Atmospheric mixing is determined by such factors as advection and diffusion, planetary boundary layer (PBL) depth, and cloud transport-together they determine an effective 'mixing volume' to which CO2 is then added or removed. In this study a mathematical model is developed to quantify the key factors determining the local 'mixing volume' and hence the seasonal cycle of atmospheric CO2. It was found that the seasonal variation in horizontal advection is more important than the seasonal variation in PBL depth in determining the local mean and seasonal variation in atmospheric CO2 and that cloud transport plays a minor role. ¿ 1998 American Geophysical Union