Water, salt, organics, and minerals: improved understanding of aerosol microphysics from a nanoscale basis

Abstract

Aerosols are liquid and solid particles suspended in the atmosphere that significantly impact human health, the environment, and the climate. However, current models used to characterize aerosols in air quality and climate studies are considerably uncertain, mainly due to a lack of understanding of aerosol microphysical processes, especially at the nanometer scale, including new particle formation, condensational growth, and activation. This thesis aims to reduce these uncertainties by investigating the impact of water, a crucial yet poorly understood constituent, on nano-aerosol microphysical properties using molecular dynamics (MD) simulations. Specifically, this thesis systematically examines the kinetic and energetic properties of sea salt, organic, and dust aerosols at the nanometer scale. This thesis also establishes a methodological framework for using MD simulations to examine the kinetic and energetic properties of nano-aerosol droplets with highly fluctuating interfaces and various morphologies. New results obtained in this study are used to identify limitations of classical theories (e.g., the Kelvin equation and Köhler theory) at sub-4 nm scales and develop parametric models of water properties in organic aerosols at different sizes and relative humidity (RH) conditions. One particularly notable conclusion is that the distinct properties of water in sub-4 nm scale clusters can enhance new particle formation of organic aerosols by 3 orders of magnitude at RH > 80 %. In the case of mineral dust aerosols, results obtained in the work provide key information required to differentiate mineral-water interactions by mineral type and parameterize the hygroscopic growth accordingly. Overall, the new fundamental insight on aerosol-water microphysics generated in this thesis can help reduce significant uncertainties in the abundance of aerosol cloud condensation nuclei and in the evolution and transport of mineral dust aerosols, thus enabling more accurate predictions of the impact of aerosols on Earth’s climate and on human health.