生单个气溶胶液滴，并在不同温度和相对湿度下将其捕获在 EDB 中，以研究其理化性质的动态变化。研究发现，由于颗粒粘度，在 0% 相对湿度下，甲苯和 β-石竹烯 SOA 颗粒中有机成分的挥发受到动力学限制。新建 CFR 的非常规使用，结合全面的离线化学表征和单颗粒测量，提供了一种独特的方法来进一步了解 SOA 形成条件、化学成分和理化性质之间的关系。
Abstract. Gas-particle equilibrium partitioning is a fundamental concept used to describe the growth and loss of secondary organic aerosol (SOA). However, recent literature has suggested that gas-particle partitioning may be kinetically limited, preventing volatilization from the aerosol phase as a result of the physical state of the aerosol (e.g. glassy, viscous). Experimental measurements of diffusion constants within viscous aerosol are limited and do not represent the complex chemical composition observed in SOA (i.e. multicomponent mixtures). Motivated by the need to address fundamental questions regarding the effect of the physical state and chemical composition of a particle on gas-particle partitioning, we present the design and operation of a newly built 0.3 m3 continuous-flow reactor (CFR), which can be used as a tool to gain considerable insights into the composition and physical state of SOA. The CFR was used to generate SOA from the photo-oxidation of α-pinene, limonene, β-caryophyllene and toluene under different experimental conditions (i.e. relative humidity, VOC and VOC∕NOx ratios). Up to 102 mg of SOA mass was collected per experiment, allowing the use of highly accurate compositional- and single-particle analysis techniques, which are not usually accessible due to the large quantity of organic aerosol mass required for analysis. A suite of offline analytical techniques was used to determine the chemical composition and physical state of the generated SOA, including attenuated total reflectance infrared spectroscopy; carbon, hydrogen, nitrogen, and sulfur (CHNS) elemental analysis; 1H and 1H-13C nuclear magnetic resonance spectroscopy (NMR); ultra-performance liquid chromatography ultra-high-resolution mass spectrometry (UHRMS); high-performance liquid chromatography ion-trap mass spectrometry (HPLC-ITMS); and an electrodynamic balance (EDB). The oxygen-to-carbon (O∕C) and hydrogen-to-carbon (H∕C) ratios of generated SOA samples (determined using a CHNS elemental analyser) displayed good agreement with literature values and were consistent with the characteristic Van Krevelen diagram trajectory, with an observed slope of −0.41. The elemental composition of two SOA samples formed in separate replicate experiments displayed excellent reproducibility, with the O∕C and H∕C ratios of the SOA samples observed to be within error of the analytical instrumentation (instrument accuracy ±0.15 % to a reference standard). The ability to use a highly accurate CHNS elemental analyser to determine the elemental composition of the SOA samples allowed us to evaluate the accuracy of reported SOA elemental compositions using UHRMS (a commonly used technique). In all of the experiments investigated, the SOA O∕C ratios obtained for each SOA sample using UHRMS were lower than the O∕C ratios obtained from the CHNS analyser (the more accurate and non-selective technique). The average difference in the ΔO∕C ratios ranged from 19 % to 45 % depending on the SOA precursor and formation conditions. α-pinene SOA standards were generated from the collected SOA mass using semi-preparative HPLC-ITMS coupled to an automated fraction collector, followed by 1H NMR spectroscopy. Up to 35.8±1.6 % (propagated error of the uncertainty in the slope of the calibrations graphs) of α-pinene SOA was quantified using this method; a considerable improvement from most previous studies. Single aerosol droplets were generated from the collected SOA samples and trapped within an EDB at different temperatures and relative humidities to investigate the dynamic changes in their physiochemical properties. The volatilization of organic components from toluene and β-caryophyllene SOA particles at 0 % relative humidity was found to be kinetically limited, owing to particle viscosity. The unconventional use of a newly built CFR, combined with comprehensive offline chemical characterization and single-particle measurements, offers a unique approach to further our understanding of the relationship between SOA formation conditions, chemical composition and physiochemical properties.