Marak Katherine E, Roebuck Jilian H, Chong Esther, Poitras Haley, Freedman Miriam Arak
J Phys Chem A. 2022 Sep 8;126(35):5965-5973. doi: 10.1021/acs.jpca.2c03063. Epub 2022 Aug 26.
Aerosol particles can facilitate heterogeneous ice formation in the troposphere and stratosphere by acting as ice-nucleating particles, modulating cloud formation/dissipation, precipitation, and their microphysical properties. Heterogeneous ice nucleation is driven by ice embryo formation on the particle surface, which can be influenced by features of the surface such as crystallinity, surface structure, lattice structure, defects, and functional groups. To characterize the effect of crystallinity, pores, and surface functional groups toward ice nucleation, samples of comparable silica systems, specifically, quartz, ordered and nonordered porous amorphous silica samples with a range of pore sizes (2-11 nm), and nonporous functionalized silica spheres, were used as models for mineral dust aerosol particles. The ice nucleation activity of these samples was investigated by using an immersion freezing chamber. The results suggest that crystallinity has a larger effect than porosity on ice nucleation activity, as all of the porous silica samples investigated had lower onset freezing temperatures and lower ice nucleation activities than quartz. Our findings also suggest that pores alone are not sufficient to serve as effective active sites and need some additional chemical or physical property, like crystallinity, to nucleate ice in immersion mode freezing. The addition of a low density of organic functional groups to nonporous samples showed little enhancement compared to the inherent nucleation activity of silica with native surface hydroxyl groups. The density of functional groups investigated in this work suggests that a different arrangement of surface groups may be needed for enhanced immersion mode ice nucleation activity. In summary, crystallinity dictates the ice nucleation activity of silica samples rather than porosity or low-density surface functional groups. This work has broader implications regarding the climate impacts resulting from ice cloud formation.
气溶胶颗粒可通过充当冰核颗粒，调节云的形成/消散、降水及其微物理性质，从而促进对流层和平流层中的异质冰形成。异质冰核形成是由颗粒表面的冰胚胎形成驱动的，这可能会受到表面特征的影响，如结晶度、表面结构、晶格结构、缺陷和官能团。为了表征结晶度、孔隙和表面官能团对冰核形成的影响，使用了具有可比二氧化硅系统的样品，具体而言，石英、具有一系列孔径（2-11纳米）的有序和无序多孔无定形二氧化硅样品，以及无孔功能化二氧化硅球，作为矿物粉尘气溶胶颗粒的模型。通过使用浸入式冷冻室研究了这些样品的冰核活性。结果表明，结晶度对冰核活性的影响比孔隙率更大，因为所有研究的多孔二氧化硅样品的起始冻结温度和冰核活性都低于石英。我们的研究结果还表明，仅孔隙不足以作为有效的活性位点，需要一些额外的化学或物理性质，如结晶度，才能在浸入式冷冻模式下使冰成核。与具有天然表面羟基的二氧化硅的固有成核活性相比，向无孔样品中添加低密度有机官能团几乎没有增强作用。这项工作中研究的官能团密度表明，可能需要不同的表面基团排列来增强浸入式冰核活性。总之，结晶度决定了二氧化硅样品的冰核活性，而不是孔隙率或低密度表面官能团。这项工作对冰云形成所产生的气候影响具有更广泛的意义。
