Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 2 2005 10.5194/acpd-5-1383-2005 http://www.atmos-chem-phys-discuss.net/5/1383/2005/ http://www.atmos-chem-phys-discuss.net/5/1383/2005/acpd-5-1383-2005.html http://www.atmos-chem-phys-discuss.net/5/1383/2005/acpd-5-1383-2005.pdf 1383 1419 2005-03-10 Black carbon ageing in the Canadian Centre for Climate modelling and analysis atmospheric general circulation model B. Croft U. Lohmann K. von Salzen Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada Institute of Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland Canadian Centre for Climate modelling and analysis, Meteorological Service of Canada, Victoria, Canada Black carbon (BC) particles in the atmosphere have important impacts on climate. The amount of BC in the atmosphere must be carefully quantified to allow evaluation of the climate effects of this type of aerosol. In this study, we present the treatment of 5 BC aerosol in the developmental version of the 4th generation Canadian Centre for Climate modelling and analysis (CCCma) atmospheric general circulation model (AGCM). The focus of this work is on the conversion of insoluble BC to soluble/mixed BC. Four separate parameterizations of this ageing process are compared to a control simulation that assumes no ageing occurs. These simulations use 1) an exponential 10 decay with a fixed 24 h half-life, 2) a condensation and coagulation scheme, 3) an oxidative scheme, and 4) a linear combination of the latter two ageing treatments. Global BC burdens are 2.15, 0.15, 0.11, 0.21, and 0.11 Tg C for the control run, and four ageing schemes, respectively. The BC lifetimes are 98.1, 6.6, 5.0, 9.5, and 4.9 days, respectively. A computationally efficient parameterization that represents the 15 processes of condensation, coagulation and oxidation is shown to simulate BC ageing well in the CCCma AGCM. As opposed to the globally fixed ageing time scale, this treatment of BC ageing is responsive to varying atmospheric composition.