1Department of Physics, P.O. Box 64, University of Helsinki, 00014 Helsinki, Finland
2Department of Applied Physics, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
3Finnish Meteorological Institute, Kuopio Unit, P.O. Box 1627, 70211 Kuopio, Finland
4Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU, UK
5Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
6Max Planck Institute for Meteorology, Bundesstr. 53, 20146 Hamburg, Germany
Abstract. 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 CO2. 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 a global climate model. The nitric acid contribution to the present-day cloud albedo effect was found to be −0.32 W m−2 and the total indirect effect −0.46 W m−2. The contribution to the cloud albedo effect is shown to increase to −0.37 W m−2 by 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 SO2 emissions and increasing NOx and greenhouse gases.