New particle formation in boreal forests of Siberia, Finland and Estonia

New particle formation (NPF) is an important atmospheric process where secondary aerosol particles are formed from gas phase precursors. We analyzed the NPF events that were observed in 2016–2018 at three boreal forest stations: the Fonovaya station in Siberia, Russia; the SMEAR II station in Central Finland; and the SMEAR Estonia station in Estonia. NPF events were found to occur less often in Siberia as compared with the European sites. In general, NPF occurred more frequently during daytime under clear sky conditions or in the presence of optically thin clouds, while NPF events were rather rare when the incoming solar radiation was low. Another factor influencing NPF in Siberia is linked to the availability of low-volatility vapors and their precursors. Particularly, a larger number of NPF events was detected under high SO2 concentrations, indicating the importance of sulfuric acid formed from SO2. The concentration of SO2 at the Siberian station is about an order of magnitude higher compared with the two other measurement sites. The concentration of NOx is also higher at Fonovaya station. During the NPF event days at Fonovaya, the wind was mostly from south-south-west (SSW) and south — the directions associated with the large industrial city Novosibirsk and industrial areas of eastern Kazakhstan. These areas are presumably the main source of anthropogenic trace gases measured at the Fonovaya station. In contrast to the European sites, relative humidity at Fonovaya did not differ significantly between the NPF and non-event days. The condensation sink was higher on non-event days at the European measurement sites but not always at Fonovaya. Air mass back trajectories showed that, oppositely to the European sites, there are no particular sectors around the Siberian site favoring NPF. Our results suggest that solar radiation and subsequent atmospheric chemistry govern the NPF processes in Siberia.