Non-equilibrium interplay between gas–particle partitioning and multiphase chemical reactions of semi-volatile compounds: mechanistic insights and practical implications for atmospheric modeling of polycyclic aromatic hydrocarbons
Non-equilibrium interplay between gas–particle partitioning and multiphase chemical reactions of semi-volatile compounds: mechanistic insights and practical implications for atmospheric modeling of polycyclic aromatic hydrocarbons
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DOI：
10.5194/acp-21-6175-2021
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发表时间：
2021-04
影响因子：
6.3
通讯作者：
Jake Wilson;U. Pöschl;M. Shiraiwa;T. Berkemeier
中科院分区：
文献类型：
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作者：
Jake Wilson;U. Pöschl;M. Shiraiwa;T. Berkemeier
抽象。多环芳烃（PAHs）是一种致癌的空气污染物。多环芳烃在大气中的扩散受气粒分配和化学损失的影响。这些过程密切相关，可能发生在截然不同的时间尺度上，这使得化学迁移模型中的数学描述变得复杂。在这里，我们使用一个动力学模型，明确解决了在气相和颗粒相的质量传输和化学反应，描述和探索的动态和非平衡的相互作用的气体颗粒分区和化学损失的多环芳烃烟尘颗粒。我们定义气粒分配的平衡时标τeq为系统弛豫到分配平衡的e折叠时间。我们发现这个指标的跨度从几秒到几小时，取决于温度，颗粒表面积和PAH的类型。平衡时间可以用一个与时间无关的方程来近似，τ eq = 1 k des + k ads，它取决于解吸速率系数kdes和吸附速率系数kads，这两个系数都可以从实验上获得的参数计算出来。该模型揭示了两个制度，其中不同的物理过程控制的平衡时间尺度：解吸控制和吸附控制制度。在与PAH芘的案例研究中，我们说明了如何化学损失可以扰动的平衡颗粒分数在典型的大气浓度的O3和OH。对于与O3的表面反应，扰动是显着的，并增加与气相浓度的O3。相反，与OH自由基反应的扰动较小，OH自由基与颗粒表面和气相中的芘反应。全球和区域化学输运模式通常采用瞬时平衡方法近似气体-颗粒分配。我们强调的情况下，这些近似偏离明确耦合处理的气体-颗粒的分区和化学在这项研究中提出的。我们发现，解之间的差异取决于算子分裂时间步长和时间步长的选择可以帮助最小化的差异。在这项工作中提出的调查结果和技术不仅是相关的多环芳烃，但也可以适用于其他半挥发性物质，进行化学反应和气相和颗粒相之间的质量传输。
Abstract. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic air pollutants. The dispersion of PAHs in the atmosphere is influenced by gas–particle partitioning and chemical loss. These processes are closely interlinked and may occur at vastly differing timescales, which complicates their mathematical description in chemical transport models. Here, we use a kinetic model that explicitly resolves mass transport and chemical reactions in the gas and particle phases to describe and explore the dynamic and non-equilibrium interplay of gas–particle partitioning and chemical losses of PAHs on soot particles. We define the equilibration timescale τeq of gas–particle partitioning as the e -folding time for relaxation of the system to the partitioning equilibrium. We find this metric to span from seconds to hours depending on temperature, particle surface area, and the type of PAH. The equilibration time can be approximated using a time-independent equation, τ eq ≈ 1 k des + k ads , which depends on the desorption rate coefficient kdes and adsorption rate coefficient kads , both of which can be calculated from experimentally accessible parameters. The model reveals two regimes in which different physical processes control the equilibration timescale: a desorption-controlled and an adsorption-controlled regime. In a case study with the PAH pyrene, we illustrate how chemical loss can perturb the equilibrium particulate fraction at typical atmospheric concentrations of O3 and OH . For the surface reaction with O3 , the perturbation is significant and increases with the gas-phase concentration of O3 . Conversely, perturbations are smaller for reaction with the OH radical, which reacts with pyrene on both the surface of particles and in the gas phase. Global and regional chemical transport models typically approximate gas–particle partitioning with instantaneous-equilibration approaches. We highlight scenarios in which these approximations deviate from the explicitly coupled treatment of gas–particle partitioning and chemistry presented in this study. We find that the discrepancy between solutions depends on the operator-splitting time step and the choice of time step can help to minimize the discrepancy. The findings and techniques presented in this work not only are relevant for PAHs but can also be applied to other semi-volatile substances that undergo chemical reactions and mass transport between the gas and particle phase.