Drivers governing the seasonality of new particle formation in the Arctic

Abstract. 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. 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). The peaks in NPF strength coincide with peaks in the 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 in June. We show that the combined influence of solar radiation and the surface area of pre-existing aerosols (i.e. condensation sink, CS) are strong predictors 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 NPF events occur during the polar night (i.e. when the Sun does not pass above horizon), and speculate that these events are linked to high altitude air masses. Furthermore, we detail the likely geographic origins of nucleation within the Arctic, as measured at ZEP. We show that NPF events are considerably more likely to originate from the marine regions towards the west of Svalbard, particularly the Greenland Sea which presented the greatest likelihood that arriving air masses from this marine region would be linked to an NPF day. We also remark on the proportion of the Aitken mode particles within the Arctic that could originate from NPF; we show that NPF events lead to an increase in the number of Aitken mode particles. We measure over 50 NPF events where the nucleation mode particles 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 lifecycle of nucleation mode particles in the Arctic, including the effect wet scavenging has in reducing the condensation sink, which in turn encourages NPF events to occur.