Our understanding of atmospheric fine particles, aerosols, is gradually growing. Published in Nature, a new study provides novel insight into the interaction of aerosols and cloud droplets. The study showed that reduced surface tension caused by aerosols acting as cloud condensation nuclei enhances cloud formation.
Photo: Eija Vallinheimo
In order to explain global climate change, increasingly accurate understanding of aerosols and their interaction with clouds is needed. Even today, the role of aerosols continues to be the most important uncertainty factor relating to climate change. Unlike greenhouse gases, aerosols cool down the climate, but estimates of their cooling effect remain rather inaccurate. Aerosols are known to form cloud droplets, and clouds are known to affect the radiation balance of the atmosphere. For years, scientists have been keen to uncover factors contributing to the formation of cloud droplets. The new study published in Nature now takes this line of research one step further.
Each cloud droplet contains at least one aerosol particle. In other words, cloud droplets need a condensation nucleus.
"For us, the most important thing has been to identify which aerosol particles eventually become cloud droplets and which don't," says Professor Ari Laaksonen from the University of Eastern Finland and the Finnish Meteorological Institute.
"Very small particles are usually not very effective condensation nuclei due to the high surface tension of water. However, the situation changes if these particles contain compounds that reduce surface tension sufficiently."
An article published in Nature in 1999 observed that when a large amount of mist water was collected in a container, organic compounds found in water were effective in reducing the water's surface tension, which led scientists to conclude that the reduced surface tension also affected the formation of mist droplets. However, Laaksonen and his colleagues noticed that the findings contained a shortcoming which the authors of the article hadn't taken into consideration.
"Compounds that reduce surface tension – surfactants – are of course effective on the water surface. If water in a laboratory container is broken down into tiny droplets, the water's surface area grows significantly. Consequently, the concentration of organic compounds on the water surface becomes so low that it may no longer have much effect on the water's surface tension," Laaksonen says.
"According to theoretical calculations, however, certain kinds of "super" surfactants could affect the formation of cloud droplets, but up until now, scientists hadn't been able to find such compounds among atmospheric aerosols."
Now, however, scientists have discovered aerosols that are very effective cloud nuclei. These aerosols were found in measurements carried out for the new study in the North Atlantic. The aerosols contained sulphate and organic compounds, and their effects on cloud formation could not be explained by anything else than reduced surface tension. Although the organic molecules within the aerosols could not be identified precisely, the measurements provided sufficient data for modelling of the phenomenon. This made it possible to conclude that due to droplet phase separation, the droplet is divided into two parts, and only the surface of the droplet has organic compounds on it. This is enough to reduce the surface tension, making it easier for the condensation nucleus to grow into a cloud droplet.
"We have confirmed that in some cases, reduced surface tension can help droplets turn into cloud droplets. This means that the number of cloud droplets that form can be significantly higher than predicted by current climate models. Next, we'll study whether this phenomenon exists globally and how this new information affects climate change modelling."
Research Professor Ari Laaksonen, tel. +358 40 513 7900, ari.laaksonen(at)fmi.fi
Surface tension prevails over solute effect in organic-influenced cloud droplet activation. Jurgita Ovadnevaite, Andreas Zuend, Ari Laaksonen, Kevin J. Sanchez, Greg Roberts, Darius Ceburnis, Stefano Decesari, Matteo Rinaldi, Natasha Hodas, Maria Cristina Facchini, John H. Seinfeld & Colin O' Dowd Nature (2017) doi:10.1038/nature22806