The role of H2SO4-NH3 anion clusters in ion-induced aerosol nucleation mechanisms in the boreal forest
Résumé
New particle formation (NPF) provides a large source of atmospheric aerosols, which affect the climate and human health. In recent chamber studies, ion-induced nu-cleation (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 Fin-land for three 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 (3 < no.S < 13): 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 nucle-ation (IIN) occurrence, which reached almost 100 % when the largest observable cluster contained six 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 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 other NPF mechanisms also 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, the IIN mechanism ought to vary depending on conditions and seasons.
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