New particle formation (NPF) provides a large source of atmospheric aerosols, which affect the climate and human health. In recent chamber studies, ion-induced nucleation (IIN) has been discovered as an important pathway of forming particles; however, atmospheric investigation remains incomplete. For this study, we investigated the air anion compositions in the boreal forest in southern Finland for three consecutive springs, with a special focus on H₂SO₄-NH₃ anion clusters. We found that the ratio between the concentrations of highly oxygenated organic molecules (HOMs) and H₂SO₄ controlled the appearance of H₂SO₄-NH₃ clusters (): all such clusters were observed when [HOM] ∕ [H₂SO₄] was smaller than 30. The number of H₂SO₄ molecules in the largest observable cluster correlated with the probability of ion-induced nucleation (IIN) occurrence, which reached almost 100 % when the largest observable cluster contained six or more H₂SO₄ molecules. During selected cases when the time evolution of H₂SO₄-NH₃ clusters could be tracked, the calculated ion growth rates exhibited good agreement across measurement methods and cluster (particle) sizes. In these cases, H₂SO₄-NH₃ clusters alone could explain ion growth up to 3 nm (mobility diameter). IIN events also occurred in the absence of H₂SO₄-NH₃, implying that other NPF mechanisms also prevail at this site, most likely involving HOMs. It seems that H₂SO₄ and HOMs both affect the occurrence of an IIN event, but their ratio ([HOMs] ∕ [H₂SO₄]) defines the primary mechanism of the event. Since that ratio is strongly influenced by solar radiation and temperature, the IIN mechanism ought to vary depending on conditions and seasons.