A gap in the clouds

While much of our climate is well characterised and modelled, there is something of a gaping hole in our understanding – the nuts and bolts of how a cloud actually begins to form. Now, an experiment based at an institution more known for smashing sub-atomic particles together promises a breakthrough in our understanding of these floating companions.    The cloud. A visual trace of the underlying convulsions driving the Earth’s weather systems, painted across the sky by the shifting balance of atmospheric temperature, pressure and water content. As a species we have pondered the existence of clouds for millennia – as scientists we have studied them for less than 200 years. And while ­in that time cloud physicists and climatologists have learnt an incredible amount of the physical processes that lead to the formation, growth and precipitation of clouds, there is, perhaps surprisingly, something of a gap in our understanding. There is a mystery hidden in their coalescence – we simply don’t know how they form. And it is a mystery that desperately needs a solution. Clouds represent the largest source of uncertainty in present climate models. According to the latest report of the Intergovernmental Panel on Climate Change much of this uncertainty surrounding clouds' effect on climate stems from the complexity of their formation. If we don’t know how they form say the IPCC, then we can’t reliably know when and where they will form and therefore how much they will influence climate. Cloud droplets form when water vapour in the atmosphere condenses onto tiny particles. These particles can be emitted directly from natural sources or human activity, but there is another, more mysterious, source.  Particles can form from precursors emitted originally as gaseous pollutants. These are the particles which literally generate from thin air, and it is the mechanism of this transformation of gas molecules into clusters and then into particles – a process called nucleation – which has proved doggedly problematic. And it really is a problem for atmospheric modelling when you consider more than half of the particles that seed cloud formation around the world today are formed by nucleation. There have been tantalising hints at this mysterious process however. Researchers have observed that the nucleation process nearly always involves sulfuric acid ­– but they also observe that sulfuric acid concentrations aren't high enough to explain the rate of new particle formation that occurs in the atmosphere. So there has to be something else at play here; an important factor as yet unfound. To find this factor, a rather special experiment was needed. An experiment which had to precisely reproduce the typical balance of atmospheric components whilst measuring the rate at which new, freshly nucleated, particles were created; all with extreme precision. [caption id="attachment_38753" align="alignright" width="200"] The CLOUD experiment at CERN[/caption] This experiment is CLOUD (Cosmics Leaving OUtdoor Droplets) – its task is to tease apart the complexity of cloud formation and it is based, perhaps fittingly, at CERN – an institution no stranger to the teasing apart of complexity. And with good reason; for there is another factor which the research team had to take into account which only the power of CERN’s particle colliders could realistically recreate – a shower of extraterrestrial rays. Galactic cosmic rays – the immensely high-energy radiation which originates mainly outside the Solar System – were thought to play an important role in the mechanism of cloud formation, and it was vital they were included in any experimental attempt to understand the process. And such experimental attention to detail is beginning to pay off. In a new paper published in Science, the CLOUD team claim they have found an indispensable ingredient to the long sought-after recipe of cloud formation – highly oxidised organic compounds. "Our measurements connect oxidised organics directly, and in detail, with the very first steps of new particle formation and growth,” explains Neil Donahue, CLOUD team member and professor of chemistry and chemical engineering at the Cernegie Mellon University in the US. "We had no idea a year ago that this chemistry was happening. There's a whole branch of oxidation chemistry that we didn't really understand. It's an exciting time." The organic compounds in question are the biogenic vapours emitted by trees. In fact, the atmosphere is chock-full of organic compounds – tiny liquid or solid particles that come from hundreds of sources including trees, volcanoes, cars, trucks and wood fires. Once they enter the atmosphere however, these organics start to change. In research published in the Proceedings of the National Academy of Sciences in 2012, Donahue and colleagues showed conclusively that organic molecules given off by pine trees, called alpha-pinene, are chemically transformed multiple times in the highly oxidising environment of the atmosphere. Other research has suggested that such oxidised organics might take part in nucleation – both in new particle formation and in their subsequent growth. These two strands of evidence now needed pulling together, and the CLOUD chamber was the ideal place to do so. The team filled the chamber with sulfur dioxide and pinnanediol (an oxidation product of alpha-pinene) and then generated hydroxyl radicals – the dominant oxidant in Earth's atmosphere. Then, using very high-resolution mass spectrometry, they watched the oxidation chemistry unfold. As predicted the two strands of evidence combined perfectly. Incredibly, the team was able to observe particles growing molecule by molecule. Single, gaseous molecules began to collide forming clusters of up to 10 molecules and, as Donahue notes, it seemed that sulfuric acid and biogenic aerosols made for ideal bedfellows. "It turns out that sulfuric acid and these oxidised organic compounds are unusually attracted to each other. This remarkably strong association may be a big part of why organics are really drawn to sulfuric acid under modern polluted conditions.” The findings represent a real breakthrough in our understanding of cloud formation. The team had found that oxidised biogenic vapors, sulphuric acid and galactic cosmic waves make for a heady atmospheric cocktail indeed. A mix of pollutant, natural emission and extraterrestrial energy source resulting in embryonic particles which can then grow to become the seeds on which cloud droplets form. And it was a breakthrough that was only possible due to the remarkable mix of expertise and equipment that makes up CERN. "The reason why it has taken so long to understand the vapours responsible for new particle formation in the atmosphere is that they are present in minute amounts near one molecule per trillion air molecules,” says CLOUD spokesperson and experimental particle physicist Jasper Kirkby. "Reaching this level of cleanliness and control in a laboratory experiment is at the limit of current technology, and CERN know-how has been crucial for CLOUD being the first experiment to achieve this performance." It may have taken a while to understand, but it does seem as if the complexity of cloud formation is now within reach. Undoubtedly impressive ­– but to really earn its keep this new evidence had to be incorporated into one of the all-important climate models. And more importantly the models had to respond and make reliable predictions. Armed with their new evidence, and no doubt a fair amount of nervous tension, the team fed their findings into a global particle formation model. They needn’t have worried. This fine-tuned model not only predicted nucleation rates more accurately but also predicted the increases and decreases of nucleation observed in field experiments over the course of a year, especially for measurements near forests. An embodiment of freedom, a symbol of our inclination to ponder – clouds play a unique and rather simple role in our imaginative worlds. Yet their role in the physical world is a more complex affair. From the delicate wisps of the cirrus to the towering edifices of the cumulous – some are tranquil, while others appear to threaten violence on the very fabric of the atmosphere in which they were formed. Now, thanks to the world’s most famous particle collider and the inspirational work of a band of researchers with their heads firmly in the clouds, we know the specifics of the formation of these floating enigmas, and as such have a better understanding of this incredibly complex role they play in our climate. Author Phil Prime is editor of Laboratory News.