A three-dimensional tracer transport model is used to investigate the temporal trend and the annual cycle of atmospheric CO2. The model transport was driven by a semi-Lagrangian scheme, using winds from 1992, analyzed at the European Centre for Medium-Range Weather Forecasts. A simple planetary boundary layer (PBL) formulation was used to investigate the sensitivity of the model results. Two source/sink distributions were used independently: fixed emissions of 5.3 Gt C/yr, estimated from fossil fuel consumption (fossil run), and a source representing net exchange with the terrestrial biosphere (vegetation run). This terrestrial flux represents a growing season net flux of 14.5 Gt C. The simulated interhemispheric difference between the mean surface concentrations, in terms of north pole minus south pole, is about 4 ppm for the fossil run and 2 ppm for the vegetation run. These results suggest that the so-called missing sink should be found in the northern hemisphere. On the other hand, the interhemispheric difference in the vegetation run was changed by 40%, while the fossil run changed by only 4% when the instantaneous PBL height was replaced by monthly averages of PBL height. This result suggests that the monthly average PBL height can be used in place of the instantaneous PBL height only in limited cases, if it is scaled with an appropriate factor. ¿ American Geophysical Union 1996