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Abstract. Measurements of atmospheric O2have been used to quantify large-scalefluxes of carbon between the oceans, atmosphere and landsince 1992 (Keeling and Shertz, 1992). With time,datasets have grownand estimates of fluxes have become more precise, buta key uncertainty in these calculations is the exchange ratioof O2and CO2 associated with the net land carbon sink(αB). We present measurements of atmosphericO2 and CO2 collected overa 6-year period from a mixed deciduous forest in centralMassachusetts, USA (42.537∘ N, 72.171∘ W).Using a differential fuel-cell-basedinstrument for O2 and a nondispersive infrared analyzer forCO2, we analyzed airstreams collected within and ∼5 m above the forest canopy. Averaged over the entireperiod of record, we find these two species covary with a slope of -1.081±0.007 mol of O2 per mole ofCO2 (themean and standard error of 6 h periods).If we limit the data to values collected on summer days within thecanopy, the slope is -1.03±0.01. These are the conditions in whichbiotic influences are most likely to dominate.This result is significantlydifferent from the value of −1.1 widely used in O2-basedcalculations of the global carbon budget, suggesting the need for a deeper understanding of the exchange ratios of the various fluxes and pools comprising the net sink.