It’s believed that air pollution affects how reflective a cloud is, as surfactant films may ‘shrink wrap’ the water droplets, preventing them from growing to form rain droplets or shrinking to evaporate. A team from Royal Holloway are using neutrons at ISIS to look at the rates of different chemical reactions which remove these surfactant films, in order to pin down the chemistry which is important in the atmosphere.
Formation of a cloud requires a large mass of air moving upwards, which cools. As it cools, water vapour condenses to form droplets, and every droplet must form on a particle of matter, or an ‘ick’ in the atmosphere. Some of these particles are covered in surfactant films. Like the grease skittering across the surface of a greasy pan in your sink when washing up liquid is added, surfactants act as an oily lipid layer found on cloud droplets, and it is these surfactant films which are thought in part to control not only the size of the water droplets in the cloud, but whether the cloud rains, its reflectivity and how long it lives. The whole cloud dynamic can be changed by what’s in the cloud droplets.
However, the surfactant films and their control over cloud dynamics are not long lived. Anything that is released into the atmosphere is oxidised – the atmosphere acts like a dilute fuel, low temperature combustion system. The lipid layer on the cloud droplet will oxidise with ozone naturally found in the atmosphere, which frees the cloud droplet of its layer, liberating it to either take on extra water or shrink and evaporate.
Dr Martin King and his team from Royal Holloway, University of London are conducting complementary studies between lasers at the Central Laser Facility (CLF) and neutron scattering at ISIS to map out cloud chemistry. Holding a water droplet in the focus of a laser, they can oxidise it and follow the size change very easily, and using neutrons at ISIS they can measure the rate of loss of the film by monitoring the reflectivity of organic lipid films during chemical reactions.
Using a Teflon bath filled with water and a one molecule thick lipid layer, simulating the surfactant film surrounding a water droplet, the team scattered neutrons off the film using the SURF instrument. The reflectivity of the film is related to the amount of lipid at the interface. By introducing an oxidant found in the atmosphere above the water and film, the team could replicate the oxidation of the lipid layer, and so the stripping away of the surfactant film in a way which may happen on a cloud.
The experimental set-up on ISIS instrument, SURF. The image shows the Teflon bath with water and a lipid layer being oxidised by ozone, which is made by splitting atmospheric oxygen with UV light. View full-size image
Unlike many users at ISIS who use neutrons to define atomic structure and interactions between molecules, the Royal Holloway team are interested in the rate of reactions, i.e. how quickly the film disappears. This fundamental piece of knowledge can help determine which chemistry is important in the atmosphere. If the lipid film’s lifetime is under three days, which is a characteristic time for particles to live in the atmosphere, the reaction will occur in the atmosphere. However, if the reaction takes 10 days, or even a year, it is not a relevant piece of cloud chemistry. This way, the team can identify fundamental atmospheric chemistry, which can be used in a cloud model.
“Neutrons are perfect – because what you can do is reflect neutrons off a sample, which allows us to build up a depth profile” commented Martin.
“You can also play a ‘trick’ with hydrogen and deuterium, which are chemically the same, but have a different number of neutrons. Using selective deuteration, we can make the water disappear so it is effectively invisible, and then we can deuterate the chemical we're interested in so it has a very high reflectivity. Where neutrons are absolutely brilliant is we can play these tricks and make things appear very strong signals and make other things disappear. We are very fortunate in that we can make the water droplet disappear and the film highly reflective, which is an advantage over any lab technique.”
Unexpectedly, the team have found that some lipid films are quite resilient to change – there are unreactive lipids which are showing resistance to oxidation, so would theoretically have a longer lifetime in the atmosphere. This discovery, along with many other oxidants to study and the scientific unknown of cloud and aerosol effects, gives Martin and his team several reasons to return to ISIS in the future.
Martin King et al.
Research date: June 2013
Oxidation of oleic acid at the air–water interface and its potential effects on cloud critical supersaturations
Laser Tweezers Raman Study of Optically Trapped Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud-Droplet Properties
Oxidation of biogenic and water-soluble compounds in aqueous and organic aerosol droplets by ozone: a kinetic and product analysis approach using laser Raman tweezers
Interaction of nitrogen dioxide (NO2) with a monolayer of oleic acid at the air-water interface - A simple proxy for atmospheric aerosol