1Department of Physics, University of Helsinki, Helsinki, Finland
2Institute of Physics, University of Tartu, Tartu, Estonia
3Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland
4Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada
5Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
6Department of Applied Environmental Science and Bert Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
Abstract. The capability to accurately yet efficiently represent atmospheric nanoparticle growth by biogenic and anthropogenic secondary organics is a challenge for current atmospheric large-scale models. It is, however, crucial to predict nanoparticle growth accurately in order to reliably estimate the atmospheric cloud condensation nuclei (CCN) concentrations. In this work we introduce a~simple semi-empirical parameterization for sub-20 nm particle growth that distributes secondary organics to the nanoparticles according to their size and is therefore able to reproduce particle growth observed in the atmosphere. The parameterization includes particle growth by sulfuric acid, secondary organics from monoterpene oxidation (SORGMT) and an additional condensable non-monoterpene organics ("background"). The performance of the proposed parameterization was investigated using ambient data on particle growth rates in three size ranges (1.5–3 nm, 3–7 nm and 7–20 nm). The growth rate data was acquired from particle/air ion number size distribution measurements at six continental sites over Europe. The longest time series of 7 yr (2003 to 2009) was obtained from a boreal forest site in Hyytiälä, Finland, while about one year of data (2008–2009) was used for the other stations. The extensive ambient measurements made it possible to test how well the parameterization captures the seasonal cycle observed in sub-20 nm particle growth and to determine the weighing factors for distributing the SORGMT for different sized particles as well as the background mass flux (/concentration). Besides the monoterpene oxidation products, background organics with a concentration comparable to SORGMT, around 6 × 107 cm−3 (consistent with an additional global SOA yield of 100 Tg yr−1) was needed to reproduce the observed nanoparticle growth. Simulations with global models suggest that the "background" could be linked to secondary biogenic organics that are formed in the presence of anthropogenic pollution.