An illustration of surface tension effect.
Jun 21 2017 Posted: 16:46 IST

An international team of US, Canadian, Italian, Finnish, French and Irish-based researchers, led by Professor Colin O’Dowd from NUI Galway’s School of Physics and Ryan Institute’s Centre for Climate and Air Pollution Studies, have conclusively shown that surfactants can significantly enhance cloud formation, ultimately increasing the cooling effect of clouds. 

It is the first time a team of researchers have confirmed this hypothesis under natural environmental conditions, and encapsulated it in a robust theoretical framework. The study was published this week in the globally prestigious scientific journal Nature.

Clouds and greenhouse gases act in tandem to balance the Earth’s energy budget thereby controlling climate. While greenhouse gases keep the heat within the earth system, whereby leading to warming, clouds reduce the amount of incoming energy into the system leading to cooling.

An increase in availability of cloud nuclei (typically in the form of airborne haze particles) leads to more droplets in the cloud, making it more reflective and longer-lived, thus increasing its cooling effect.  

Such enhancement in cloud nuclei abundance can occur through an increase in either their absolute profusion, or, their efficiency at forming droplets at lower water vapour humidities in the air. The most common and generally most efficient form of cloud nuclei found are water soluble inorganic salts (such as sea salt and sulphates)  however, if those were mixed or entirely made of organic compounds they would possess low water solubility and suppress the nuclei activity.

The game changes, however, if surfactants are present in the organic mix. Surfactants are 'wetting agents' that lower the surface tension of water. They are also called surface-active agents, a substance such as a detergent that, when added to a liquid, reduces its surface tension. Although the role of surfactants in promoting cloud droplet formation was proposed two decades ago, it has been disputed for almost as long, with one camp promoting a significant effect and the other camp claiming that the surface tension effect is cancelled by the simultaneous reduction in the solute, or Raoult, effect which is driven by the dissolution of the salt ions in the solution. Current theories simply find that these two effects counteract each other so suppression of droplet formation by less-soluble organics dominates.  

The international team pushed the experimental and theoretical boundaries of atmospheric science research to elucidate this phenomenon using state-of-the-art aerosol mass spectrometer in conjunction with the most advanced thermodynamic droplet model. In  simulating the cloud droplet activation process using mixed organic-inorganic nuclei, they revealed that surface tension can be lowered without triggering changes in the Raoult (solute) effect through a process known as liquid-liquid phase separation (essentially an organic-rich layer on the drop’s surface keeps the surfactants separated from the internal aqueous solution occupying the core of the droplet). The model was able to explain the tenfold increases in cloud droplet number concentration observed. They concluded that this phenomenon could be detected in many diverse environments throughout the world, reinforcing its role in cloud brightening and global climate cooling.

Dr Jurgita Ovadnevaite, scientist at the School of Physics at NUI Galway and lead author of the paper, said: “This study represents a major breakthrough in our understanding of cloud droplet formation from both an experimental and theoretical perspective. The next challenge is to scale up this nanometer scale finding to the global level through the incorporation of the surface tension effect into global climate models.”

Dr Darius Ceburnis, Mace Head Operations Manager at NUI Galway, added: “These advanced breakthroughs are only achievable through investment in continuous, realtime, and state-of-the-art measurements of Essential Climate Variables and Air Pollution at stations such as Mace Head, which is endorsed by the World Meteorological Organisation’s Global Atmosphere Watch programme, and is one of the most advanced stations of its kind in the world. Mace Head is strategically located in a remote area to monitor how dirty the cleanest air has become. The publication in the most prestigious journal globally, Nature, is a reward for such an investment and is the second one in as many years.”

The study was funded by the European Commission and the Environmental Protection Agency and the research was hosted at NUI Galway’s Climate and Air Pollution Research Facility at Mace Head in Carna, Co Galway, on the Galway-Atlantic coastline.

To read the full study in Nature visit http://www.nature.com/nature. 

For more information on Mace Head, visit: www.macehead.org.

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