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，它可以散射和吸收太阳光和地面红外辐射，影响辐射收支，对气候有直接影响。它们还充当水可以凝结的核，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冷凝形成二次有机气溶胶（SOA）。SOA的组成富含氧化的有机化合物，并且可以包含高分子量的有机分子。当大气干燥或寒冷时，SOA颗粒可以是高粘性的;事实上，已经表明SOA可以以玻璃状颗粒存在。因此，由水形成或由高粘性SOA形成的液滴在其粘度上可以相差超过15个数量级。云滴）具有低粘度、容易流动，并且在沉积时变形和扩散。当暴露于相对湿度和温度的变化时，它们可以快速响应环境的变化，失去或获得水分以及任何半挥发性或挥发性有机化合物。实质上，它们与周围的气相在组成上是平衡的。对于接近玻璃化转变的颗粒，颗粒不变形并且具有固体的机械性质。它们只能缓慢地对环境的变化作出反应，失去或获得水分、半挥发性和挥发性有机成分的速度非常缓慢。事实上，可以估计，这样的粒子原则上可能需要许多天才能平衡，这表明SOA可以以动力学停滞/受阻状态存在于大气中。预测气溶胶在大气中的性质和影响依赖于知道气溶胶质量是否处于热力学平衡或是否受到动力学限制，这对于理解大气中气溶胶的质量以及气溶胶形成液滴或冰晶的能力具有重要意义。在这个项目中，我们将使用单粒子测量，模型和模拟，以确定环境颗粒的粘度和水和有机成分在粘性气溶胶中的扩散动力学。测量将使用气溶胶光学镊子或电动天平中捕获的单个颗粒进行。光散射测量可以准确测定液滴大小和折射率，将用于检查颗粒对环境条件变化的响应。根据这些变化的时间依赖性，可以确定颗粒内分子的扩散。粘度可以通过聚结两个颗粒并确定复合颗粒的形状松弛成球形的时间尺度来直接测量。对简单和复杂成分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，改进后的模型将用于评估大气中粘性气溶胶的特性。最初，粘性气溶胶的作用将在一个详细的模型中进行评估的过程中发生的气溶胶室测量设计来模拟大气气溶胶。这将允许的准确性进行评估，与非平衡动力学限制的气溶胶过程可以被捕获和如何敏感的腔室测量是非平衡效应。最后，我们将利用一个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，我们将寻求更好地确定气溶胶何时可以被认为是处于平衡状态，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 andhaving 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.