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Formation of nano-scale clusters from atmospheric vapors

Periodic Reporting for period 2 - nanoCAVa (Formation of nano-scale clusters from atmospheric vapors)

Reporting period: 2017-08-01 to 2018-07-31

Formation of new aerosol particles in the atmosphere occurs when vapour molecules collide with each other forming molecular clusters, and the conditions are favourable for these clusters to stay together and continue growing in the atmosphere. Approximately half of global cloud condensation nuclei result from new particle formation, so understanding this process is relevant for estimating the climate impact of aerosol particles.

The aim of this project is to characterize the sources and concentrations of atmospheric clusters and nano-particles, study their spatial and temporal variability in different environments, and to find out how and when the newly formed clusters grow into larger aerosol particles. A secondary objective is to provide training for the Experienced Researcher (ER) funded by the project in order for her to reach an independent academic research career.

We concluded that clusters have many pre-cursors in the atmosphere, including both biogenic and anthropogenic vapours, and thus their composition and concentration varies greatly between different environments. We found several new mechanisms that can form and grow clusters and aerosol particles. Overall, the action was thus very successful both scientifically, and also for enhancing the career prospects of the ER.
During the reporting period the ER have performed and analyzed both laboratory experiments and field measurements together with colleagues from Paul Scherrer Institute and University of Helsinki to characterize nano-particles in the atmosphere and processes leading to their formation.

We collected and reviewed field measurements of the concentration of 1-3 nm particles from 9 sites around the world. Further measurements were performed in an alpine environment at Jungfraujoch high-alpine research station in Switzerland and at SMEAR stations in Finnish boreal forest. Measurements were also made onboard a research icebreaker at Southern and Atlantic Oceans to study the aerosol concentrations in clean marine environment. The ER also participated in the analysis of other existing atmospheric data sets.

We performed laboratory experiments in the CLOUD chamber at CERN. By adjusting the chamber conditions (T, RH, ionization, UV-light) and trace gas concentrations (e.g. O3, SO2, NOx, VOC), we can simulate different kinds of environments and altitudes of the atmosphere. During 3 intensive campaigns we studied particle formation both from biogenic and anthropogenic organic vapours, together with inorganic pollutants like NOx, SO2 and NH3. The ER had a major role in planning and coordinating the experiments focused on simulating new particle formation in boreal forest conditions. We also finalized the analysis of several earlier experiments. Additional measurements were conducted to characterize and compare the nano-particle instrumentation used during the project.

The main results are summarized below:
from field observations we concluded that 1) atmospheric cluster formation is connected to photochemistry (Kontkanen et al. 2017, Jokinen et al. 2017) and 2) the concentrations are mainly driven by the availability of pre-cursor vapours rather than limited by the condensation sink (Kontkanen et al. 2016; 2017). 3) Sulphuric acid and industrial activities were identified as potential sources of atmospheric nano-particles (Sarnela et al. 2015, Kontkanen et al. 2016). 4) Ion clusters in the boreal forest can form both from pure biogenic precursors (Rose et al. 2018) or may consist of sulfuric-acid and ammonia (Yan et al. 2018).

Based on laboratory experiments we have: 4) revealed a new growth mechanism of nano-particles by cluster-cluster collisions (Lehtipalo et al. 2016) and 5) quantified the magnitude of a charge enhancement on nucleation the early growth process (Lehtipalo et al. 2016, Wagner et al. 2017). Furthermore, we have proven 6) that particle formation and early growth can happen solely from biogenic vapours (Kirkby et al. 2016, Tröstl et al. 2016) and 7) that growth rates exhibit a clear size dependency, explained by the Kelvin effect and the volatility distribution of the organic pre-cursor vapors (Tröstl et al. 2016, Stolzenburg et al. 2018).

The results have been and will be exploited in developing parametrizations, which can describe new particle formation and following CCN production in global aerosol models to constrain the climate impact of aerosols.
The project has clearly increased our understanding of clusters and nano-particles in the atmosphere and their formation on a molecular level.

During the project, the ER has contributed to 33 peer-reviewed scientific articles (+several more are submitted or in preparation). The ER was active in supervising MSc and PhD students, organizing a conference, participating in workshops, project meetings and conferences, giving altogether 17 presentations, peer-reviewing articles and doing science outreach.

These results have several wider implications. Many other groups worldwide have started measurements of sub-3 nm particles, further increasing the knowledge on their concentrations both in the atmosphere and also in other contexts, like indoor environments. Identifications of the new nucleation and growth mechanisms enable including them in global aerosol models, and thus refining the estimates of the radiative forcing due to aerosols. Gordon et al. (2016) already showed that including pure biogenic nucleation could lead to a reduction of 27% in estimates of anthropogenic aerosol radiative forcing.

Additionally, the project has had a remarkable impact on the academic career prospects of the ER. The mobility period allowed learning new methods and starting new collaborations. The returning phase was successful in integrating the ER back to Univ. Helsinki, where she started as associate professor after the completion of the project.

Gordon, H. et al. Reduced anthropogenic aerosol radiative forcing caused by biogenic new particle formation. PNAS, 2016
Jokinen, T. et al. Solar eclipse demonstrating the importance of photochemistry in new particle formation, Scientific Reports, 2017
Kirkby, J., et al. Ion-induced nucleation of pure biogenic particles. Nature, 2016
Kontkanen, J. et al. High concentrations of sub-3 nm clusters and frequent new particle formation observed in the Po Valley, Italy, during the PEGASOS 2012 campaign, Atmos. Chem. Phys., 2016
Kontkanen, J. et al. Measurements of sub-3202fnm particles using a particle size magnifier in different environments: from clean mountain top to polluted megacities, Atmos. Chem. Phys., 2017
Lehtipalo, K. et al. The effect of acid-base clustering and ions on the growth of atmospheric nano-particles. Nat. Commun., 2016
Rose, C. et al. Observations of biogenic ion-induced cluster formation in the atmosphere. Science Advances, 2018
Sarnela, N. et al. Sulphuric acid and aerosol particle production in the vicinity of an oil refinery, Atmos. Environ., 2015
Stolzenburg, D. et al. Rapid growth of organic aerosol nanoparticles over a wide tropospheric temperature range. PNAS, 2018
Tröstl, J. et al. Low-volatility organic compounds are key to initial particle growth in the atmosphere. Nature, 2016
Wagner R. et al. The role of ions in new particle formation in the CLOUD chamber, Atmos. Chem. Phys., 2017
Yan C. et al. The role of H2SO4-NH3 anion clusters in ion-induced aerosol nucleation mechanisms in the boreal forest, Atmos. Chem. Phys., 2018
Scematics of the cluster growth process in acid-base system.