Two studies published in Nature this week outline exciting breakthroughs in our understanding of how aerosol particles are formed in the atmosphere, and overturn the long held belief that sulphur-containing molecules are needed to form new particles.
The research comes from the EU-funded CLOUD consortium which includes University of Leeds academic Professor Ken Carslaw and PhD student Kamalika Sengupta.
Professor Carslaw said “The CERN CLOUD experiment is uniquely capable of measuring how individual clusters of molecules form and grow into particles that eventually affect clouds and climate. We have shown that particles can form just from the organic molecules emitted by natural vegetation. This means that sulphur-containing pollutants are not needed.”
Piers Forster outlines the exciting breakthrough:
“We know that mankind’s sulphur dioxide emissions from coal burning form small aerosol particles in the atmosphere, which in turn grow to act as cloud seeds. This in turn has made clouds brighter and more reflective, which has had a cooling effect on the Earth since the industrial revolution. This cooling effect is potentially large. Historically, nearly as large as the warming effect from carbon dioxide but it is also very uncertain, one of the largest uncertainties in climate science.
“We know little about the way these aerosol particles are formed and grow. Previous work has suggested that man-made sulphur particles were nearly always involved in particle formation, suggesting that the pre-industrial atmosphere would often have been very clean with few cloud seeds compared to today. This is what most climate models also assume and thus they typically model large changes in particles since pre-industrial times, which produce correspondingly large increases in cloud brightness and a large cooling effect.
“This work is something of a game changer as it revises what happens in pristine pre-industrial environments. It shows that organic molecules can form and grow particles independently of sulphur. This has been measured in laboratory studies at CERN and also observed in clean air on the Jungfrau1. These results mean that there may well have been more cloud seeds available in the past than we thought, potentially lessening their increase over the last century. When these new results are worked through into our simulations we may find a smaller aerosol-cloud cooling effect over the last century.
“If the change in the aerosol-cloud effect is significant it is good news for climate change. Firstly, it may mean that the Earth is less sensitive to greenhouse gas changes than we previously thought. Secondly, it may mean that cleaning up sulphur dioxide emissions may not lead to such a large drop in cloud reflectance and corresponding warming that some feared might happen. There is still a lot of research that needs doing to test these theories though.”
Professor Carslaw adds that “It’s going to be very challenging to observe such natural aerosol formation in today’s polluted atmosphere. But hopefully these results will encourage scientists to understand the natural atmosphere, which is so important to our understanding of how human activities affect the climate”.
The CLOUD Marie Curie Initial Training Network is a multi-site network of eight PhD students and two post-docs at nine partner institutions across Europe. The network investigates various aspects of the interactions of cosmic rays with aerosols and clouds, which bears on the possibility of a “solar indirect” contribution to climate change. Besides the individual research of the PhD students and post-docs at their hosting institutions, the major focus of the network is common experiments on aerosol nucleation carried out at CERN. These experiments are conducted at an aerosol chamber that is exposed to a CERN elementary particle beam where the effects of cosmic rays on aerosol and cloud formation can be efficiently simulated.
1) A brief CLOUD tour, available at: https://cds.cern.ch/record/2154271?
2) A description of the Nature results, which includes animations: https://cds.cern.ch/record/2155289?
Kirkby, J. et al. Ion-induced nucleation of pure biogenic particles. Nature, doi 10.1038/nature17953 (2016).
Tröstl, J. et al. The role of low-volatility organic compounds in initial particle growth in the atmosphere. Nature, doi 10.1038/nature18271 (2016).