Abstract
Abstract. Measurements of atmospheric O2 have been used to quantify large-scale fluxes of carbon between the oceans, atmosphere and land since 1992 (Keeling and Shertz, 1992). With time, datasets have grown and estimates of fluxes have become more precise, but a key uncertainty in these calculations is the exchange ratio of O2 and CO2 associated with the net land carbon sink (αB). We present measurements of atmospheric O2 and CO2 collected over a 6-year period from a mixed deciduous forest in central Massachusetts, USA (42.537∘ N, 72.171∘ W). Using a differential fuel-cell-based instrument for O2 and a nondispersive infrared analyzer for CO2, we analyzed airstreams collected within and ∼5 m above the forest canopy. Averaged over the entire period of record, we find these two species covary with a slope of -1.081±0.007 mol of O2 per mole of CO2 (the mean and standard error of 6 h periods). If we limit the data to values collected on summer days within the canopy, the slope is -1.03±0.01. These are the conditions in which biotic influences are most likely to dominate. This result is significantly different from the value of −1.1 widely used in O2-based calculations 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.
Highlights
Since the pioneering work of Keeling and Shertz (1992), measurements of the abundance of atmospheric O2 and CO2 have been used extensively for constraining fluxes of carbon to and from the land biosphere and the oceans
Having measured changes in atmospheric CO2 and O2 in a mixed deciduous, midlatitude forest with a precision of a few micromoles per mole, we find that these species covary with an all-data average molar ratio over 6 h periods of −1.081 ± 0.007 (O2 : CO2)
In the absence of fossil fuel influences, our measured ratios reflect the local signature of photosynthesis and respiration (LBER)
Summary
Since the pioneering work of Keeling and Shertz (1992), measurements of the abundance of atmospheric O2 and CO2 have been used extensively for constraining fluxes of carbon to and from the land biosphere and the oceans. Knowing fff and αff from industrial inventories, calculating Zocean from changes in ocean heat storage, and measuring the changes in O2 and CO2, we can solve these two equations for fland and focean in terms of αB (for a rigorous treatment of these equations, see for example Keeling and Manning, 2014). This O2-based method has become increasingly sophisticated with refinements in methodology, and increasingly precise with better instruments and ever-longer datasets (Keeling and Manning, 2014). All estimates of global carbon fluxes based on atmospheric O2 require a value for αB
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