Ammonia, which is often released from agricultural activities, can mix into the atmosphere and play an important role in air pollution. In Southeast Asia, the summer monsoon—a massive seasonal wind system—carries these ammonia-laden air masses high into the atmosphere. This journey takes the pollutants up to the upper troposphere and lower stratosphere, an area known as the UTLS. Once there, these air masses accumulate, creating conditions for new particle formation. This process contributes to what scientists call the Asian Tropopause Aerosol Layer, or ATAL, a region of tiny particles floating in the atmosphere from the East Mediterranean Sea to West China.

Although scientists know that ammonia and its reaction products can influence air pollution, we still don’t fully understand the ATAL. That’s why in this study we use advanced Earth system modelling to explore how ammonia influences the formation and growth of particles in this mysterious layer of the atmosphere.

In a groundbreaking aspect of this research, for the first time, the synergistic effects of ammonia combined with nitric acid and sulphuric acid under upper tropospheric conditions were explored. This synergy, recently discovered by experiments at the CERN CLOUD chamber, reveals that when these three chemicals interact in the cold upper atmosphere, they can significantly enhance particle formation. This finding adds important understanding of how ammonia contributes to the ATAL and highlights the intricate chemical processes that occur high above our heads.

Using an Earth system model, and incorporating the cutting-edge data from the CERN CLOUD chamber, the researchers investigated how ammonia impacts particle formation, especially during the day-night cycle. The findings were striking: during daylight hours, the rate of new particle formation in the atmosphere increases tenfold in the UTLS region during the South Asian monsoon. This dramatic increase is primarily due to deep convection—the powerful upward movement of warm air—that brings high ammonia levels from the ground up to the UTLS.

So, what does this mean for the environment? The study revealed that this surge in ammonia-driven particle formation leads to a significant rise—up to 80%—in the number of cloud condensation nuclei, the tiny particles around which clouds form. This increase in cloud condensation nuclei affects cloud properties, such as their ability to reflect sunlight, which has broader implications for Earth’s climate.

Interestingly, while ammonia significantly influences the composition of the ATAL, particularly by boosting nitrate levels, its overall effect on the mass of aerosols (tiny particles) in the ATAL is only small. However, it strongly increases the reflection of sunlight from the atmosphere, affecting the brightness of the sky and, consequently, the temperature at the Earth’s surface.

In summary, this study links laboratory experiments to the real atmosphere through modelling, highlighting the profound impact of ammonia on the ATAL. Understanding these processes is crucial as we continue to explore the intricate ways human activities affect our planet’s climate and air quality.

The lead author, as well as many contributors are early stage researches in the CLOUD-DOC Marie Curie Doctoral Training Network, a multi-site network of 12 PhD students at 12 partner institutions across Europe. The network investigates various aspects of the interactions of cosmic rays with aerosols and clouds. Besides the individual research of the PhD students at their hosting institutions, the major focus of the network are sets of common experiments on ion-induced nucleation and ion-aerosol interaction carried out at CERN. These experiments are conducted in an aerosol chamber exposed to a CERN elementary particle beam where the effects of cosmic rays on aerosol and cloud formation can be efficiently simulated.