Clouds play a key role in cooling the Earth's surface through dispersing rain and reflecting sunlight back into space. Through studying the role of secondary aerosol particles in cloud formation, EU-funded researchers hope to help sharpen future climate projections based on global models.
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When clouds form, water condenses around aerosol particles tiny liquid or solid particles suspended in the atmosphere which act as cloud condensation nuclei. Around half of the aerosol particles in the atmosphere form from condensable vapours that are present only in minute quantities.
The number of particles in the air can influence the properties of clouds: for example, the amount of sunlight scattered depends on the number of cloud droplets in a cloud. Changes in cloud structure can therefore have implications for our climate.
We wanted to find out which trace gases are involved in aerosol particle formation and how these trace gases arise, explains CLOUD-TRAIN project coordinator Joachim Curtius from the Goethe University Frankfurt, Germany. A key focus of our project was to determine the role of ions (electrically charged molecules) in this process.
Rebuilding climate models
Through use of the novel CLOUD research facility at CERN in Geneva, entirely new cloud-forming processes have been discovered. The CLOUD-TRAIN team found for example that new aerosol particles can develop from organic compounds, called terpenes, which are given off by plants, without any need for additional vapours such as sulphuric acid.
Here we were able to quantify the formation and growth processes, identify the role of ions for these processes, and understand these processes at the molecular level, says Curtius. The project also quantified the role of amines, which are derivatives of ammonia, in atmospheric particle formation processes.
By including these results in a global climate model, the CLOUD-TRAIN team has been able to discern which processes dominate aerosol formation, and in which region of the atmosphere.
We have also been able to more accurately predict the prevalence of aerosols in the atmosphere during pre-industrial times, says Curtius. We estimate that there were more aerosol particles than previously assumed in the past, and therefore less of a difference between present and pre-industrial particle concentrations.
These results have since been published in peer-reviewed journals such as Nature, Science and PNAS.
The project marks an important watershed in understanding aerosol nucleation and growth based on experimental data that has been effectively incorporated in a global climate model. The findings correspond with with observations.
We anticipate that various other climate models will include our experimental results and processes in the near future, says Curtius. This research will also contribute to better descriptions of shifts in regional and local climate due to strong shifts in regional pollution levels, which will be important in countries like China and India.
Precision research in Europe
The research involved using a pion beam from CERN to create ions in a chamber, simulating galactic cosmic rays in the atmosphere.
Several aspects of this research could only have been studied at the CLOUD facility, which is unique in the world, says Curtius. The facility enabled us to study aerosol formation and cloud formation under precisely controlled laboratory conditions. All parameters, such as exact temperature, particle concentration and cloud droplet properties, could be measured while processes were taking place.
This has several advantages. In the atmosphere, conditions such as temperature and gas concentrations are constantly changing, and processes are simply too complex to untangle. Because these fundamental processes are not well understood, aerosol formation is not well represented in models.
A comprehensive experimental investigation under laboratory conditions was therefore extremely beneficial, says Curtius.
A further key benefit was that all groups involved in the COULD-TRAIN training network cooperated to perform one joint experiment at CERN.
CLOUD-TRAIN was not merely a number of students each working at home in their individual labs and meeting just for summer school, but was truly a joint experimental effort involving excellent cooperation and exchange across institutions, says Curtius.