Seasonal Variability of Stratospheric Methane: Implications for Constraining Tropospheric Methane Budgets Using Total Column Observations.

Global and regional methane budgets are uncertain due to the large number of highly variable sources. Current estimates are derived by bridging emissions inventories with atmospheric observations using chemical transport models. This approach requires accurately simulating advection and chemical loss to produce valid distributions of methane concentrations resulting from surface fluxes when assimilating total column measurements. To assess the impact of model stratospheric errors on inversions that assimilate total columns, we compare the agreement between Total Carbon Column Observing Network (TCCON) and GEOS-Chem total and tropospheric column-averaged mole fractions of methane. We find both a mismatch in the Northern Hemisphere stratospheric contribution that increases as the tropopause altitude decreases and a temporal phase lag in the model's tropospheric seasonality driven by transport errors. These tropospheric errors particularly compensate the stratospheric discrepancies between measurements and models, thereby producing agreement in the total columns masking inconsistencies in methane vertical profiles. These errors alias into source attribution resulting from model inversions. We estimate that the tropospheric time lag leads to large errors in posterior wetland emissions in the high latitudes of the Northern Hemisphere. [ABSTRACT FROM AUTHOR]