We have simulated upper tropospheric conditions in the CLOUD chamber and studied the formation and growth of new particles from novel mixtures of vapours. In the upper troposphere, particle formation is enhanced by the low air temperature but limited by the availability of suitable vapours. Nevertheless, newly-formed particles are persistently observed over almost all regions of Earth’s upper troposphere. However, the vapours and mechanisms that drive the formation of these particles are not understood.

We have found a new mechanism for extremely rapid particle formation and growth in the upper troposphere via an unexpected synergy between nitric acid, sulfuric acid and ammonia vapours. We have measured that these three vapours together form new particles 10– 1,000 times faster than sulfuric acid–ammonia nucleation alone, which has been previously shown by CLOUD to be rate-limited by the scarce sulfuric acid in the upper troposphere. Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to larger sizes where they can seed clouds. The resultant particles - even with trace amounts of sulfuric acid as low as 1% - are highly efficient ice nuclei, comparable to the most efficient ice nuclei known in the atmosphere.

Importance for Atmosphere and Climate

The new mechanism may be the dominant source of new particles in areas of the upper troposphere where ammonia is efficiently convected, such as over the Asian monsoon regions. Indeed, abundant ammonium nitrate particles have recently been reported in the Asian tropopause aerosol layer. Our global model simulation shows that particles formed from this synergistic mechanism can spread across the mid-latitude Northern Hemisphere, influencing Earth’s climate on an intercontinental scale. In the upper troposphere, nitric acid is abundant from lightning while ammonia originates from surface emissions – livestock and fertilizer – and is carried aloft by convective clouds and then released when droplets freeze. The atmospheric concentrations of all three vapours were much lower in the pre-industrial era, and each is likely to follow different trajectories under future air pollution controls. The new CLOUD results can inform policies for anthropogenic pollution regulations as well as improve the ability of global models to predict how the climate will change in future.

CERN CLOUD experiment

CLOUD is studying how new particles form in the atmosphere from trace gases, which then grow to modify clouds and climate. Using a particle beam from the CERN Proton Synchrotron, CLOUD is also investigating whether these processes are affected by ionisation from galactic cosmic rays. Atmospheric aerosol particles cool the climate by reflecting sunlight and by forming more numerous but smaller cloud droplets, making clouds brighter and more long-lasting. Accurate projections of climate change are limited by the uncertainty in how much aerosols and clouds have increased since pristine pre-industrial times and how they may continue to change in the future as anthropogenic emissions are reduced. Particle formation processes are especially important in the upper troposphere because they are the source of most of the seeds for global clouds. Especially important are rare particles known as ice nuclei which can turn supercooled liquid droplets into ice, dramatically increasing the transmission of visible light through the clouds and initiating precipitation.

Using CERN know-how, the CLOUD chamber has achieved much lower contaminants than previous experiments, allowing us to measure particle nucleation and growth from precisely controlled mixtures of vapours at atmospheric concentrations well below one molecule per trillion. A special feature of CLOUD is its ability to measure nucleation enhanced by ionisation from galactic cosmic rays between ground level and, using a CERN pion beam, the top of the troposphere - or with all the effects of ionisation completely suppressed by an internal electric field. During experimental campaigns at CERN, our team assembles an unparalleled array of state-of-the-art instruments to characterise the physical and chemical state of the particles and vapours in the CLOUD chamber, as well as the ability of the particles to nucleate ice. As with other experiments at CERN, CLOUD combines fundamental experiments and modelling - in our case, atmospheric processes and global climate - within a single team of international researchers.


Publication by the CLOUD collaboration: Wang, M. et al. Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation. Nature, doi:10.1038/s41586-022-04605-4 (2022).