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 BC aerosol in the developmental version of the 4th generation Canadian Centre 5 for Climate modelling and analysis (CCCma) atmospheric general circulation model (AGCM). The focus of this work is on the conversion of insoluble BC to

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... d 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 decay with a fixed 24 h half-life, 2) a condensation and coagulation scheme, 3) an 10 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 processes of condensation, coagulation and oxidation is shown to simulate BC ageing 15 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. 20 impacting radiative transfer through the atmosphere (direct and semi-direct effects). Additionally, aerosols act as cloud condensation and ice nuclei, which influence cloud properties (indirect effects). The first indirect effect is based on the premise that for a given amount of cloud liquid water content, increased aerosol number implies more and smaller cloud droplets, and hence a more strongly reflective cloud. The second 25 indirect effect suggests that a cloud with more but smaller cloud droplets will be less 1384 ACPD