Valenzuela Antonio, Chu Fenghong, Haddrell Allen E, Cotterell Michael I, Walker Jim S, Orr-Ewing Andrew J, Reid Jonathan P
Department of Applied Physics, University of Granada, Granada 18071, Spain.
Andalusian Institute for Earth System Research (IISTA-CEAMA), Granada 18006, Spain.
J Phys Chem A. 2021 Jan 14;125(1):394-405. doi: 10.1021/acs.jpca.0c09213. Epub 2020 Dec 23.
Optical trapping is a well-established technique to manipulate and levitate micro- and nanoscale particles and droplets. However, optical traps for single aerosol studies are most often limited to trapping spherical nonabsorbing droplets, and a universal optical trap for the stable confinement of particles regardless of their absorption strength and morphology is not established. Instead, new opportunities arise from levitating droplets using electrodynamic traps. Here, using a combined electrodynamic linear quadrupole trap and a cavity ring-down spectrometer, we demonstrate that it is possible to trap single droplets and simultaneously measure their extinction cross sections and elastic scattering phase functions over extended periods of time. To test the novel setup, we evaluated the evaporation of 1,2,6-hexanetriol under low-humidity conditions, and the evolution of aqueous (NH)SO and NaCl droplets experiencing changing environmental conditions. Our studies extended beyond spherical droplets and we measured particle extinction cross sections after the efflorescence (crystallization) of the inorganic salt particles. Comparison of measured cross sections for crystallized particles with light scattering model predictions (using Mie theory or the T-matrix/extended boundary-condition method (EBCM) implementations for random orientation, with either the spheroid or superellipsoid parameterizations) enables information on particle shape to be inferred. Specifically, we find that cross sections for dry (NH)SO particles are accounted for by Mie theory and, thus, particle shape is represented well by a sphere. Conversely, the cross sections for dry NaCl particles are only reconciled with light scattering models pertaining to nonspherical shapes. These results will have implications for accurate remote sensing retrievals of dry salt optical properties and for parameterizations implemented in radiative forcing calculations with changing humidity. Moreover, our new platform for precise and accurate measurement of optical properties of micron-scale and sub-micron particles has potential applications in a range of areas of atmospheric science, such as precise light scattering measurements for ice crystals and mineral dust. It represents a promising step toward accurate characterizations of optical properties for nonspherical and light-absorbing aerosols.
光镊是一种成熟的技术，用于操纵和悬浮微米级和纳米级的颗粒与液滴。然而，用于单个气溶胶研究的光镊大多仅限于捕获球形非吸收性液滴，尚未建立一种能稳定捕获颗粒而不考虑其吸收强度和形态的通用光镊。相反，利用电动阱悬浮液滴带来了新的机遇。在此，我们使用电动线性四极阱和腔衰荡光谱仪相结合的方法，证明了可以捕获单个液滴，并能在较长时间内同时测量其消光截面和弹性散射相函数。为了测试这个新装置，我们评估了低湿度条件下1,2,6 -己三醇的蒸发情况，以及经历环境条件变化的硫酸铵（(NH₄)₂SO₄）和氯化钠（NaCl）水溶液液滴的演变。我们的研究超越了球形液滴，还测量了无机盐颗粒风化（结晶）后的颗粒消光截面。将结晶颗粒的测量截面与光散射模型预测结果（使用米氏理论或针对随机取向采用椭球体或超椭球体参数化的T矩阵/扩展边界条件方法（EBCM）实现）进行比较，能够推断出颗粒形状的信息。具体而言，我们发现米氏理论能够解释干燥的硫酸铵颗粒的截面，因此颗粒形状可以很好地用球体来表示。相反，干燥的氯化钠颗粒的截面仅与非球形形状的光散射模型相符。这些结果将对准确遥感反演干盐光学特性以及在湿度变化的辐射强迫计算中所采用的参数化方法产生影响。此外，我们用于精确测量微米级和亚微米级颗粒光学特性的新平台在大气科学的一系列领域具有潜在应用，例如对冰晶和矿物粉尘进行精确的光散射测量。这代表了朝着准确表征非球形和光吸收性气溶胶光学特性迈出的有希望的一步。
