Atmospheric globe-trotters

Aerosols, suspended particles in the air, play a key role in the energy balance of the Earth’s atmosphere, primarily through their impact on cloud formation. In order to properly mitigate global warming, it is important to understand the processes that affect the atmospheric energy balance, so that the correct preventative measures can be taken.

New particles are formed in the atmosphere when supersaturated vapors nucleate. Once these particles grow large enough to form cloud droplets, they are known as cloud condensation nuclei. Half of global cloud condensation nuclei are formed by nucleation in the upper troposphere. Yet, until now, the reason why such a large proportion of cloud droplets were forming in this part of the atmosphere was a mystery. Although the low temperatures of the upper troposphere promote particle formation, concentrations of known nucleating species (e.g., sulfuric acid and ammonia) are too low to explain the high concentration of particles consistently observed here.

The CLOUD chamber, situated at the end of one of the beamlines of the CERN proton synchrotron, provides a means to study the processes underlying cloud formation, such as nucleation. An international collaboration of scientists, including the laboratory of atmospheric chemistry at PSI, have now discovered a new mechanism by which new particles in the atmosphere are formed. It involves a synergy between nitric acid, sulfuric acid, and ammonia. This is particularly interesting for understanding particles in the upper troposphere, since the rates of nucleation can be up to 1000 x higher than with other mechanisms. A previous publication in Nature from the CLOUD collaboration showed that at low temperatures ammonium nitrate salt can form rapidly on particles, and thus grow them to large sizes. Building on this knowledge, the collaboration could now go on to show that once particles are formed in the upper troposphere by nitric acid, sulfuric acid, and ammonia, the presence of nitric acid and ammonia allows them to grow rapidly to a size at which they condense into cloud droplets.

The source for this ammonia is manmade: typically fertilisers and livestock. The ammonia released is then convected high into the upper troposphere, where they can play their role in the particle formation process. With regions rich in ammonia and strong convective clouds, Asian monsoons provide the perfect conditions for this mechanism to occur. Models of global weather systems show that the particles, when formed in an Asian monsoon, can be transported globally – making it from Asia to North America in just 3 days, impacting the global climate.

“This particle formation mechanism gives a clear demonstration of how pollutants can rapidly affect the climate on the other side of the world, and why it is so crucially important to monitor them” explains Imad El Haddad, leader of the PSI team for whom understanding the role of pollution in particle formation and growth is a key focus area.

Text: Paul Scherrer Institute / Imad El Haddad & Miriam Arrell