Abstract. 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 (3<no.S<13): 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.
The role of H<sub>2</sub>SO<sub>4</sub>-NH<sub>3</sub> anion clusters in ion-induced aerosol nucleation mechanisms in the boreal forest
