Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 11107–11124, 2016

Special issue: NETCARE (Network on Aerosols and Climate: Addressing Key Uncertainties...

Atmos. Chem. Phys., 16, 11107–11124, 2016

Research article 08 Sep 2016

Research article | 08 Sep 2016

Effects of 20–100 nm particles on liquid clouds in the clean summertime Arctic

W. Richard Leaitch1, Alexei Korolev1, Amir A. Aliabadi1,a, Julia Burkart2, Megan D. Willis2, Jonathan P. D. Abbatt2, Heiko Bozem3, Peter Hoor3, Franziska Köllner4, Johannes Schneider4, Andreas Herber5, Christian Konrad5, and Ralf Brauner6 W. Richard Leaitch et al.
  • 1Environment and Climate Change Canada, Toronto, Canada
  • 2Department of Chemistry, University of Toronto, Toronto, Canada
  • 3Institute for Atmospheric Physics, University of Mainz, Mainz, Germany
  • 4Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
  • 5Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 6Department of Maritime and Logistics Studies, Jade University, Elsfleth, Germany
  • anow at: Environmental Engineering, University of Guelph, Guelph, Canada

Abstract. Observations addressing effects of aerosol particles on summertime Arctic clouds are limited. An airborne study, carried out during July 2014 from Resolute Bay, Nunavut, Canada, as part of the Canadian NETCARE project, provides a comprehensive in situ look into some effects of aerosol particles on liquid clouds in the clean environment of the Arctic summer. Median cloud droplet number concentrations (CDNC) from 62 cloud samples are 10 cm−3 for low-altitude cloud (clouds topped below 200 m) and 101 cm−3 for higher-altitude cloud (clouds based above 200 m). The lower activation size of aerosol particles is  ≤  50 nm diameter in about 40 % of the cases. Particles as small as 20 nm activated in the higher-altitude clouds consistent with higher supersaturations (S) for those clouds inferred from comparison of the CDNC with cloud condensation nucleus (CCN) measurements. Over 60 % of the low-altitude cloud samples fall into the CCN-limited regime of Mauritsen et al. (2011), within which increases in CDNC may increase liquid water and warm the surface. These first observations of that CCN-limited regime indicate a positive association of the liquid water content (LWC) and CDNC, but no association of either the CDNC or LWC with aerosol variations. Above the Mauritsen limit, where aerosol indirect cooling may result, changes in particles with diameters from 20 to 100 nm exert a relatively strong influence on the CDNC. Within this exceedingly clean environment, as defined by low carbon monoxide and low concentrations of larger particles, the background CDNC are estimated to range between 16 and 160 cm−3, where higher values are due to activation of particles  ≤  50 nm that likely derive from natural sources. These observations offer the first wide-ranging reference for the aerosol cloud albedo effect in the summertime Arctic.

Short summary
Thought to be mostly unimportant for summertime Arctic liquid-water clouds, airborne observations show that atmospheric aerosol particles 50 nm in diameter or smaller and most likely from natural sources are often involved in cloud formation in the pristine Arctic summer. The result expands the reference for aerosol forcing of climate. Further, for extremely low droplet concentrations, no evidence is found for a connection between cloud liquid water and aerosol particle concentrations.
Final-revised paper