Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors
Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors
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DOI：
10.5194/acp-19-12515-2019
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发表时间：
2019-04
影响因子：
6.3
通讯作者：
M. Song;Adrian M. Maclean;Yuanzhou Huang;Natalie R. Smith;Sandra L. Blair;J. Laskin;A. Laskin;W. W. Derieux-W.;Ying Li;M. Shiraiwa;S. Nizkorodov;A. Bertram
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文献类型：
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作者：
M. Song;Adrian M. Maclean;Yuanzhou Huang;Natalie R. Smith;Sandra L. Blair;J. Laskin;A. Laskin;W. W. Derieux-W.;Ying Li;M. Shiraiwa;S. Nizkorodov;A. Bertram
摘要。二级有机气溶胶（SOA）中的液-液相分离（LLPS）和粘度（或扩散）信息是改进大气中颗粒大小、质量、反应性和云成核性质的预测所必需的。在这里，我们报告了柴油燃料蒸气光氧化产生的LLPS和SOA内的粘度。柴油含有多种挥发性有机化合物，柴油蒸气光氧化产生的SOA可以很好地代表人为排放产生的SOA。在我们的实验中，LLPS发生在相对湿度（RH）为~ 70%至~ 100%的范围内，导致富有机物的外相和富水的内相。这些结果可能对预测人为SOA的云核性质有影响，因为在高rh值下富含有机物的外相的存在可以降低云滴形成所需的水的过饱和。在RH≤10%时，粘度≥1×108 Pa s，与沥青粘度大致对应。在38% - 50%相对湿度下，粘度范围为1×108至3×105 Pa s。这些测量的粘度与基于氧碳元素比（O:C）和摩尔质量的预测以及基于碳、氢和氧原子数量的预测一致。根据测量的粘度和stoks - einstein关系，在≤10% RH时，柴油SOA中有机物的扩散系数≤5.4×10-17 cm2 s -1, 200 nm柴油SOA颗粒内有机物的混合时间（τ混合）为50 h。这些小的扩散系数和大的混合时间在实验室实验中可能很重要，在实验室实验中，SOA通常在低RH条件下产生和研究，时间尺度为分钟到小时。在38% - 50% RH下，计算得到的有机扩散系数范围为5.4×10-17至1.8×10-13 cm2 s−1，计算得到的τ混合值范围为~ 0.01 h至~ 50 h。这些值为人为SOA的物理化学性质提供了重要的约束条件。
Abstract. Information on liquid–liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of ∼70 % to ∼100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At ≤10 % RH, the viscosity was ≥1×108 Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %–50 % RH, the viscosity was in the range of 1×108 to 3×105 Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (O:C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at ≤10 % RH diffusion coefficients of organics within diesel fuel SOA is ≤5.4×10-17 cm2 s−1 and the mixing time of organics within 200 nm diesel fuel SOA particles (τmixing) is 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low-RH conditions and on timescales of minutes to hours. At 38 %–50 % RH, the calculated organic diffusion coefficients are in the range of 5.4×10-17 to 1.8×10-13 cm2 s−1 and calculated τmixing values are in the range of ∼0.01 h to ∼50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.