Quantifying the interplay of particle size, composition and phase separation: development of size-dependent aerosol thermodynamics and dynamics models for improved simulations of air quality and aerosol-cloud interactions

大气气溶胶是悬浮在空气中的微小的、看不见的颗粒。高质量浓度的气溶胶导致糟糕的空气质量--这是全球过早死亡的主要原因。在正在进行的新冠肺炎大流行期间，感染者呼出的气雾剂也因其在病毒传播中的潜在作用而获得了广泛的公众兴趣和负面报道。气溶胶也是地球气候系统中复杂的辐射强迫因素。它们是气候预测中最大的不确定性来源之一。在良好的湿度条件下，气溶胶粒子能否达到激活成云滴所需的临界尺寸，很大程度上取决于它的化学成分和大小。成功激活成云滴的临界干燥尺寸通常约为40至60 nm直径。颗粒的比表面积与体积比成为这个超细尺寸范围内的一个重要因素，构成了热力学平衡组成和相态的尺寸依赖关系。位于空气/颗粒界面的分子，即“表面”，与颗粒内部(块体)相比，与邻近分子的相互作用不同，这导致能量损失表现为表面张力或更一般的界面能。了解和量化尺寸对分子在颗粒体、不同液体之间的表面和潜在内部界面之间的分配的影响是至关重要的，因此这是本研究计划的目标。周围的气溶胶在化学上非常复杂，通常表现出所谓的液-液相分离，根据最近的发现，可能会有三个或更多的液体相分离。最近的高影响研究还表明，相分离可以对气溶胶成分和云的形成产生重要影响，但与尺寸效应对相变和颗粒特征有关的一些具有挑战性的方面仍然知之甚少。这项研究计划旨在利用我们在大气化学和物理领域的广泛专业知识，通过开发和应用独特的、高度先进的热力学和动力学模型来贡献原创性的科学研究。我们将重点模拟表面和界面对气溶胶颗粒组成、相分离和整个大气颗粒尺寸范围内吸水率的影响。我们将进一步在详细模式和简化模式之间架起桥梁；后者用于大规模空气质量和化学气候模式。拟议的研究将为麦吉尔大学的学生提供极好的培训机会，并有望产生高影响力的科学研究。该计划将建立在国家和国际合作的基础上，导致与加拿大社区的知识交流，并为全球的气溶胶和空气质量专家提供新的工具。

Atmospheric aerosols are microscopically tiny, invisible particles suspended in the air. High mass concentrations of aerosols lead to poor air quality - a leading cause of premature deaths globally. During the ongoing COVID-19 pandemic, aerosols from exhalation of infected people have also gained broad public interest and bad press due to their potential role in virus transmission. Aerosols are also complex radiative forcing agents in Earth's climate system. They constitute one of the largest sources of uncertainty in climate projections. Whether an aerosol particle can reach a critical size necessary to activate into a cloud droplet under favorable humidity conditions depends strongly on its chemical composition and size. The critical dry size for successful activation into a cloud droplet is usually around 40 to 60 nm diameter. The surface area to volume ratio of particles becomes an important factor in this ultrafine size range, constituting a size dependence of the thermodynamic equilibrium composition and phase state. Molecules situated at the air/particle interface, i.e. the "surface", experience differing interactions with neighboring molecules compared to the particle interior (bulk), which leads to an energetic penalty manifesting itself as surface tension or, more generally, interfacial energy. Understanding and quantifying size effects on the partitioning of molecules between the particle bulk, the surface and potential interior interfaces between different liquid phases, is critical - hence a goal of this research program. Ambient aerosols are chemically very complex, often exhibiting so called liquid-liquid phases separation and, according to recent findings, potentially three or more liquid phases. Recent high-impact studies have also shown that phase separation can be an important influence on aerosol composition and cloud formation, yet a number of challenging aspects related to size effects on phase transitions and particle characteristics remain poorly understood. This research program aims to contribute original scientific research with the development and application of unique, highly advanced thermodynamic and dynamic models, drawing from our extensive expertise in this field of atmospheric chemistry and physics. We will focus on modeling the effects of surfaces and interfaces on aerosol particle composition, phase separation and water uptake over the whole range of particle sizes of atmospheric interest. We will further build bridges between detailed and reduced-complexity models; the latter for use in large-scale air quality and chemistry climate models. The proposed research will offer excellent opportunities for the training of students at McGill University with the prospect of generating high-impact scientific studies. The program will build upon national and international collaborations, lead to knowledge exchange with the community in Canada, and provide new tools for aerosol and air quality experts around the globe.