Chemistry and microphysics of ice in the upper troposphere and lower stratosphere

The properties and action of clouds in the atmosphere are of central importance for our understanding and prediction of climate change. This project contributes to the solution of key questions in current atmospheric research concerning ice clouds in the upper troposphere and lower stratosphere (UT/LS). The overall aim is to better understand the formation, microphysics and chemistry of ice in the UT/LS.

The project focuses on an improved description of cloud processes – a prerequisite for successful prediction of future climate change. Specific goals include to:

1. develop a molecular level description of water interactions with ice and aerosol particles in the UT/LS,
2. characterize key chemical interactions between ice particles and gas components including nitric acid, sulphuric acid and organic compounds,
3. develop a fundamental understanding of heterogeneous freezing of cloud droplets including contact and evaporation freezing.

A unifying theme to achieve these goals is the need for a detailed understanding of ice surface processes on the microscopic level. We are using a unique molecular beam apparatus to study the interaction of atoms and molecules with ice. Molecular beam experiments provide detailed information about the dynamics and kinetics of surface processes including sticking, desorption, diffusion and surface reactions. The method is also used to study the structure and morphology of the surface. Molecular beam experiments have traditionally been carried out under ultra-high vacuum. The present setup is, however, unique in allowing experiments at higher pressure, and the technique may be termed “environmental” molecular beam studies in analogy with environmental electron microscopy that is carried out at elevated pressures. We also employ large-scale molecular dynamics simulations to further improve the molecular-level understanding of ice processes. Particle levitation techniques are used to study freezing processes in single micrometer-sized water droplets and key processes are parameterized for use in atmospheric models.

The project is funded by The Swedish Research Council (VR).