Atmos. Chem. Phys. Discuss., 10, 11795-11850, 2010
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On the roles of sulphuric acid and low-volatility organic vapours in the initial steps of atmospheric new particle formation
P. Paasonen1, T. Nieminen1, E. Asmi2, H. E. Manninen1, T. Petäjä1, C. Plass-Dülmer3, H. Flentje3, W. Birmili4, A. Wiedensohler4, U. Hõrrak5, A. Metzger6, A. Hamed7, A. Laaksonen2,7, M. C. Facchini8, V.-M. Kerminen1,2, and M. Kulmala1,9
1University of Helsinki, Helsinki, Finland
2Finnish Meteorology Institute, Helsinki, Finland
3Meteorological Observatory of Hohenpeissenberg, Hohenpeissenberg, Germany
4Institute for Tropospheric Research, Leipzig, Germany
5University of Tartu, Tartu, Estonia
6Ionicon Analytik GmbH, Innsbruck, Austria
7University of Eastern Finland, Kuopio, Finland
8Institute Atmospheric Sciences and Climate, Bologna, Italy
9Stockholm University, Stockholm, Sweden

Abstract. Sulphuric acid and organic vapours have been identified as the key components in the ubiquitous secondary new particle formation in the atmosphere. In order to assess their relative contribution and spatial variability, we analyzed altogether 36 new particle formation events observed at four European measurement sites during EUCAARI campaigns in 2007–2009. We tested models of several different nucleation mechanisms coupling the formation rate of neutral particles (J) with the concentration of sulphuric acid ([H2SO4]) or low-volatility organic vapours ([org]) condensing on sub-4 nm particles, or with a combination of both concentrations. Furthermore, we determined the related nucleation coefficients connecting the neutral nucleation rate J with the vapour concentrations in each mechanism. The main goal of the study was to identify the mechanism of new particle formation and subsequent growth that minimizes the difference between the modeled and measured nucleation rates. At three out of four measurement sites – Hyytiälä (Finland), Melpitz (Germany) and San Pietro Capofiume (Italy) – the nucleation rate was closely connected to squared sulphuric acid concentration, whereas in Hohenpeissenberg (Germany) the low-volatility organic vapours were observed to be dominant. However, the nucleation rate at the sulphuric acid dominant sites could not be described with sulphuric acid concentration and a single value of the nucleation coefficient, as K in J=K [H2SO4]2, but the coefficients seemed to vary between the sites. This inter-site variation was substantially smaller when the heteromolecular homogenous nucleation between H2SO4 and organic vapours was assumed to take place in addition to homogenous nucleation of H2SO4 alone, i.e. J=KSA1 [H2SO4]2+KSA2 [H2SO4][org]. By adding in this equation a term describing homomolecular organic vapour nucleation, Ks3 [org]2, equally good results were achieved. In general, our results suggest that organic vapours do play a role, not only in the condensational growth of the particles, but as well in the nucleation process, with a site specific degree.

Citation: Paasonen, P., Nieminen, T., Asmi, E., Manninen, H. E., Petäjä, T., Plass-Dülmer, C., Flentje, H., Birmili, W., Wiedensohler, A., Hõrrak, U., Metzger, A., Hamed, A., Laaksonen, A., Facchini, M. C., Kerminen, V.-M., and Kulmala, M.: On the roles of sulphuric acid and low-volatility organic vapours in the initial steps of atmospheric new particle formation, Atmos. Chem. Phys. Discuss., 10, 11795-11850, doi:10.5194/acpd-10-11795-2010, 2010.
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