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

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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.