Diffusivity measurements of volatile organics in levitated viscous aerosol particles
Diffusivity measurements of volatile organics in levitated viscous aerosol particles
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DOI：
10.5194/acp-17-8453-2017
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发表时间：
2017-07-11
影响因子：
6.3
通讯作者：
Peter, Thomas
中科院分区：
文献类型：
--
作者：
Bastelberger, Sandra;Krieger, Ulrich K.;Peter, Thomas
现场测量表明，大气二次有机气溶胶（SOA）颗粒可以存在于一个高粘度，玻璃态已经刺激了许多研究解决玻璃气溶胶中的水的低扩散率。这些研究的重点是吸湿增长的动力学限制和水的增塑作用。相反，对有机分子和氧化剂在粘性基质中的扩散限制知之甚少。这些可能影响大气化学和具有不同挥发性成分的复杂混合物的气体-颗粒分配。在这项研究中，我们量化的粘性基质中的挥发性有机物的扩散率。在控制相对湿度（RH）和温度的电动天平中研究了蔗糖和少量挥发性四甘醇（PEG-4）的水溶液中产生的单个颗粒的蒸发。通过米氏共振光谱法测定的PEG-4的蒸发损失与径向分辨扩散模型结合使用，以检索PEG-4的平移扩散系数。实验得出的扩散系数与粘度估计的三元系的比较揭示了斯托克斯-爱因斯坦的关系，这往往被调用来推断粘度的扩散系数的故障。PEG-4的蒸发显示出明显的RH和温度依赖性，并且当RH小于或接近30%时，其蒸发受到严重抑制，对应于在< 15 ℃的温度下扩散系数< 10（-14）cm（2）s（-1）。温度依赖性很强，表明扩散活化能约为300 kJ mol（-1）。我们的结论是，大气中的挥发性有机化合物可以受到严重的扩散限制在粘性有机气溶胶颗粒。这可能使有机材料，包括污染物分子，如多环芳烃（PAH）的一个重要的长距离传输机制。
Field measurements indicating that atmospheric secondary organic aerosol (SOA) particles can be present in a highly viscous, glassy state have spurred numerous studies addressing low diffusivities of water in glassy aerosols. The focus of these studies is on kinetic limitations of hygro-scopic growth and the plasticizing effect of water. In contrast, much less is known about diffusion limitations of organic molecules and oxidants in viscous matrices. These may affect atmospheric chemistry and gas-particle partitioning of complex mixtures with constituents of different volatility. In this study, we quantify the diffusivity of a volatile organic in a viscous matrix. Evaporation of single particles generated from an aqueous solution of sucrose and small amounts of volatile tetraethylene glycol (PEG-4) is investigated in an electrodynamic balance at controlled relative humidity (RH) and temperature. The evaporative loss of PEG-4 as determined by Mie resonance spectroscopy is used in conjunction with a radially resolved diffusion model to retrieve translational diffusion coefficients of PEG-4. Comparison of the experimentally derived diffusivities with viscosity estimates for the ternary system reveals a breakdown of the Stokes-Einstein relationship, which has often been invoked to infer diffusivity from viscosity. The evaporation of PEG-4 shows pronounced RH and temperature dependencies and is severely depressed for RH less than or similar to 30 %, corresponding to diffusivities < 10(-14) cm(2) s(-1) at temperatures < 15 degrees C. The temperature dependence is strong, suggesting a diffusion activation energy of about 300 kJ mol(-1). We conclude that atmospheric volatile organic compounds can be subject to severe diffusion limitations in viscous organic aerosol particles. This may enable an important long-range transport mechanism for organic material, including pollutant molecules such as polycyclic aromatic hydrocarbons (PAHs).