ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-7625-2012 Brightening of the global cloud field by nitric acid and the associated radiative forcing Makkonen R. 1 7 Romakkaniemi S. 2 Kokkola H. 3 Stier P. 4 Räisänen P. 5 Rast S. 6 Feichter J. 6 Kulmala M. 1 Laaksonen A. 2 5 Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, 70211, Kuopio, Finland Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, UK Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany now at: Department of Geosciences, University of Oslo, P.O. Box 1047, 0316 Oslo, Norway 22 08 2012 12 16 7625 7633 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.net/12/7625/2012/acp-12-7625-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/7625/2012/acp-12-7625-2012.pdf

Clouds cool Earth's climate by reflecting 20% of the incoming solar energy, while also trapping part of the outgoing radiation. The effect of human activities on clouds is poorly understood, but the present-day anthropogenic cooling via changes of cloud albedo and lifetime could be of the same order as warming from anthropogenic addition in CO<sub>2</sub>. Soluble trace gases can increase water condensation to particles, possibly leading to activation of smaller aerosols and more numerous cloud droplets. We have studied the effect of nitric acid on the aerosol indirect effect with the global aerosol-climate model ECHAM5.5-HAM2. Including the nitric acid effect in the model increases cloud droplet number concentrations globally by 7%. The nitric acid contribution to the present-day cloud albedo effect was found to be −0.32 W m<sup>−2</sup> and to the total indirect effect −0.46 W m<sup>−2</sup>. The contribution to the cloud albedo effect is shown to increase to −0.37 W m<sup>−2</sup> by the year 2100, if considering only the reductions in available cloud condensation nuclei. Overall, the effect of nitric acid can play a large part in aerosol cooling during the following decades with decreasing SO<sub>2</sub> emissions and increasing NO<sub>x</sub> and greenhouse gases.

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