Iodine Drives New Particle Formation in the Central Arctic Ocean

The Arctic is warming faster than Earth on average (Arctic amplification) and the extent of the sea ice coverage has decreased dramatically over the past decades, especially in summer. However, the underlying processes behind this amplification are not well understood because of many feedback mechanisms between the ocean, the atmosphere and the cryosphere. Clouds are an important player in the Arctic radiative budget, as they almost always have a net surface warming effect. However, models struggle to reproduce Arctic clouds partly because aerosol processes and aerosol sources in this area are poorly understood. This is particularly true in the central Arctic Ocean where aerosols can reach extremely low concentrations and clouds may even become limited by cloud condensation nuclei (CCN) availability. New particle formation (NPF) has been identified as a potential source of CCN in this region, and a modelling study showed that NPF is needed to explain the observed Aitken mode particle concentration. However, no conclusive results were reported concerning the nature of these NPF events and the chemical composition of the nucleating vapours. In this contribution we present the first comprehensive physicochemical characterization of several NPF events in the central Arctic Ocean during the Arctic Ocean 2018 expedition. We identified a marked increase of the iodic acid concentration towards the end of the summer, potentially associated with the onset of the freeze-up season and a concurrent increase of the ozone concentration. Moreover, our results indicate that iodic acid is a main driver for the formation of new particles and their early growth. The frequency of new particle formation events follows the same seasonal trend as the ultrafine particle concentration, which is one order of magnitude higher in fall compared to summer. In addition, we developed a simple parametrization that describes iodic acid variability, providing an estimate of the iodine precursor emission rates. Finally, we also provide direct evidences that Aitken mode particles activate as cloud condensation nuclei suggesting that iodine NPF can influence the formation of clouds and their properties in the central Arctic.