Price Chelsea L, Bain Alison, Wallace Brandon J, Preston Thomas C, Davies James F
Department of Chemistry, University of California Riverside, Riverside, California 92420, United States.
Department of Atmospheric and Oceanic Sciences and Department of Chemistry, McGill University, Montreal, Quebec H3A 0G4, Canada.
J Phys Chem A. 2020 Mar 5;124(9):1811-1820. doi: 10.1021/acs.jpca.9b10748. Epub 2020 Feb 26.
Single-particle trapping is an effective strategy to explore the physical and optical properties of aerosol with high precision. Laser-based methods are commonly used to probe the size, optical properties, and composition of nonlight-absorbing droplets in optical and electrodynamic traps. However, these methods cannot be applied to droplets containing photoactive chromophores, and thus, single-particle methods have been restricted to only a subset of atmospherically relevant particle compositions. In this work, we explore the application of a broadband light scattering approach, Mie resonance spectroscopy, to simultaneously probe the size and the refractive index (RI) of droplets in a linear quadrupole electrodynamic balance. We examine the evaporation of poly(ethylene glycol)s and compare the calculated vapor pressures with literature values to benchmark the size accuracy without prior constraint on the RI. We then explore the hygroscopic growth and deliquescence of sodium chloride droplets, measuring RI at the deliquescence relative humidity and demonstrating agreement to literature values. These data allow the wavelength dependence of the RI of aqueous NaCl to be determined using a first-order Cauchy equation, and we effectively reproduce literature data from multiple techniques. We finally discuss measurements from a light-absorbing aqueous droplet containing humic acid and interpret the spectra via the imaginary component of the RI. The approach described here allows the radius of nonabsorbing droplets to be determined within 0.1%, the refractive index within 0.2%, and the first-order term in the Cauchy dispersion equation within ∼5%.
单粒子捕获是一种高精度探索气溶胶物理和光学性质的有效策略。基于激光的方法通常用于探测光学和电动阱中非光吸收性液滴的尺寸、光学性质和组成。然而，这些方法不能应用于含有光活性发色团的液滴，因此，单粒子方法仅限于大气相关粒子组成的一个子集。在这项工作中，我们探索了一种宽带光散射方法——米氏共振光谱法的应用，以同时探测线性四极电动平衡中液滴的尺寸和折射率。我们研究了聚乙二醇的蒸发，并将计算出的蒸气压与文献值进行比较，以在不对折射率进行先验约束的情况下基准化尺寸精度。然后，我们探索了氯化钠液滴的吸湿增长和潮解，测量了潮解相对湿度下的折射率，并证明与文献值一致。这些数据使得可以使用一阶柯西方程确定氯化钠水溶液折射率的波长依赖性，并且我们有效地重现了来自多种技术的文献数据。我们最后讨论了含有腐殖酸的光吸收性水滴的测量结果，并通过折射率的虚部解释光谱。这里描述的方法能够在0.1%的误差范围内确定非吸收性液滴的半径，在0.2%的误差范围内确定折射率，在约5%的误差范围内确定柯西色散方程中的一阶项。
