Global CH$_4$ fluxes derived from JAXA/GOSAT lower-tropospheric partial column data and the CarbonTracker Europe-CH$_4$ atmospheric inverse model

Satellite-driven inversions provide valuable information about methane (CH$_4$) fluxes, but the assimilation of total column-averaged dry-air mole fractions of CH$_4$ (XCH$_4$) has been challenging. This study explores, for the first time, the potential of the new lower-tropospheric partial column (pXCH$_4$_LT) GOSAT data, retrieved by the Japan Aerospace Exploration Agency (JAXA), to constrain global and regional CH$_4$ fluxes. Using the CarbonTracker Europe-CH$_4$ (CTE-CH4) atmospheric inverse model, we estimated CH$_4$ fluxes between 2016–2019 by assimilating the JAXA/GOSAT pXCH$_4$_LT and XCH$_4$ data and surface CH$_4$ observations independently of each other. The Northern Hemisphere CH$_4$ fluxes derived from the pXCH$_4$_LT data were similar to the estimates derived from the surface observations but were underestimated by about 35 Tg CH$_4$ yr$^{−1}$ (∼ 6 % of the global total) using the XCH$_4$ data. For the Southern Hemisphere, the estimates from both GOSAT inversions were about 15–30 Tg CH$_4$ yr$^{−1}$ higher than those derived from surface data. The evaluations against independent data from the Atmospheric Tomography Mission aircraft campaign showed good agreement in the lower-tropospheric CH$_4$ from the inversions using the pXCH$_4$_LT and surface data. ... mehrHowever, from these inversions, the modelled north–south gradients showed significant overestimation in the upper troposphere and stratosphere, possibly due to relatively uniform inter-hemispheric OH distributions that control CH$_4$ sinks. Overall, we found that the use of the JAXA/GOSAT pXCH$_4$_LT data shows considerable potential in constraining global and regional CH$_4$ fluxes, advancing our understanding of the CH$_4$ budget.