What is new particle formation?

New particle formation describes a certain process where tiny particles are formed out of vapour molecules in the atmosphere. Those newly formed particles – or molecular clusters with sizes around 1-3 nanometers – can grow to sizes of cloud condensation nuclei (> 50 nanometers) and influence the optical properties, lifetime and precipitation of clouds, thereby connecting new particle formation to Earth’s radiative balance and climate. Approximately half of all cloud condensation nuclei in the air are formed through this process. Since these particles in the air have massive influence on climate it is crucial to understand their formation processes in different atmospheric environments in detail. This is our main research challenge, and this is why we carry our detector complexes around the globe.

Little clusters are formed in the atmosphere

In the gas-phase, little clusters form through reactions of different vapours. Compounds such as sulphuric acid and ammonia can in proper conditions stick to each other and form the initial seeds of new particles. However, there are likely several mechanisms via which new clusters are formed in the different parts of the Earths atmosphere. These mechanisms are only vaguely understood. Recently, we succeeded to resolve the cluster formation mechanism in the coastal environment where iodic acid was found the be responsible on cluster formation.

Tiny clusters become a particle

To have any climatic relevance, tiny clusters need to grow to sizes above some 50 nanometers. Vapours like sulphuric acid or extremely low volatile organic compounds are needed to ensure this growth, otherwise the little clusters would just “stick” to coarse particles in the atmosphere and “disappear”. Still not all vapours could have been identified through measurement campaigns. In our group, we investigate the new particle formation in different areas and climate zones around the globe to detect all gases involved.

With understanding the new particle formation, we are able to understand also the influence of human activities on the process. Our experimental results can be used to improve the predictive power of e.g. global climate models.

The Chemistry

Besides understanding the mechanisms of cluster and new particle formation, it is also crucial to understand the chemical reactions leading to production of the cluster / particle precursor vapours. Hydroxyl radical (OH) is an important oxidant, but our recent research have pointed out some previously disregarded or unknown chemical reaction pathways crucial for understanding the precursor formation. These include the reaction of so called stabilised Criegee intermediates with sulphur dioxide leading to sulphuric acid production as well as so called auto-oxidation reaction, producing extremely low volatile organic compounds. Still, yet unknown mechanisms are needed to explain e.g. the atmospheric formation of iodic acid.

Instrument Development

Why resolving the chemistry and physics related to cluster formation is so difficult? Why we still have open questions? It’s because the concentrations of particle precursor vapours and clusters are so small that it is challenging to analyse. Sulphuric acid, for example can be important for particle formation at the concentrations of 1 ppq (part per quadrillion). It means, that out of 10^15 (1 million billions) molecules of atmospheric air only 1 molecule would be sulphuric acid. Imagine a human hair. It’s diameter, ~100 micrometers, is ~1 ppq of the distance between the earth and sun. So it’s hard! And harder it gets when we want to detect clusters and their chemical composition. Their concentrations are namely still lower.

If the detector technology would have been on required level, all our research questions would have been answered already. But that has not been the case. Because of the challenge that the extremely low concentrations of clusters and precursor vapours create, one of the cornerstones of our research is continuous instrument and method development toward increasing sensitivity and identification capability. Some of the detection technologies we have been working with have also been commercialised via spin-off companies. Airmodus Ltd., for example, manufactures and distributes chemical ionisation sources while Karsa Ltd. develops and manufactures mass spectrometer systems for aviation security applications. Both technologies have emerged from the basic research we have been conducting within last decade.

Quantitative high-quality data

In order to study the precursors and growth of the particles we have to have quantitative methods to study them. Whereas to understand the trends and changes happening in the atmosphere we have to have long-term measurements of the atmospheric vapors and particles. ACTRIS CiGas-UHEL is a topical center hosted by University of Helsinki with a key-mission  to offer operational support for continuous long-term measurements of condensable vapours and aerosol precursors such as sulphuric acid and oxidized organic compounds in the atmosphere. The core activity of CiGas-UHEL is to ensure sustainable and traceable high-quality data and data products of in-situ measured atmospheric reactive trace gases with known uncertainty. That is accomplished by:

  • development, testing and implementing advanced measurement technologies and data evaluation algorithms,

  • testing prototypes of gas analytical devices

  • enhancing the competence of the operative personnel by training.