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Volume 11, issue 22 | Copyright

Special issue: POLARCAT (Polar Study using Aircraft, Remote Sensing, Surface...

Atmos. Chem. Phys., 11, 11807-11825, 2011
https://doi.org/10.5194/acp-11-11807-2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 29 Nov 2011

Research article | 29 Nov 2011

Hygroscopicity and composition of Alaskan Arctic CCN during April 2008

R. H. Moore1, R. Bahreini2,3, C. A. Brock2, K. D. Froyd2,3, J. Cozic2,3,*, J. S. Holloway2,3, A. M. Middlebrook2, D. M. Murphy2, and A. Nenes1,4 R. H. Moore et al.
  • 1School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
  • 2Earth Systems Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 4School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
  • *now at: Laboratoire de Glaciologie et Géophysique de l'Environnement, Grenoble, France

Abstract. We present a comprehensive characterization of cloud condensation nuclei (CCN) sampled in the Alaskan Arctic during the 2008 Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project, a component of the POLARCAT and International Polar Year (IPY) initiatives. Four distinct air mass types were sampled including a cleaner Arctic background and a relatively pristine sea ice boundary layer as well as biomass burning and anthropogenic pollution plumes. Despite differences in chemical composition, inferred aerosol hygroscopicities were fairly invariant and ranged from κ = 0.1–0.3 over the atmospherically-relevant range of water vapor supersaturations studied. Organic aerosols sampled were found to be well-oxygenated, consistent with long-range transport and aerosol aging processes. However, inferred hygroscopicities are less than would be predicted based on previous parameterizations of biogenic oxygenated organic aerosol, suggesting an upper limit on organic aerosol hygroscopicity above which κ is less sensitive to the O:C ratio. Most Arctic aerosols act as CCN above 0.1 % supersaturation, although the data suggest the presence of an externally-mixed, non-CCN-active mode comprising approximately 0–20% of the aerosol number. CCN closure was assessed using measured size distributions, bulk chemical composition, and assumed aerosol mixing states; CCN predictions tended toward overprediction, with the best agreement (±0–20 %) obtained by assuming the aerosol to be externally-mixed with soluble organics. Closure also varied with CCN concentration, and the best agreement was found for CCN concentrations above 100 cm−3 with a 1.5- to 3-fold overprediction at lower concentrations.

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