The role of atmospheric aerosols in cloud formation is a topic of crucial importance for climate science. The difference between the sunlight absorbed by the Earth and the energy radiated back into space is called radiative forcing. How aerosol concentrations affect this radiative forcing is exceptionally complex, poorly understood, and challenging to measure.
Lipopolysaccharides (LPS) are large molecules, made up of a lipid and a polysaccharide, which have been observed in marine aerosols. Researchers from the universities of Birmingham and Reading used the neutron reflectometry instrument INTER to investigate a typical glycolipid, galactocerebroside (GCB), as a proxy for these more complex lipopolysaccharides. Their pioneering experiment was explicitly designed to study mixed organic monolayers at the air-water interface. Using neutron reflectometry simultaneously with Fourier transform infrared reflection absorption spectroscopy allowed for more sophisticated observations than were previously possible. They investigated the structure at the air–water interface using complementary Brewster angle microscopy.
Their work, published in Atmosphere, looked specifically at the oxidation of organic films by nitrate radicals (NO3•), the key atmospheric oxidant present at night. Their results suggest that aerosols produced from the sea-surface microlayer at night remain covered in surfactant molecules on timescales that are atmospherically relevant. Most surprisingly, the presence of salt (CaCl2) was found to substantially extend the lifetime of the, usually reactive, tails of the surfactants that cover the aerosols, offering a new explanation for their longer lifetimes in the atmosphere. The extended atmospheric lifetimes of the surfactants may reduce the surface tension of droplets and could affect transport across the air-water interface.
Related publication: “Night-Time Oxidation of a Monolayer Model for the Air–Water Interface of Marine Aerosols—A Study by Simultaneous Neutron Reflectometry and in Situ Infra-Red Reflection Absorption Spectroscopy (IRRAS)" Atmosphere 2018, 9(12), 471, DOI: 10.3390/atmos9120471
Authors: Ben Woden (University of Reading), Maximilian W. A. Skoda (ISIS), Matthew Hagreen (University of Reading) and Christian Pfrang (University of Birmingham).