Dynamic Surface Properties of Atmospheric Aerosol and Resulting Climate Impacts

Theme: Basic processes

Start date: Cohort 1: 2019

Supervisors: Dr Bryan Bzdek (Bristol) and Dr Matthew Watson (Bristol)

The surface tension of atmospheric aerosols impacts their ability to serve as cloud droplet seeds and affect climate. This project will develop approaches to measure droplet surface tensions and better resolve dynamics at the particle surface, working closely with modellers.

Abstract

Atmospheric aerosols affect climate by direct scattering or absorption of solar radiation and indirectly, by serving as Cloud Condensation Nuclei (CCN) and forming cloud droplets. Atmospheric aerosols provide the largest negative radiative forcing, whilst remaining as the contribution with the largest uncertainty. The surface properties of atmospheric aerosol are crucial due to their high surface-to-volume ratios, whilst determining the fraction of atmospheric aerosol that may form cloud droplets. Most climate models still assume that activating CCN have a surface tension equivalent to pure water, but current experimental validation of this assumption is insufficient. Surfactants found in the atmosphere may depress the surface tension of droplets below that of water, as found in field measurements, whilst size-dependency of droplets is another contributing factor to their dynamic surface tension. The proposed research will investigate the dynamic surface tension and surface-bulk partitioning of surfactants within picolitre droplets using two experimental approaches, both used to evaluate damped oscillations observed in droplets. First, droplet coalescence using holographic optical tweezers will investigate surface tension of droplets at equilibrium surface composition and surface tension, considering size-dependency of their surface-bulk partitioning of surfactants. The second method will evaluate surface-bulk partitioning timescales for surfactants in droplets with fresh surface ages (sub-millisecond), whilst assessing the significance of the partitioning process on their dynamic surface tension. These experimental data will be shared with collaborators to modify monolayer partitioning models and thus, to improve climate modelling estimates, reducing uncertainty found in aerosol-related radiative forcing projections.