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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Dec 2018

Research article | 11 Dec 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Regional New Particle Formation as Modulators of Cloud Condensation Nuclei and Cloud Droplet Number in the Eastern Mediterranean

Panayiotis Kalkavouras1,2, Aikaterini Bougiatioti1,2, Nikos Kalivitis1, Maria Tombrou3, Athanasios Nenes2,4,5, and Nikolaos Mihalopoulos1,2 Panayiotis Kalkavouras et al.
  • 1Env. Chemical Processes Lab., Dept. of Chemistry, Univ. of Crete, Heraklion, 71003, Greece
  • 2Institute of Environmental Research & Sustainable Development, National Observatory of Athens, Palea Penteli, 15236, Greece
  • 3Department of Physics, Univ. of Athens, Athens, 15784, Greece
  • 4Laboratory of Atmospheric Processes and their Impacts, Schools of Architecture, Civil & Environmental Engineering, École Polytechnique Fédérale de Lausanne, 1015, Lausanne, Switzerland
  • 5Institute for Chemical Engineering Science, Foundation for Research and Technology Hellas, Patras, 26504, Greece

Abstract. A significant fraction of atmospheric particles that serve as cloud condensation nuclei (CCN), and furthermore as cloud droplets are thought to originate from the condensational growth of new particles formed from the gas phase. Here, particle number size distributions (< 850 nm), aerosol chemical composition and meteorological parameters were studied during 7 years of continuous measurements (June 2008 to May 2015) at a remote background site of the eastern Mediterranean. 162 NPF episodes were recorded and analyzed to assess the impact of NPF on CCN and cloud droplet number concentration (CDNC) formation. A new metric is introduced to quantitatively determine the initiation and duration of the influence of NPF on the CCN spectrum. Annually, NPF days were found to increase CCN concentrations between 40 and 50 % in the 0.2–1.0 % supersaturation range. CCN perturbations from NPF are found to occur in the afternoon, relatively later in the winter and autumn than in the summer. Introducing the observed aerosol size distributions together with chemical composition into an established cloud droplet parameterization showed that the supersaturations that develop however are much lower (below 0.1 %) for typical boundary layer dynamics (width of the vertical velocity distribution ~ 0.3 m s−1) and NPF is found to enhance CDNC by 7 to 12.5 %. This considerable contrast between CCN and CDNC response is in part from the different supersaturation levels considered, but also because supersaturation drops from increasing CCN because of water vapor competition effects. The low cloud supersaturation further delays the appearance of NPF impacts on CDNC to clouds formed in the late evening and nighttime – which carries important implications for the extend and types of indirect effects induced by NPF events. An analysis based on CCN concentrations using prescribed supersaturation can provide much different, and even misleading, conclusions and should be avoided. The proposed approach here offers a simple, yet highly effective way for a more realistic impact assessment of NPF events on cloud formation.

Panayiotis Kalkavouras et al.
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Panayiotis Kalkavouras et al.
Panayiotis Kalkavouras et al.
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Short summary
We study how new particle formation (NPF) events affect clouds throughout the year at a ground site in the E.Mediterranean. Using a new set of tools and evaluation metrics, NPF is found to affect only evening and nocturnal clouds by modestly increasing droplet number by 7 to 12 %. A conventional analysis based on CCN concentration at prescribed supersaturation levels (or aerosol size) can considerably bias the perceived influence of NPF events on regional clouds, hydrological cycle and climate.
We study how new particle formation (NPF) events affect clouds throughout the year at a ground...