Novel approaches for quantifying the highly uncertain thermodynamics and kinetics of atmospheric gas-to-particle conversion

大气气溶胶粒子既可以是人为的，也可以是生物的，仍然是地球系统中的一个主要不确定因素：它们通过直接散射和吸收太阳辐射（直接影响）以及调节云的性质（间接影响）来影响气候。在区域范围内，气溶胶颗粒是导致空气质量恶化的主要污染物之一，其对两者的影响难以量化。要减少这些关键的不确定性，就需要准确了解这些悬浮在大气中的颗粒物的化学成分、浓度和大小。不幸的是，目前在预测气溶胶这些不断变化的化学和物理特性所需的许多基本参数中存在巨大的不确定性。这阻碍了我们最终了解其真正的环境影响。大气气溶胶颗粒物的很大一部分由有机材料组成（颗粒物质量的20-90%）。不幸的是，这一部分可能包含数千种基本上未经鉴定的化合物，这些化合物具有广泛的化学性质。这在本质上造成了上文所列的不确定性。由于气溶胶粒子存在于大气中，低挥发性有机化合物的凝结改变了凝结相有机材料的数量和组成，从而改变了它们对气候和健康的影响。这种凝结是高度动态的，目前，在将这种行为从单个颗粒协调到更宽尺度方面存在3个基本限制：1）通常将气溶胶颗粒视为由多种组分组成的简单液体。然而，越来越明显的是，大气颗粒物以粘性无定形状态存在，而不是简单的液体/固体混合物。然后，在这种非晶态中，气体和凝聚相之间的分配在动力学上受到限制。传统的气溶胶模型没有考虑到这一点。这给气体/颗粒质量传递的预测增加了显著的不确定性，因为混合时间尺度最终由气溶胶中的气溶胶成分的扩散系数决定，另一方面，扩散系数与颗粒物质的粘度有关。对于典型的气溶胶大小，混合的特征时间可以从几毫秒增加到几小时甚至几天！2)除扩散率和粘度外，每种气溶胶成分的平衡蒸气压主要由其纯成分饱和蒸气压决定，而饱和蒸气压又取决于化合物的分子特性。目前对有机组分的饱和蒸汽压知之甚少，特别是对挥发性最低的化合物。已知该参数的不确定性会在预测的气溶胶质量中引入4个数量级的不确定性！3)最后，为了评估这些现象在大气中的重要性，迫切需要将有机冷凝/蒸发作为一个动态过程并将气相传输耦合到冷凝相扩散的建模方法，尽管这些方法几乎不存在。目前，根本缺乏数据和建模工具来解决这些专题问题的重要性。虽然存在粘度、扩散率和蒸气压的预测技术，但它们是为化学工程目的开发的，尚未对大气科学进行评价。在这个提案中，我们的目标是对这里列出的属性进行新的和新颖的测量，以及评估/改进现有的模型。开发一个新的动力学模型，我们将评估这些属性的灵敏度在单粒子水平，并与实际测量的单粒子生长使用光镊实验进行比较。我们亦会发展简单的参数化计划，让大型模型可以评估对气候的敏感程度。

Atmospheric aerosol particles, which can be both anthropogenic and biogenic in origin, remain a major uncertainty in the Earth system: they impact the climate by directly scattering and absorbing solar radiation (the direct effect), as well as regulating the properties of clouds (the indirect effect). On regional scales, aerosol particles are among the main pollutants deteriorating air quality, their impacts on both poorly quantified. Reducing these critical uncertainties requires accurate knowledge on the chemical composition of these particles, their concentrations and size as they are suspended in the atmosphere. Unfortunately, there are currently huge uncertainties in many fundamental parameters that are required to predict these evolving chemical and physical characteristics of aerosols. This inhibits us from ultimately understanding their true environmental impacts. A significant fraction of atmospheric aerosol particles are comprised of organic material (20-90% of particle mass). Unfortunately, this fraction could comprise thousands of, largely unidentified, compounds with a wide range of chemical properties. This, in essence, creates the uncertainties listed above. The specifics of these uncertainties are now discussed.As aerosol particles reside in the atmosphere, condensation of low volatility organic compounds changes the amount and composition of condensed phase organic material, thus their climatic and health impacts. This condensation is highly dynamic and, presently, there are 3 fundamental restrictions in reconciling this behaviour from a single particle to wider scales:1) It is common to regard aerosol particles as a simple liquid comprised of multiple components. However, it is becoming increasingly evident that atmospheric particles exist as viscous amorphous states, rather than simple liquid/solid mixtures. Partitioning between the gas and condensed phase is then kinetically limited in such amorphous states. Traditional aerosol models do not account for this. This adds significant uncertainty to predictions of gas/particle mass transfer as mixing timescales are ultimately governed by the diffusion coefficients of the aerosol constituents in the aerosol, which, on the other hand, are connected to the viscosity of the particulate matter. For typical aerosol sizes, the characteristic time for mixing could increase from a few milliseconds to hours or even days! 2) In addition to diffusivity and viscosity, the equilibrium vapour pressure of each aerosol constituent is largely determined by its pure component saturation vapour pressure, which depends on the molecular properties of the compound. Saturation vapour pressures of organic components are currently poorly known, particularly for the least volatile compounds. The uncertainty in this parameter is already known to introduce 4 orders of magnitude of uncertainty in predicted aerosol mass!3) Finally, to assess the atmospheric importance of these phenomena, modelling approaches that treat the organic condensation/evaporation as a dynamic process and couple the gas phase transport to the condensed phase diffusion are urgently needed, although these remain almost non-existent. Presently, there is a fundamental lack of data and modelling tools to resolve the importance of these topical issues. Whilst predictive techniques for viscosity, diffusivity and vapour pressure exist, they are developed for chemical engineering purposes and remain unevaluated for atmospheric science. In this proposal we aim to make new and novel measurements of the properties listed here aswell as evaluating/improving existing models. Developing a new kinetic model we will assess the sensitivity to these properties at the single particle level and compare with actual measurements of single particle growth using optical tweezer experiments. We will also develop simple parameterised schemes so that large scale models can assess the wider sensitivity to the climate.