In this paper, we investigate the participation of ion-induced nucleation in atmospheric new-particle formation. We present one year of Ion-DMPS data from the SMEAR II station in Hyytiälä, southern Finland (22 September 2005 to 22 September 2006). We measured continuously the concentrations of ions in ambient and charge equilibrated air in seven size bins over a diameter range 3–15 nm. All new-particle formation event days were classified according to observed particle charging states and analysed based on a new theoretical tool by which the measured charging states can be extrapolated down to smaller particle sizes. We investigated the contribution of ion-induced nucleation for both positively and negatively charged particles. The median contribution of ion-induced nucleation at 2 nm during the one year of measurements was around 6.4% with a median absolute deviation (MAD) of 2.0%,. The smallest contribution was 1.7% (MAD = 1.6%) whereas the maximum was 16.5% (MAD = 2.2%). We also analysed the data on a seasonal basis and found the largest contribution of ion-induced nucleation during summer (7.6%) and lower during the rest of the year (4.9%).