Diffusion and Equilibration in Viscous Atmospheric Aerosol

气溶胶是大气的重要组成部分。气溶胶被定义为分散在气相中的固体颗粒或液滴，它可以散射和吸收阳光和地面红外辐射，影响辐射收支，对气候有直接影响。它们还可以作为水凝结的核，导致云滴的形成，间接影响气候。除了有许多天然来源外，它们还可以在污染环境中由半挥发性有机化合物凝结形成二次有机气溶胶（SOA）形成。SOA的组成中含有丰富的氧化有机化合物，可以含有高分子量的有机分子。当大气干燥或寒冷时，SOA颗粒可能具有高度粘性；事实上，已经证明SOA可以以玻璃粒子的形式存在。因此，由水形成的液滴或由高粘性SOA形成的液滴的粘度差异可以超过15个数量级。大部分是水的气溶胶液滴(例如：云滴)粘度低，易于流动，沉积时变形和扩散。当暴露于相对湿度和温度的变化时，它们可以对环境的变化做出快速反应，失去或获得水以及任何半挥发性或挥发性有机化合物。实际上，它们与周围的气相在组成上处于平衡状态。对于接近玻璃化转变的颗粒，颗粒不变形，具有固体的机械性能。它们对环境变化的反应很慢，失去或获得水、半挥发性和挥发性有机成分的速度很慢。实际上，可以估计，这样的粒子原则上可能需要许多天才能达到平衡，这表明SOA可以在大气中以动力学阻滞/阻碍状态存在。预测大气中气溶胶的性质和影响依赖于了解气溶胶质量是否处于热力学平衡或是否受到动力学限制，这对了解大气中气溶胶的质量以及气溶胶形成液态云滴或冰晶的能力具有重要意义。在这个项目中，我们将使用单颗粒测量、模型和模拟的组合来表征环境颗粒的粘度以及粘性气溶胶中水和有机成分的扩散动力学。测量将使用气溶胶光学镊子或电动天平捕获的单个粒子。光散射测量允许精确测定液滴大小和折射率，将用于检查粒子对环境条件变化的响应。根据这些变化的时间依赖性，可以确定分子在粒子内的扩散。粘度可以直接测量，通过合并两个粒子，并确定复合粒子的形状松弛到一个球体的时间尺度。对组成简单和复杂的粒子的测量将用于改进气溶胶粘度和分子扩散常数的模型。在最后阶段，将使用改进的模型来评估大气中粘性气溶胶的特性。首先，粘性气溶胶的作用将在设计用于模拟大气气溶胶的气溶胶室测量中发生的过程的详细模型中进行评估。这将有助于评估捕获非平衡动力学限制气溶胶过程的准确性，以及室测量对非平衡效应的敏感性。最后，将使用更大尺度的区域模式研究大气气溶胶对非平衡效应的敏感性。总之，我们将设法更好地确定气溶胶何时可以被认为处于平衡状态，何时在大气中受到动力学限制。

Aerosols are a key component of the atmosphere. Defined as either solid particles or liquid droplets dispersed in the gas phase, aerosols can scatter and absorb sunlight and terrestrial infrared radiation influencing the radiation budget and having a direct effect on climate. They also act as nuclei on which water can condense, leading to the formation of cloud droplets, indirectly influencing the climate. As well as having many natural sources, they can form in polluted environments from the condensation of semi-volatile organic compounds forming secondary organic aerosol (SOA). The composition of SOA is rich in oxidised organic compounds and can contain organic molecules of high molecular weight. When the atmosphere is dry or cold, SOA particles can be highly viscous; indeed, it has been shown that SOA can exist as glassy particles. As such, droplets formed from water or formed from highly viscous SOA can differ in their viscosity by more than 15 orders of magnitude. Aerosol droplets that are largely water (eg. cloud droplets) have low viscosity, flow readily, and deform and spread when deposited. When exposed to changes in relative humidity and temperature, they can respond quickly to the change in the environment, losing or gaining water and also any semi-volatile or volatile organic compounds. They are, in essence, at equilibrium in composition with the surrounding gas phase. For particles approaching the glass transition, the particles do not deform and have the mechanical properties of a solid. They can only respond slowly to changes in the environment, losing or gaining water, semivolatile and volatile organic components only very slowly. Indeed, it can be estimated that such particles could in principle take many days to equilibrate and suggesting that SOA can exist in a kinetically arrested/hindered state in the atmosphere. Predicting the properties and impacts of aerosol in the atmosphere relies on knowing if the aerosol mass is in thermodynamic equilibrium or if it is kinetically limited, with significant consequences for understanding even the mass of aerosol in the atmosphere and the ability of the aerosol to form liquid cloud droplets or ice crystals. In this project, we will use a combination of single particle measurements, models and simulations to characterise the viscosity of ambient particles and the diffusion kinetics of water and organic components within viscous aerosol. Measurements will be made using individual particles captured in aerosol optical tweezers or in an electrodynamic balance. Light scattering measurements that allow the accurate determination of droplet size and refractive index will be used to examine the response of the particle to changes in environmental conditions. From the time-dependence of these changes, the diffusion of molecules within the particle can be determined. The viscosity can be measured directly by coalescing two particles and determining the timescale for the shape of the composite particle to relax to a sphere. Measurements of particles of simple and complex composition will be used to refine models of aerosol viscosity and molecular diffusion constants. In a final stage, the refined models will be used to assess the properties of viscous aerosol in the atmosphere. Initially, the role of viscous aerosol will be evaluated in a detailed model of the processes occurring in aerosol chamber measurements designed to simulate atmospheric aerosol. This will allow an assessment of the accuracy with which non-equilibrium kinetically limited aerosol processes can be captured and how sensitive the chamber measurements are to non-equilibrium effects. Finally, the sensitivity of atmospheric aerosol to non-equilibrium effects will be investigated using a wider scale regional model. In summary, we will seek to better define when aerosol can be considered to be at equilibrium and when kinetically limited in the atmosphere.