Abstract:

We conduct observing system simulation experiments (OSSEs) to determine the potential of future satellite measurements of atmospheric methane columns (TROPOMI, GeoCARB, GEO-CAPE) for constraining methane emissions down to the 25 km scale through inverse analyses. The OSSE uses the GEOS-Chem chemical transport model (0.25◦ × 0.3125◦ grid resolution) in a 1-week simulation for the Southeast US with 216 emission elements to be optimized through inversion 5 of synthetic satellite observations. Clouds contaminate 73-91% of the viewing scenes depending on pixel size. Comparison of GEOS-Chem to TCCON surface-based methane column observations indicates a model transport error standard deviation of 12 ppb, larger than the instrument errors when aggregated on the 25 km model grid scale, and with a temporal error correlation of 6 hours. We find that TROPOMI (7×7 km2 pixels, daily return time) can provide a coarse regional optimization of methane emissions, and is highly sensitive to cloud cover. The geostationary instruments can do much better and are less 10 sensitive to cloud cover, reflecting both their finer pixel resolution and more frequent observations. The information content from GeoCARB toward constraining methane emissions increases by 20-25% for each doubling of the GeoCARB measurement frequency. Temporal error correlation in the transport model moderates but does not cancel the benefit of more frequent measurements for geostationary instruments.