Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities
Timescales of secondary organic aerosols to reach equilibrium at various temperatures and relative humidities
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DOI：
10.5194/acp-19-5959-2019
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发表时间：
2019-02
影响因子：
6.3
通讯作者：
Ying Li;M. Shiraiwa
中科院分区：
文献类型：
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作者：
Ying Li;M. Shiraiwa
抽象的。二次有机气溶胶 (SOA) 占空气颗粒物的很大一部分，SOA 的形成通常是在假设快速建立气体-颗粒平衡的情况下进行建模的。在这里，我们使用最先进的动力学通量模型估计了 SOA 在各种温度和相对湿度下实现气体-粒子平衡的特征时间尺度。通过改变开放和封闭系统中有机化合物的颗粒相状态、尺寸、质量负荷和挥发性来计算平衡时间尺度。模型模拟表明，在行星边界层的大多数条件下，半挥发性化合物的平衡时间尺度约为秒或分钟，但如果颗粒在低相对湿度下采用具有高玻璃化转变温度的玻璃态或无定形固态，平衡时间可能会超过 1 小时。在温度较低的自由对流层中，即使在中等或相对较高的相对湿度下，由于高粘性颗粒中整体扩散的动力学限制，它也可能比数小时或数天更长。与封闭系统中的半挥发性化合物相比，低挥发性化合物分配成高粘性颗粒的时间尺度更短，因为它很大程度上由冷凝槽决定，因为再蒸发非常缓慢，气相和近表面本体之间相对快速地建立局部平衡。平衡时间尺度对挥发性和体积扩散率的依赖性为 SOA 分配的热力学或动力学处理提供了重要的见解，以便准确预测区域和全球化学输运模型中半挥发性化合物的气相和颗粒相浓度。
Abstract. Secondary organic aerosols (SOA) account for a substantial fraction of air particulate matter, and SOA formation is often modeled assuming rapid establishment of gas–particle equilibrium. Here, we estimate the characteristic timescale for SOA to achieve gas–particle equilibrium under a wide range of temperatures and relative humidities using a state-of-the-art kinetic flux model. Equilibration timescales were calculated by varying particle phase state, size, mass loadings, and volatility of organic compounds in open and closed systems. Model simulations suggest that the equilibration timescale for semi-volatile compounds is on the order of seconds or minutes for most conditions in the planetary boundary layer, but it can be longer than 1 h if particles adopt glassy or amorphous solid states with high glass transition temperatures at low relative humidity. In the free troposphere with lower temperatures, it can be longer than hours or days, even at moderate or relatively high relative humidities due to kinetic limitations of bulk diffusion in highly viscous particles. The timescale of partitioning of low-volatile compounds into highly viscous particles is shorter compared to semi-volatile compounds in the closed system, as it is largely determined by condensation sink due to very slow re-evaporation with relatively quick establishment of local equilibrium between the gas phase and the near-surface bulk. The dependence of equilibration timescales on both volatility and bulk diffusivity provides critical insights into thermodynamic or kinetic treatments of SOA partitioning for accurate predictions of gas- and particle-phase concentrations of semi-volatile compounds in regional and global chemical transport models.