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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 17 | Copyright
Atmos. Chem. Phys., 18, 13231-13243, 2018
https://doi.org/10.5194/acp-18-13231-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Sep 2018

Research article | 13 Sep 2018

The role of H2SO4-NH3 anion clusters in ion-induced aerosol nucleation mechanisms in the boreal forest

Chao Yan1, Lubna Dada1, Clémence Rose1, Tuija Jokinen1, Wei Nie1,2, Siegfried Schobesberger1,3, Heikki Junninen1,4, Katrianne Lehtipalo1, Nina Sarnela1, Ulla Makkonen5, Olga Garmash1, Yonghong Wang1, Qiaozhi Zha1, Pauli Paasonen1, Federico Bianchi1, Mikko Sipilä1, Mikael Ehn1, Tuukka Petäjä1,2, Veli-Matti Kerminen1, Douglas R. Worsnop1,6, and Markku Kulmala1,2,7 Chao Yan et al.
  • 1Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, P. O. Box 64, 00014, Helsinki, Finland
  • 2Joint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, Nanjing, 210046, China
  • 3Department of Applied Physics, University of Eastern Finland, 70211 Kuopio, Finland
  • 4Institute of Physics, University of Tartu, Ülikooli 18, 50090 Tartu, Estonia
  • 5Finnish Meteorological Institute, 00560 Helsinki, Finland
  • 6Aerodyne Research, Inc., Billerica, MA 01821, USA
  • 7Aerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China

Abstract. New particle formation (NPF) provides a large source of atmospheric aerosols, which affect the climate and human health. In recent chamber studies, ion-induced nucleation (IIN) has been discovered as an important pathway of forming particles; however, atmospheric investigation remains incomplete. For this study, we investigated the air anion compositions in the boreal forest in southern Finland for three consecutive springs, with a special focus on H2SO4-NH3 anion clusters. We found that the ratio between the concentrations of highly oxygenated organic molecules (HOMs) and H2SO4 controlled the appearance of H2SO4-NH3 clusters (3 < no. S < 13): all such clusters were observed when [HOM][H2SO4] was smaller than 30. The number of H2SO4 molecules in the largest observable cluster correlated with the probability of ion-induced nucleation (IIN) occurrence, which reached almost 100% when the largest observable cluster contained six or more H2SO4 molecules. During selected cases when the time evolution of H2SO4-NH3 clusters could be tracked, the calculated ion growth rates exhibited good agreement across measurement methods and cluster (particle) sizes. In these cases, H2SO4-NH3 clusters alone could explain ion growth up to 3nm (mobility diameter). IIN events also occurred in the absence of H2SO4-NH3, implying that other NPF mechanisms also prevail at this site, most likely involving HOMs. It seems that H2SO4 and HOMs both affect the occurrence of an IIN event, but their ratio ([HOMs][H2SO4]) defines the primary mechanism of the event. Since that ratio is strongly influenced by solar radiation and temperature, the IIN mechanism ought to vary depending on conditions and seasons.

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Ions can play an important role in atmospheric new particle formation by stabilizing the embryonic clusters. Such a process is called ion-induced nucleation (IIN). We found two distinct IIN mechanisms – driven by H2SO4-NH3 clusters and by organic vapors, respectively. The concentration ratio of organic vapors to H2SO4 regulates via which pathway the IIN occur. As the organic vapor concentration is influenced by temperature, a seasonal variation in the main IIN mechanism can be expected.
Ions can play an important role in atmospheric new particle formation by stabilizing the...
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