'Every cloud has a lipid lining'
Emily Mobley, Science Media Researcher for ISIS
Floating on ships in the Indian Ocean, researchers noticed that clouds drifting downwind of a city had different reflectivity to clouds which didn’t come downwind of a city. 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 and laser tweezers at the CLF 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.
The CLF’s optical tweezers system located in the Research Complex at Harwell can be used to suspend individual droplets, similar to those found in clouds, in the focus of a laser beam so that the surface chemistry of the droplet can be analysed using spectroscopy techniques. The droplets, which are doped with small levels of pollutant material, are subsequently exposed to a common atmospheric oxidant, such as ozone, in order to model atmospheric reaction processes. Changes in the chemistry of the droplet can then be measured during the reaction from analysis of the spectra. Determination of these properties provides a greater understanding of cloud droplet behaviour in the atmosphere and its impact on the climate.
Stephanie Jones
PhD student, CLF and Royal Holloway University of London
Image: 6 micron droplet levitating in the optical trap set up by two infra-red laser beams focussed through microscope lenses, shown in red. Levitating droplet is so small that it is only visible by scattering light from it's surface and is seen as a white dot in the centre of the sample cell. Click image for larger version.
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.
See here for the full story (link opens in a new window) on the ISIS website
Useful links:
Laser Tweezers Raman Study of Optically Trapped Aerosol Droplets of Seawater and Oleic Acid Reacting with Ozone: Implications for Cloud-Droplet Properties (PDF - link opens in a new window)DOI: 10.1021/ja044717o
Oxidation of oleic acid at the air–water interface and its potential effects on cloud critical supersaturations (PDF - link opens in a new window) DOI: 10.1039/b906517b
CLF news article on additional application of laser tweezers: CLF's laser tweezers to lead to faster breakthroughs in biological research (link opens in a new window)
