Madawala Chamika K, Lee Hansol D, Kaluarachchi Chathuri P, Tivanski Alexei V
Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States.
Anal Chem. 2023 Oct 3;95(39):14566-14572. doi: 10.1021/acs.analchem.3c01835. Epub 2023 Sep 23.
Atmospheric aerosols' viscosities can vary significantly depending on their composition, mixing states, relative humidity (RH) and temperature. The diffusion time scale of atmospheric gases into an aerosol is largely governed by its viscosity, which in turn influences heterogeneous chemistry and climate-relevant aerosol effects. Quantifying the viscosity of aerosols in the semisolid phase state is particularly important as they are prevalent in the atmosphere and have a wide range of viscosities. Currently, direct viscosity measurements of submicrometer individual atmospheric aerosols are limited, largely due to the inherent size limitations of existing experimental techniques. Herein, we present a method that utilizes atomic force microscopy (AFM) to directly quantify the viscosity of substrate-deposited individual submicrometer semisolid aerosol particles as a function of RH. The method is based on AFM force spectroscopy measurements coupled with the Kelvin-Voigt viscoelastic model. Using glucose, sucrose, and raffinose as model systems, we demonstrate the accuracy of the AFM method within the viscosity range of ∼10-10 Pa s. The method is applicable to individual particles with sizes ranging from tens of nanometers to several micrometers. Furthermore, the method does not require prior knowledge on the composition of studied particles. We anticipate future measurements utilizing the AFM method on atmospheric aerosols at various RH to aid in our understanding of the range of aerosols' viscosities, the extent of particle-to-particle viscosity variability, and how these contribute to the particle diversity observable in the atmosphere.
大气气溶胶的粘度会因其成分、混合状态、相对湿度（RH）和温度而有显著变化。大气气体扩散到气溶胶中的时间尺度在很大程度上由其粘度决定，而粘度又会影响非均相化学和与气候相关的气溶胶效应。量化半固相状态下气溶胶的粘度尤为重要，因为它们在大气中普遍存在且粘度范围广泛。目前，对亚微米级单个大气气溶胶的直接粘度测量受到限制，这主要是由于现有实验技术固有的尺寸限制。在此，我们提出一种利用原子力显微镜（AFM）直接量化沉积在基底上的单个亚微米级半固生气溶胶颗粒粘度随相对湿度变化的方法。该方法基于AFM力谱测量并结合开尔文 - 沃伊特粘弹性模型。以葡萄糖、蔗糖和棉子糖作为模型体系，我们证明了AFM方法在约10⁻¹⁰帕秒的粘度范围内的准确性。该方法适用于尺寸从几十纳米到几微米的单个颗粒。此外，该方法不需要预先了解所研究颗粒的成分。我们预计未来利用AFM方法在不同相对湿度下对大气气溶胶进行测量，将有助于我们了解气溶胶粘度范围、颗粒间粘度变化程度以及这些如何导致大气中可观测到的颗粒多样性。
