Articles | Volume 11, issue 3
Atmos. Chem. Phys., 11, 1051–1064, 2011
Atmos. Chem. Phys., 11, 1051–1064, 2011

Research article 07 Feb 2011

Research article | 07 Feb 2011

Soot microphysical effects on liquid clouds, a multi-model investigation

D. Koch1,2,*, Y. Balkanski3, S. E. Bauer1,2, R. C. Easter8, S. Ferrachat4, S. J. Ghan8, C. Hoose7,10, T. Iversen5, A. Kirkevåg5, J. E. Kristjansson10, X. Liu8, U. Lohmann4, S. Menon9, J. Quaas6, M. Schulz3,10, Ø. Seland5, T. Takemura11, and N. Yan3 D. Koch et al.
  • 1Columbia University, New York, NY, USA
  • 2NASA GISS, New York, NY, USA
  • 3Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France
  • 4Institute of Atmospheric and Climate Science, ETH Zurich, Switzerland
  • 5Norwegian Meteorological Institute, Oslo, Norway
  • 6Max Planck Institute for Meteorology, Hamburg, Germany
  • 7Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Karlsruhe, Germany
  • 8Pacific Northwest National Laboratory, Richland, USA
  • 9Lawrence Berkeley National Laboratory, USA
  • 10Department of Geosciences, University of Oslo, Oslo, Norway
  • 11Kyushu University, Fukuoka, Japan
  • *now at: Department of Energy, DC, USA

Abstract. We use global models to explore the microphysical effects of carbonaceous aerosols on liquid clouds. Although absorption of solar radiation by soot warms the atmosphere, soot may cause climate cooling due to its contribution to cloud condensation nuclei (CCN) and therefore cloud brightness. Six global models conducted three soot experiments; four of the models had detailed aerosol microphysical schemes. The average cloud radiative response to biofuel soot (black and organic carbon), including both indirect and semi-direct effects, is −0.11 Wm−2, comparable in size but opposite in sign to the respective direct effect. In a more idealized fossil fuel black carbon experiment, some models calculated a positive cloud response because soot provides a deposition sink for sulfuric and nitric acids and secondary organics, decreasing nucleation and evolution of viable CCN. Biofuel soot particles were also typically assumed to be larger and more hygroscopic than for fossil fuel soot and therefore caused more negative forcing, as also found in previous studies. Diesel soot (black and organic carbon) experiments had relatively smaller cloud impacts with five of the models <±0.06 Wm−2 from clouds. The results are subject to the caveats that variability among models, and regional and interrannual variability for each model, are large. This comparison together with previously published results stresses the need to further constrain aerosol microphysical schemes. The non-linearities resulting from the competition of opposing effects on the CCN population make it difficult to extrapolate from idealized experiments to likely impacts of realistic potential emission changes.

Final-revised paper