Constraints on methane emissions in North America from future geostationary remote sensing measurements

Abstract

The success of future geostationary (GEO) satellite observation missions depends on our ability to design instruments that address their key scientific objectives. In this study, an Observation System Simulation Experiment (OSSE) is performed to quantify the constraints on methane (CH4) emissions in North America obtained from Short Wave Infrared (SWIR), Thermal Infrared (TIR) and multi-spectral measurements in geostationary orbit compared to existing SWIR low earth (LEO) measurements. A stochastic algorithm is used to compute the information content of a variational inversion at high spatial resolution (0.5° × 0.7°) using the GEOS-Chem chemical transport model and its adjoint. Both the SWIR LEO and TIR GEO configurations generally provide poor constraints on CH4 emissions (error reduction <30 %), with the exception of a few hotspots (e.g., Los Angeles, Toronto urban areas and Appalachian Mountains) where the error reduction is greater than 50 %. On weekly time scales and for a GEO orbit, the degree of freedom for signal (DOFs) of the inversion from multi-spectral observations (500) is a factor of two higher than that obtained from a SWIR instrument (255) due to the increase in measurement sensitivity to boundary layer concentrations in the multi-spectral case. On a monthly time scale and for a GEO orbit, a SWIR instrument would reduce error in emission estimates by more than 70 % for hotspots of CH4 sources (emissions > 4 × 105 kg day-1 grid-1) at model grid scale, while a TIR instrument would provide a relative error reduction of 25-60 % over those areas. While performing similarly for monthly inversions, a multi-spectral instrument would allow for more than 70 % error reduction for these emissions for 7 or 3 day inversions. Sensitivity of the inversions to error in boundary conditions are found to be negligible. Moreover, estimates of the model resolution matrix over significant emitting regions (CH4 emissions > 2 × 105 kg day-1 grid-1) show that for all instrument configurations in GEO orbit the inversion is able to independently constrain CH4 sources at spatial scales smaller than 200 km. These results highlight the importance of using observations sensitive to boundary layer concentrations (i.e., SWIR) to achieve significant improvements in constraining CH4 sources compared to current LEO capabilities.