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. S ome models simulate an excess of more than 2.5 ppmv CO2 in the remote North ern Hemisphere due to this "rectification" of an annually balanced terrestr ial 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 recti fier signal with current ideas about the global carbon budget. A rectifier effect of 2.5 ppmv at northern flask sampling stations implies an unreasona bly strong northern sink of atmospheric CO2, and a corresponding source in the tropics or Southern Hemisphere.