The biggest unknown factor in climate change is the way that aerosols (microscopic liquid and solid particles) interact with clouds, and how this interaction might feed back into climate change processes. Aerosols are thought to extend cloud lifetimes, while also making the clouds more reflective of incoming sunlight. However, the processes underlying this are not fully understood, and much scientific effort is currently being spent on teasing out the intricacies of these phenomena.

Further complicating the chemical and physical interactions of these particles is the fact that many aerosol particles, particularly microscopic water droplets, attract certain molecules specifically to their surface. Coating these particles in a thin film  can radically alter the way  they grow and coagulate into clouds, and how they interact with the rest of the atmosphere. These molecules, called surfactants, are key components e.g. in cleaning products because they are composed of “heads", which are attracted to water, and “tails", which are attracted to greasy material. The same feature that makes them so good at cleaning also predisposes them to accumulate on the surface of water droplets and form the films that influence the behaviour of those droplets.​

Research carried out at ISIS Neutron and Muon Source over the last few years has focused on a few particularly important surfactants such as fatty acids - familiar to most people as the basic components of everyday fats such as olive oil. Cooking meat releases a significant quantity of these weak organic acids into the atmosphere, and more is released via sea spray. ​

Atmospheric Chemistry's Engine

Recent work has focused on studying the way these surfactant films react with two of the three primary drivers of atmospheric chemistry: ozone molecules and nitrate radicals. These two oxidising gases are present in very small quantities in the troposphere, but their high reactivity means they are—together with hydroxyl radicals during daytime—the driving force behind most important chemical reactions in the lower atmosphere.​

The way the fatty acids under study​​​ react with these atmospherically important oxidants in the gas phase is fairly well understood. When fatty acids are arranged into a thin layer on the surface of a water droplet their molecules are presented in a particular manner to the atmospheric oxidant molecules that is distinct from the random arrangements of free molecules present in the gas phase.  As a result, studying the way these molecules react with atmospheric oxidants in the specific case of a thin film on water is of great importance for understanding both for how long the water droplets will be affected by the surfactants, and how the arrangement of the surfactants into surface films affects their behaviour and atmospheric impact.​

Augmenting Neutron Science with Infrared Spectroscopy

Neutron reflectometry has been used in the past to study these systems, but it can only track a single surfactant.  A recent technique development carried out at ISIS Neutron and Muon Source has pioneered the inclusion of an infrared light beam reflecting off the water surface at right-angles to the neutron beam. A bespoke reaction chamber and mirror system has been designed and built in order to introduce and contain oxidising gases while allowing both Infrared Spectroscopy (IR) and neutron beams to pass through simultaneously.​

The combination of these two techniques has allowed, for the first time, detailed study of the way these surfactants act in multi-surfactant configurations. The IR setup has been deployed on the INTER reflectometer and the additional information provided has allowed the research team to begin delving deeper into the more complex morphology and reactions of mixed films. The research team was led by Dr Skoda (ISIS Neutron and Muon Source) and  Dr Pfrang (University of Reading) with joint ISIS/NERC-funded PhD student Ben Thomas,  and their paper can be found here.

The additional information that IR spectroscopy provides has been crucial for the study of the mixed films. When the neutron signal is unable to detect one of the film components or to distinguish between the two species, the IR signal provides the molecular information necessary to obtain the full picture of the oxidation process.

For example, it initially appeared as if  the presence of an unreactive surfactant (stearic acid) slowed down the reaction of a reactive surfactant (oleic acid) with ozone. However, further investigation with the aid of IR data revealed that the presence of the unreactive surfactant disturbs the structure of the film and that, if this is not taken into account, neutron measurements of the reactive component would be affected. Therefore, it may be that the unreactive surfactant does not slow the reaction at all, once structural considerations are taken into account in analysis.

Further work on these systems is ongoing, and they form an ideal case study of the importance of both f tracking the component not measured by the neutron beam and f taking into account structural considerations when analysing neutron data. Both these forms of critical information can be provided by infrared spectroscopy, making it a key complementary technique for studying complex systems.

​Ben Thomas, Max Skoda and Christian Pfrang

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Further information

The research paper can be found here​. ​