Drivers governing the seasonality of new particle formation in the Arctic

New particle formation (NPF) is the phenomenon wherein gaseous precursors form critical clusters of barely a few nanometres in diameter, after which, under favourable conditions, these particles can grow to climate-relevant sizes. Here we present measurements from 2022 to 2024 of particle and ion number size distributions from the Zeppelin Observatory (ZEP), an Arctic research station situated on the western edge of Svalbard atop a mountain. NPF events begin in April and continue occurring into November. The events at the start of the NPF season (i.e. April/May) are considerably stronger (i.e. a larger production of nucleation mode particles) compared with other months of the year. The peaks in NPF strength coincide with peaks in the accumulated solar insolation experienced by arriving air masses. During the summer period NPF events occur on 20 %–40 % of days each month; however, there is a consistent decline starting in June. We show that the combined influence of solar insolation and the surface area of pre-existing aerosols (i.e. condensation sink, CS) is a strong predictor for the likelihood of NPF. We develop a simplified predictive model which matches the frequency of NPF events identified via the classification schemes used in this study; we show that the ratio of solar insolation over CS corresponds well to the frequency of NPF events ( R 2 of 0.78). We show that NPF events occur during the polar night (i.e. when the sun does not pass above the horizon) and that these events are linked to high-altitude air masses. Furthermore, we detail the likely geographic origins of nucleation mode particles as measured at ZEP. We show that NPF events are considerably more likely to originate from marine regions towards the west of Svalbard, particularly the Greenland Sea, which is the marine region with the greatest likelihood of originating air masses linked to an NPF day. We also show that NPF events lead to an increase in the number of Aitken mode particles, indicating that potentially a significant proportion of the Aitken mode particles originate from NPF. Of the measured NPF events, 37 % exhibited nucleation mode particles that grew beyond 25 nm (a diameter representing the minimum activation diameter for particles to act as cloud condensation nuclei). Overall, we present a concise picture of the life cycle of nucleation mode particles in the Arctic, including the effect wet scavenging has in reducing the condensation sink, which in turn promotes the occurrence of NPF events.