title: Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3.
abstract: Homogeneous nucleation and subsequent cluster growth leads to the formation of new aerosol particles in the atmosphere. The nucleation of sulfuric acid and organic vapours is thought to be responsible for the formation of new particles over continents, whereas iodine oxide vapours have been implicated in particle formation over coastal regions. The molecular clustering pathways that are involved in atmospheric particle formation have been elucidated in controlled laboratory studies of chemically simple systems, but direct molecular-level observations of nucleation in atmospheric field conditions that involve sulfuric acid, organic or iodine oxide vapours have yet to be reported. Here we present field data from Mace Head, Ireland, and supporting data from northern Greenland and Queen Maud Land, Antarctica, that enable us to identify the molecular steps involved in new particle formation in an iodine-rich, coastal atmospheric environment. We find that the formation and initial growth process is almost exclusively driven by iodine oxoacids and iodine oxide vapours, with average oxygen-to-iodine ratios of 2.4 found in the clusters. On the basis of this high ratio, together with the high concentrations of iodic acid (HIO3) observed, we suggest that cluster formation primarily proceeds by sequential addition of HIO3, followed by intracluster restructuring to I2O5and recycling of water either in the atmosphere or on dehydration. Our study provides ambient atmospheric molecular-level observations of nucleation, supporting the previously suggested role of iodine-containing species in the formation of new aerosol particles, and identifies the key nucleating compound.
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Mikko
authors: M
authors: Sipilä
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Nina
authors: N
authors: Sarnela
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Tuija
authors: T
authors: Jokinen
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Henning
authors: H
authors: Henschel
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Heikki
authors: H
authors: Junninen
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Jenni
authors: J
authors: Kontkanen
authors: Leibniz-Institute for Tropospheric Research (TROPOS), Leipzig, Germany.
authors: Stefanie
authors: S
authors: Richters
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Juha
authors: J
authors: Kangasluoma
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Alessandro
authors: A
authors: Franchin
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Otso
authors: O
authors: Peräkylä
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Matti P
authors: MP
authors: Rissanen
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Mikael
authors: M
authors: Ehn
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Hanna
authors: H
authors: Vehkamäki
authors: Department of Chemistry, University of Helsinki, Helsinki, Finland.
authors: Theo
authors: T
authors: Kurten
authors: Leibniz-Institute for Tropospheric Research (TROPOS), Leipzig, Germany.
authors: Torsten
authors: T
authors: Berndt
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Tuukka
authors: T
authors: Petäjä
authors: Department of Physics, University of Helsinki, Helsinki, Finland.|Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.|Aerodyne Research Inc., Billerica, Massachusetts 01821, USA.|Finnish Meteorological Institute, Helsinki, Finland.
authors: Douglas
authors: D
authors: Worsnop
authors: School of Physics and Centre for Climate &Air Pollution Studies, Ryan Institute, National University of Ireland Galway, University Road, Galway, Ireland.
authors: Darius
authors: D
authors: Ceburnis
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Veli-Matti
authors: VM
authors: Kerminen
authors: Department of Physics, University of Helsinki, Helsinki, Finland.
authors: Markku
authors: M
authors: Kulmala
authors: School of Physics and Centre for Climate &Air Pollution Studies, Ryan Institute, National University of Ireland Galway, University Road, Galway, Ireland.
authors: Colin
authors: C
authors: O'Dowd