The Atmospheric Significance of Thin Films on Aerosol Interfaces

Atmospheric aerosols are tiny solid or liquid particulates suspended in the atmosphere that can affect weather and climate by nucleating clouds and scattering sunlight. The presence of thin films at the interfaces of aerosol particles can alter their cloud-nucleating and light-scattering properties, ultimately changing their impact on the atmosphere.

The work presented here measures the thickness and oxidation of films at the air–water and mineral–water interfaces, evaluates whether the lifetimes of these films are sufficient to justify their inclusion in atmospheric models, and models the effect such films may have on the light scattering of individual particles and the top-of-atmosphere albedo.

Surfactant material extracted from several environments, as well as proxy material, was used to form thin interfacial films at air–water and mineral–water interfaces. These films were characterised predominantly using neutron reflectometry. Material extracted from different environments generally formed films with thicknesses of one to a few nanometres. The only exception was material extracted from woodsmoke, which formed thick, multilayered films at the air–water interface exceeding 20 nm. This is an unprecedented observation in reflectometry-based studies of atmospherically relevant films.

The capacity of oxidants to remove thin films from the measured interfaces was used to assess film lifetimes in the atmosphere. Gas-phase sulfur dioxide, and both gas- and aqueous-phase ozone, were not effective in removing films formed from environmentally extracted material. However, gas-phase and aqueous-phase hydroxyl radicals demonstrated some capacity to remove these films. Gas-phase hydroxyl radicals partially removed the films from the air–water interface, while aqueous-phase hydroxyl radicals completely removed them from the mineral–water interface.

Calculated half-lives of these films, based on their reaction with hydroxyl radicals, were estimated to be on the order of one hour for daytime hydroxyl radical concentrations and one day for nighttime concentrations. These lifetimes are significant relative to the physical lifetime of aerosol particles in the atmosphere and therefore justify the inclusion of such films in atmospheric aerosol models.

Mie light-scattering and one-dimensional atmospheric radiative transfer models were used to assess the radiative impact of thin interfacial films on aerosol particles. Films only one to a few nanometres thick at the air–water and mineral–water interfaces were generally found to decrease the single scattering albedo (indicating a warming effect) and increase the asymmetry parameter of core–shell aerosol particles (enhancing forward scattering of light).

The modified single scattering albedo and asymmetry parameter values were applied to aerosol particles within the planetary boundary layer in a one-dimensional radiative transfer model. The presence of thin interfacial films on atmospheric aerosol particles resulted in a decrease in top-of-atmosphere albedo of approximately 0.01 for wavelengths between 300–800 nm, with similar effects observed for films at both the air–water and mineral–water interfaces.

Overall, the measured thickness, lifetime, and radiative impact of thin films on aerosol interfaces demonstrate their atmospheric significance and provide strong justification for their inclusion in more complex, spatially and temporally resolved atmospheric aerosol chemistry models.