Using NO2 satellite observations to constrain ffCO2

Success of the Paris Agreement relies on rapid reductions in fossil fuel CO₂ (ffCO₂) emissions, which can be independently verified using atmospheric data. However, estimating changes in ffCO₂ from atmospheric CO₂ is challenging due to large and variable contributions from natural fluxes and background concentrations. Nitrogen oxides (NO_x = NO + NO₂), which are a major contributor to surface air pollution that have adverse effects on human health, are co-emitted with CO₂ during incomplete fossil fuel combustion. Because atmospheric NO_x has a relatively short lifetime (hours to days), low background concentrations, and limited natural sources, it is possible to link elevated NO₂ satellite columns to their parent emissions.

We present results from an Ensemble Kalman Filter (EnKF) based model inversion using the GEOS-Chem atmospheric chemistry and transport model, along with NO₂ TROPOMI observations, to estimate NO_x emissions across mainland Europe. Leveraging sector-specific CO₂:NO_x emission ratios, we then convert the NO_x posterior dataset to ffCO₂. Additionally, we present preliminary findings for an alternative methodology that relies less on prior knowledge of emission ratios. This approach uses a combined CO₂:NO_x inversion, integrating TROPOMI NO₂ and OCO-2 CO₂ measurements to directly constrain ffCO₂.

Our results describe a more accurate and direct approach for estimating fossil fuel CO₂ emissions, which we anticipate will offer valuable insights for verifying national emission reductions and informing global climate mitigation strategies.