The role of nanoparticles in Arctic cloud formation

Abstract. To constrain uncertainties in radiative forcings associated with aerosol--cloud interactions, improved understanding of Arctic cloud formation is required, yet long-term measurements of the relevant cloud and aerosol properties remain sparse. We present the first long-term study of cloud residuals, i.e. particles that were involved in cloud formation, and ambient aerosol particles in Arctic low-level clouds measured at Zeppelin Observatory, Svalbard. A detailed evaluation of the ground-based counter-flow virtual impactor inlet system is also presented. Cloud residuals as small as 15 nm are routinely observed especially during the dark period and are potentially linked to ice, supporting prior work suggesting that classical droplet activation is not the only relevant process in the formation of Arctic low-level clouds. The reported measurements and findings provide a new basis for improving our understanding of Arctic clouds and for developing robust parameterisations of mixed-phase clouds in Earth system models.