CO 2 concentrations and related environmental factors were measured in an Asian tropical rainforest located in a small valley in Xishuangbanna, SW China, with the aim of investigating the CO 2 pooling effect and its mechanism of formation. Pooling of CO 2 was observed during the evening (1600-2200 hours local time); the accumulated CO 2 subsequently flowed away after dusk. We consider that along-slope drainage flow, soil CO 2 efflux, and temperature inversion contribute to the development of CO 2 pooling. A new model is proposed to track the mechanism of the formation and dissipation of CO 2 pooling (e.g., drainage flow, compensatory mechanisms). Given its influence on the storage term, we suggest that CO 2 pooling and subsequent disappearance should be taken into account when calculating eddy covariance and other micrometeorological measurements of carbon flux for valley sites.