title: Comparing simulated and experimental molecular cluster distributions.
abstract: Formation of secondary atmospheric aerosol particles starts with gas phase molecules forming small molecular clusters. High-resolution mass spectrometry enables the detection and chemical characterization of electrically charged clusters from the molecular scale upward, whereas the experimental detection of electrically neutral clusters, especially as a chemical composition measurement, down to 1 nm in diameter and beyond still remains challenging. In this work we simulated a set of both electrically neutral and charged small molecular clusters, consisting of sulfuric acid and ammonia molecules, with a dynamic collision and evaporation model. Collision frequencies between the clusters were calculated according to classical kinetics, and evaporation rates were derived from first principles quantum chemical calculations with no fitting parameters. We found a good agreement between the modeled steady-state concentrations of negative cluster ions and experimental results measured with the state-of-the-art Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF) in the CLOUD chamber experiments at CERN. The model can be used to interpret experimental results and give information on neutral clusters that cannot be directly measured.
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland. tinja.olenius@helsinki.fi
authors.forename: Tinja
authors.initials: T
authors.lastname: Olenius
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Siegfried
authors.initials: S
authors.lastname: Schobesberger
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Oona
authors.initials: O
authors.lastname: Kupiainen-Määttä
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Franchin
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Heikki
authors.initials: H
authors.lastname: Junninen
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Ismael K
authors.initials: IK
authors.lastname: Ortega
authors.affiliation: Department of Chemistry, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Theo
authors.initials: T
authors.lastname: Kurtén
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Ville
authors.initials: V
authors.lastname: Loukonen
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Worsnop
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Markku
authors.initials: M
authors.lastname: Kulmala
authors.affiliation: Department of Physics, University of Helsinki, FIN-00014 Helsinki, Finland.
authors.forename: Hanna
authors.initials: H
authors.lastname: Vehkamäki