Mishra Ashmi, Kilchhofer Kevin, Iezzi Lucia, Pöschl Ulrich, Alpert Peter A, Ammann Markus, Berkemeier Thomas
Multiphase Chemistry Department, Max Planck Institute for Chemistry, 55128 Mainz, Germany.
PSI Center for Energy and Environmental Sciences, 5232 Villigen, Switzerland.
ACS Earth Space Chem. 2025 Feb 25;9(3):689-698. doi: 10.1021/acsearthspacechem.4c00364. eCollection 2025 Mar 20.
Iron contained in atmospheric aerosol particles can form complexes with organic ligands and initiate photochemical reactions that alter the composition and physicochemical properties of the particles. Depending on the temperature and humidity, organic particles exist in different phase states, which affects reactant diffusivity and chemical reaction rates. We performed coated-wall flow-tube experiments using citric acid films doped with iron as proxies for secondary organic aerosols. We quantified the CO production under UV irradiation as a function of time and relative humidity (RH) and observed a pronounced decrease of CO production with decreasing RH. The kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) and a Monte Carlo-based global optimization method were applied to all measured data to determine the underlying effects of mass transport and chemical reactions. The model analysis revealed that after an initial rapid reaction, photooxidation becomes limited by the reoxidation of Fe. Under dry conditions (RH < 65%), the reoxidation of Fe is kinetically limited by the supply of O, as slow diffusion in the viscous organic matrix leads to anoxia in the interior of the film. At high humidity (RH > 85%), mass transport limitations cease, resulting in full O saturation, and photooxidation becomes limited by the chemical reaction of Fe with oxidants. Reactive oxygen species play a key role in Fe reoxidation and thus in perpetuating photooxidation chemistry. A single O molecule triggers a redox cascade from O to HO, HO, and OH, leading to ≈3 cycles of the Fe/Fe redox pair. Our model and kinetic parameters provide new insights and constraints in the interplay of microphysical properties and photochemical aging of mixed organic-inorganic aerosol particles, which may influence their effects on air quality, climate, and public health.
大气气溶胶颗粒中所含的铁可与有机配体形成络合物，并引发光化学反应，从而改变颗粒的组成和物理化学性质。根据温度和湿度的不同，有机颗粒存在于不同的相态中，这会影响反应物的扩散率和化学反应速率。我们使用掺杂铁的柠檬酸薄膜作为二次有机气溶胶的替代物，进行了涂壁流动管实验。我们量化了紫外线照射下一氧化碳的生成量随时间和相对湿度（RH）的变化情况，并观察到随着相对湿度的降低，一氧化碳的生成量显著减少。将气溶胶表面和体相化学动力学多层模型（KM-SUB）以及基于蒙特卡洛的全局优化方法应用于所有测量数据，以确定传质和化学反应的潜在影响。模型分析表明，在初始快速反应之后，光氧化作用受到铁再氧化的限制。在干燥条件下（RH < 65%），铁的再氧化在动力学上受到氧供应的限制，因为在粘性有机基质中的缓慢扩散导致薄膜内部缺氧。在高湿度条件下（RH > 85%），传质限制消失，导致氧完全饱和，光氧化作用受到铁与氧化剂化学反应的限制。活性氧物种在铁的再氧化中起着关键作用，从而在维持光氧化化学过程中发挥作用。单个氧分子引发从O到HO、HO和OH的氧化还原级联反应，导致Fe/Fe氧化还原对循环约3次。我们的模型和动力学参数为混合有机-无机气溶胶颗粒的微观物理性质与光化学老化之间的相互作用提供了新的见解和限制条件，这可能会影响它们对空气质量、气候和公众健康的影响。
