Global CH₄ Fluxes Derived from JAXA/GOSAT Lower Tropospheric Partial Column Data and the CTE-CH₄ Atmospheric Inverse Model

Abstract. Satellite-driven inversions provide valuable information about methane (CH₄) fluxes, but the assimilation of total column-averaged dry-air mole fractions of CH₄ (XCH₄) has been challenging. This study explores, for the first time, the potential of the new lower tropospheric partial column (pXCH₄_LT) GOSAT data, retrieved by the Japan Aerospace Exploration Agency (JAXA), to constrain global and regional CH₄ fluxes. Using the CarbonTracker Europe-CH₄ atmospheric inverse model, we estimated CH₄ fluxes between 2016–2019 by assimilating the JAXA/GOSAT pXCH₄_LT and XCH₄ data and surface CH₄ observations, independently of each other. The Northern Hemisphere CH₄ fluxes derived from the JAXA/GOSAT pXCH₄_LT data were similar to the estimates derived from the surface observations, but was underestimated by about 35 Tg CH₄ year^-1 (∼6 % of the global total) using the JAXA/GOSAT XCH₄ data. For the Southern Hemisphere, the estimates from the both GOSAT inversions were about 15–30 Tg CH₄ year^-1 higher than that derived from surface data. The evaluations against independent data from the Atmospheric Tomography Mission aircraft campaign showed good agreement in the lower tropospheric CH₄ from the inversions using the JAXA/GOSAT pXCH₄_LT and surface data. However, 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 CH₄ sinks. Overall, we found that the use of the JAXA/GOSAT pXCH₄_LT data shows considerable potential in constraining global and regional CH₄ fluxes, advancing our understanding of the CH₄ budget.