The role of H 2 SO 4 -NH 3 anion clusters in ion-induced aerosol nucleation mechanisms in the boreal forest

New particle formation (NPF) provides a large source of atmospheric aerosols, which affect the climate and human health. Ion-induce nucleation (IIN) has been discovered as an important pathway of forming particles within recent chamber studies, however, atmospheric investigation remains incomplete. For this study, we investigated the air anion compositions in the boreal forest in Southern Finland for 3 consecutive springs, with a special focus on H 2 SO 4 -NH 3 anion clusters. We found that the ratio between the concentrations of highly oxygenated organic molecules (HOMs) and H 2 SO 4 controlled the appearance of H 2 SO 4 -NH 3 clusters (#S > 3): All such clusters were observed when [HOM] / [H 2 SO 4 ] was smaller than 30. The number of H 2 SO 4 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 6 or more H 2 SO 4 molecules. During selected cases when the time evolution of H 2 SO 4 -NH 3 clusters could be tracked, the calculated ion growth rates exhibited a good agreement across measurement methods and cluster (particle) sizes. In these cases, H 2 SO 4 -NH 3 clusters alone could explain ion growth up to 3 nm (mobility diameter). IIN events also occurred in the absence of H 2 SO 4 -NH 3 , implying that also other NPF mechanisms prevail at this site, most likely involving HOMs. It seems that H 2 SO 4 and HOMs both affect the occurrence of an IIN event, but their ratio ([HOMs] / [H 2 SO 4 ]) defines the primary mechanism of the event. Since that ratio is strongly influenced by solar radiation and temperature, IIN mechanism ought to vary depending on conditions and seasons.