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Abstract. A significant fraction of atmospheric particles that serve as cloudcondensation nuclei (CCN) are thought to originate from the condensationalgrowth of new particle formation (NPF) from the gas phase. Here, 7 years ofcontinuous aerosol and meteorological measurements (June 2008 to May 2015)at a remote background site of the eastern Mediterranean were recorded andanalyzed to assess the impact of NPF (of 162 episodes identified) on CCN andcloud droplet number concentration (CDNC) formation in the region. A newmetric is introduced to quantitatively determine the initiation and durationof the influence of NPF on the CCN spectrum. NPF days were found to increaseCCN concentrations (from 0.10 % to 1.00 % supersaturation) between 29 %and 77 %. Enhanced CCN concentrations from NPF are mostly observed, asexpected, under low preexisting particle concentrations and occur in theafternoon, relatively later in the winter and autumn than in the summer.Potential impacts of NPF on cloud formation were quantified by introducingthe observed aerosol size distributions and chemical composition into anestablished cloud droplet parameterization. We find that thesupersaturations that develop are very low (ranging between 0.03 % and0.27 %) for typical boundary layer dynamics ( σw ∼0.3  m s −1 ) and NPF is found to enhance CDNC by a modest13 %. This considerable contrast between CCN and CDNC response is in partfrom the different supersaturation levels considered, but also becausesupersaturation drops from increasing CCN because of water vapor competitioneffects during the process of droplet formation. The low cloudsupersaturation further delays the appearance of NPF impacts on CDNC toclouds formed in the late evening and nighttime – which has importantimplications for the extent and types of indirect effects induced by NPFevents. An analysis based on CCN concentrations using prescribedsupersaturation can provide very different, even misleading, conclusions andshould therefore be avoided. The proposed approach here offers a simple, yethighly effective way for a more realistic impact assessment of NPF events oncloud formation.

Regional new particle formation as modulators of cloud condensation nuclei and cloud droplet number in the eastern Mediterranean