Aerosol particles represent unique chemical environments because of their high surface area-to-volume ratio that promotes the effects of interfacial chemistry in confined environments. Properties such as viscosity, diffusivity, water content, pH, and morphology—following liquid–liquid phase separation—can strongly alter how a particle interacts with condensable vapors and reactive trace gases, thus modifying its continual evolution and environmental effects. Our understanding of this chemical evolution of atmospheric particulate matter and its environmental impacts is largely limited by our ability to directly observe how these critical particle properties respond to the addition or reactive uptake of new chemical components. Aerosol optical tweezers (AOT) stably trap particles in focused laser beams, providing positional control and the retrieval of many of these critical properties required to understand and predict the chemistry of aerosolized microdroplets. The analytical power of the AOT stems from the retrieval of the cavity-enhanced Raman spectrum induced by the trapping laser. Analysis of the whispering gallery modes (WGMs) that resonate as a standing wave around the droplet’s interface, provide high accuracy measurements of the droplet’s size, refractive index (and thus a measurement of composition), and can distinguish between core–shell, partially engulfed, and homogeneous morphologies. We have advanced the ability to determine the properties of the core and shell phases in biphasic droplets, including obtaining high-accuracy pH measurements. These capabilities were applied to perform AOT physical chemistry experiments on authentic secondary organic aerosol (SOA) produced directly in the AOT chamber by ozonolysis of terpene vapors. The propensity of the SOA to phase separate as a shell from a wide range of nonpolar to polar core phases was observed, along with the discovery of a stable emulsified state of SOA particles in an aqueous salt droplet. Micron-thick SOA shells did not impede the gain or loss of water or squalane from the core to the surrounding air, indicating no significant diffusional limitations to condensational growth or partitioning even under dry conditions. These experiments formed the foundation of a new framework that predicts how the phase-separated morphology of complex aerosols containing organic carbon evolves during continual atmospheric oxidation processes. Increases in oxidation state will quickly drive conversion from a partially engulfed to core–shell morphology that has dramatically different chemical reactivity since the core phase is completely concealed by the shell. The recent advances in the experimental capabilities of the AOT technique such as presented here enable novel experimental methodologies that provide insights into the chemistry and multidimensional properties of aerosol microdroplets, and how these coevolve and respond to continual chemical reactions.

中文翻译：

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气溶胶光学镊子阐明了大气微滴的化学性质，酸度，相分离和形态

气雾剂颗粒具有独特的化学环境，因为它们的高表面积体积比可促进密闭环境中界面化学的作用。诸如液相，液相分离之后，诸如粘度，扩散系数，水含量，pH值和形态等属性可以极大地改变颗粒与可冷凝蒸气和反应性微量气体的相互作用，从而改变其持续演化和环境影响。我们对大气颗粒物的化学演化及其环境影响的理解在很大程度上受到我们直接观察这些关键颗粒特性如何响应新化学成分的添加或反应性吸收的能力的限制。气溶胶光学镊子（AOT）稳定地将粒子捕获在聚焦的激光束中，提供位置控制以及对理解和预测雾化微滴化学反应所需的许多关键特性的检索。AOT的分析能力源于对俘获激光引起的腔增强拉曼光谱的恢复。对耳语通道模式（WGM）的分析会在液滴界面周围作为驻波产生共振，从而可以对液滴的大小，折射率进行高精度测量（从而对成分进行测量），并且可以区分部分被吞噬的核壳结构和均质形态。我们已经具有确定双相液滴中核相和壳相性质的能力，包括获得高精度的pH测量值。这些功能被用于对通过萜烯蒸气的臭氧分解直接在AOT室中产生的真实二级有机气溶胶（SOA）进行AOT物理化学实验。观察到SOA作为壳从多种非极性到极性核相中相分离的倾向，并发现了盐水溶液中SOA颗粒的稳定乳化状态。微米级厚的SOA外壳不会阻止水或角鲨烷从内核到周围空气的获取或损失，这表明即使在干燥条件下，冷凝水的生长或分配也没有明显的扩散限制。这些实验形成了一个新框架的基础，该框架预测了在连续的大气氧化过程中，包含有机碳的复杂气溶胶的相分离形态将如何演变。氧化态的增加将迅速促使部分吞噬转变为核-壳形态，该化学形态具有显着不同的化学反应性，因为核相完全被壳所掩盖。如本文所述，AOT技术的实验能力的最新进展使新颖的实验方法能够提供对气溶胶微滴的化学性质和多维性质以及它们如何进化和响应连续化学反应的见解。