Factors influencing the contribution of ion-induced nucleation in a boreal forest, Finland

We present the longest series of measurements so far (2 years and 7 months) made with an Ion-DMPS at the SMEAR II measurement station in Hyytiälä, Southern Finland. We show that the classification of overcharged (implying some participation of ion-induced nucleation) and undercharged (implying no or very little participation of ion-induced nucleation) days based on Ion-DMPS measurements agrees with the fraction of ion-induced nucleation based on NAIS measurements. We analyzed the influence of different parameters on the contribution of ion-induced nucleation to the total particle formation rate. We found that the fraction of ion-induced nucleation is typically higher on warmer, drier and sunnier days compared to colder days with less solar radiation and a higher relative humidity. Also, we observed that bigger concentrations of new particles were produced on days with a smaller fraction of ion-induced nucleation. Moreover, sulfuric acid concentrations were smaller for days with a bigger fraction of ion-induced nucleation. Finally, we propose tentative explanations on how these different parameters influence the different nucleation mechanisms, and show that the different mechanisms seem to take place at the same time during an event. The relative contribution of the different mechanisms seems to vary depending on the surrounding conditions.