Can a strong atmospheric CO₂ rectifier effect be reconciled with a 'reasonable' carbon budget?

Atmospheric CO₂ accumulates near the Earth's surface because of relatively deeper vertical mixing when photosynthesis is active than when it is not. Some models simulate an excess of more than 2.5 ppmv CO₂ in the remote Northern Hemisphere due to this 'rectification' of an annually balanced terrestrial carbon cycle. The covariance between CO₂ flux and vertical mixing, and the resulting vertical structure of CO₂ are generally consistent with field data at local scales, but it is difficult to reconcile such a strong rectifier signal with current ideas about the global carbon budget. A rectifier effect of 2.5 ppmv at northern flask sampling stations implies an unreasonably strong northern sink at atmospheric CO₂, and a corresponding source in the tropics or Southern Hemisphere.