Direct measurement of bidirectional turbulent fluxes of atmospheric cluster ions

Abstract Atmospheric ions can play a role in atmospheric processes such as new particle formation. Turbulent flux measurements can help to quantify source and sink processes of atmospheric ions. In this study, we present for the first time eddy covariance flux measurements of the vertical turbulent transport of atmospheric cluster ions before, during and after new particle formation events above a forest canopy in Central Europe in the summer of 2022. Typical positive cluster ion concentrations ranged from approximately 200 to 400 cm −3 , and negative cluster ion concentrations ranged from 20 to 200 cm −3 . Interestingly, the turbulent fluxes of positive and negative cluster ions were bidirectional: The net turbulent flux of negative cluster ions was mostly positive (median value: +4.8 × 10 6 m −2 ·s −1 ), that is, directed from the surface to the atmosphere, while the net turbulent flux of positive cluster ions was mostly negative (median value: −4.2 × 10 6 m −2 ·s −1 ), that is, directed from the atmosphere to the surface. For negative cluster ions, the median vertical transport velocity in the atmospheric electric field was v el = −1.5 cm·s −1 , less than 25% of the median turbulent transfer velocity v tur = −6.6 cm·s −1 . For positive cluster ions, v el = +1.4 cm·s −1 and v tur = +1.3 cm·s −1 were almost equal. Subtracting the parameterized deposition flux from the measured net turbulent flux of cluster ions yields a positive ion source flux for both positive and negative cluster ions. Overall, direct eddy covariance flux measurements of cluster ions provide interesting insights into ion dynamics and bear the potential for further application in future studies.