University of Helsinki: Nitric acid may influence the formation of aerosol particles in the upper troposphere

Aerosol particles have an impact on both human health and the climate. This impact on the climate is particularly notable in the upper troposphere (approximately 10 km above ground), as the particles serve as the nuclei for cloud droplets and ice crystals, which means that they can play a role in cloud formation. It was previously believed that the particles are formed in the upper troposphere primarily from sulphuric acid and water. The new study establishes that at cold temperatures, nitric acid, sulphuric acid and ammonia can form particles together.

– This mechanism generates particles up to 10–100 times faster than sulphuric acid and water. There are few other particle sources in the upper troposphere, so this new mechanism may be highly significant in terms of the amount of particles, says Katrianne Lehtipalo, Associate Professor at the Institute for Atmospheric and Earth System Research at the University of Helsinki and the Finnish Meteorological Institute.

Previously, nitric acid was thought to have no major role in the formation of aerosol particles, even though nitratesare often present in larger particles. The group’s previous research showed that in cold air, nitric acid may significantly promote the growth of particles, and, at temperatures under -15 degrees Celsius, even form particles together with ammonia.

– This is noteworthy, as the amounts of nitric acid and ammonia in the lower atmosphere are up to thousand times those of sulphuric acid, says Lehtipalo.

Nitric acid is generated also at higher altitudes in the atmosphere as a result of lightning, whereas ammonia, largely derived from agriculture, occasionally floats there from lower layers of the atmosphere. The amount of ammonia in the upper troposphere is still poorly understood.

The recent study was able to confirm a mechanism through which sulphuric acid, nitric acid and ammonia together participate in the formation of particles at the molecular level. It was also established that the resulting particles are effective ice nuclei. Modelling results suggest that this mechanism has a particular effect in the monsoon areas of Asia, where the ammonia rises higher thanks to convection, increasing the amount of particles in the upper troposphere. However, the resulting particles may be carried over long distances in the atmosphere, effecting the amount of particles in a large area.

The study was conducted by simulating the conditions of the upper troposphere in a laboratory. The CLOUD chamber at CERN makes it possible to study the formation and growth of aerosol particles with unprecedented precision. The chamber can be cooled down to temperatures below -50 degrees Celsius, and the Proton Synchrotron can be used to simulate cosmic radiation at different layers of the atmosphere. The Finnish participants in the study were the University of Helsinki’s Institute for Atmospheric and Earth System Research (INAR) and the Helsinki Institute of Physics (HIP), as well as the Finnish Meteorological Institute.

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