14 Jun 2023
 | 14 Jun 2023

Atmospheric oxygen as a tracer for fossil fuel carbon dioxide: a sensitivity study in the UK

Hannah Chawner, Karina E. Adcock, Tim Arnold, Yuri Artioli, Caroline Dylag, Grant L. Forster, Anita Ganesan, Heather Graven, Gennadi Lessin, Peter Levy, Ingrid T. Luijx, Alistair Manning, Penelope A. Pickers, Chris Rennick, Christian Rödenbeck, and Matthew Rigby

Abstract. We investigate the use of oxygen (O2) and carbon dioxide (CO2) measurements for the estimation of the fossil fuel component of atmospheric CO2 in the UK. Atmospheric potential oxygen (APO) – a tracer that combines O2 and CO2, minimising the influence of terrestrial biosphere fluxes – is simulated at three sites in the UK, two of which make atmospheric APO measurements. We present a set of model experiments that estimate the sensitivity of APO simulations to key inputs: fluxes from the ocean, fossil fuel flux magnitude and distribution, the APO baseline, and the ratio of O2 to CO2 fluxes from fossil fuel combustion and the terrestrial biosphere. To estimate the influence of uncertainties in ocean fluxes, we compared three ocean O2 flux estimates, from the NEMO – ERSEM and ECCO-Darwin ocean models, and the Jena CarboScope APO inversion. The sensitivity of APO to fossil fuel emission magnitudes and to terrestrial biosphere and fossil fuel exchange ratios was investigated through Monte Carlo sampling within literature uncertainty ranges, and by comparing different inventory estimates. Of the factors that could potentially compromise APO-derived fossil fuel CO2 estimates, we find that the ocean O2 flux estimate has the largest overall influence at the three sites in the UK. At times, this influence is comparable to the contribution to APO of simulated fossil fuel CO2. We find that simulations using different ocean fluxes differ from each other substantially, with no single model estimate, or a simulation with zero ocean flux, providing a significantly closer fit to the observations. Furthermore, the uncertainty in the ocean contribution to APO could lead to uncertainty in defining an appropriate regional background from the data. Our findings suggest that the contribution of non-terrestrial sources need to be well accounted for, in order to reduce their potential influence on inferred fossil fuel CO2.

Hannah Chawner et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-385', Anonymous Referee #2, 06 Jul 2023
  • RC2: 'Comment on egusphere-2023-385', Anonymous Referee #4, 15 Oct 2023

Hannah Chawner et al.


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Short summary
As O2 is uptaken during combustion it can be used to trace fossil fuel (ff) emissions. We combine CO2 and O2 to minimise the biospheric impact using a quantity called atmospheric potential oxygen (APO), providing a method to isolate ff emissions. We model APO and compare with observations, focusing mainly on a site in Norfolk, UK. We attempt to use this to estimate emissions of ffCO2. We find large uncertainty in oceanic O2 emissions estimates, impacting both our model and estimates of ffCO2.