ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-6999-2011 Global cloud condensation nuclei influenced by carbonaceous combustion aerosol Spracklen D. V. 1 Carslaw K. S. 1 Pöschl U. 2 Rap A. 1 Forster P. M. 1 Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK Biogeochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany 01 03 2011 11 3 6999 7044 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/11/6999/2011/acpd-11-6999-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/6999/2011/acpd-11-6999-2011.pdf

Black carbon in carbonaceous combustion aerosol warms the climate by absorbing solar radiation, meaning reductions in black carbon emissions are often perceived as an attractive global warming mitigation option. However, carbonaceous combustion aerosol can also act as cloud condensation nuclei (particles upon which cloud drops form) so they also cool the climate by increasing cloud albedo. The net radiative effect of carbonaceous combustion aerosol is uncertain because their contribution to cloud drops has not been evaluated on the global scale. By combining extensive observations of cloud condensation nuclei concentrations and a global aerosol model, we show that carbonaceous combustion aerosol accounts for more than half of global cloud condensation nuclei. The evaluated model predicts that wildfire and pollution (fossil fuel and biofuel) carbonaceous combustion aerosol causes a global mean aerosol indirect effect of −0.34 W m<sup>−2</sup> due to changes in cloud albedo, with pollution sources alone causing a global mean aerosol indirect effect of −0.23 W m<sup>−2</sup>. The small size of carbonaceous combustion particles from pollution sources means that whilst they account for only one-third of the emitted mass from these sources they cause two-thirds of the cloud albedo indirect effect that is due to carbonaceous combustion aerosol. This cooling effect must be accounted for to ensure that black carbon emissions controls that reduce the high number concentrations of small pollution particles have the desired net effect on climate.

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