| Summary: | During the last decade, advances in the remote sensing of greenhouse gas (GHG) concentrations by the Greenhouse Gases Observing SATellite-1 (GOSAT-1), GOSAT-2, and Orbiting Carbon Observatory-2 (OCO-2) have produced finer-resolution atmospheric carbon dioxide (CO<sub>2</sub>) datasets. These data are applicable for a top-down approach towards the verification of anthropogenic CO<sub>2</sub> emissions from megacities and updating of the inventory. However, great uncertainties regarding natural CO<sub>2</sub> flux estimates remain when back-casting CO<sub>2</sub> emissions from concentration data, making accurate disaggregation of urban CO<sub>2</sub> sources difficult. For this study, we used Moderate Resolution Imaging Spectroradiometer (MODIS) land products, meso-scale meteorological data, SoilGrids250 m soil profile data, and sub-daily soil moisture datasets to calculate hourly photosynthetic CO<sub>2</sub> uptake and biogenic CO<sub>2</sub> emissions with 500 m resolution for the Kantō Plain, Japan, at the center of which is the Tokyo metropolis. Our hourly integrated modeling results obtained for the period 2010–2018 suggest that, collectively, the vegetated land within the Greater Tokyo Area served as a daytime carbon sink year-round, where the hourly integrated net atmospheric CO<sub>2</sub> removal was up to 14.15 ± 4.24% of hourly integrated anthropogenic emissions in winter and up to 55.42 ± 10.39% in summer. At night, plants and soil in the Greater Tokyo Area were natural carbon sources, with hourly integrated biogenic CO<sub>2</sub> emissions equivalent to 2.27 ± 0.11%–4.97 ± 1.17% of the anthropogenic emissions in winter and 13.71 ± 2.44%–23.62 ± 3.13% in summer. Between January and July, the hourly integrated biogenic CO<sub>2</sub> emissions of the Greater Tokyo Area increased sixfold, whereas the amplitude of the midday hourly integrated photosynthetic CO<sub>2</sub> uptake was enhanced by nearly five times and could offset up to 79.04 ± 12.31% of the hourly integrated anthropogenic CO<sub>2</sub> emissions in summer. The gridded hourly photosynthetic CO<sub>2</sub> uptake and biogenic respiration estimates not only provide reference data for the estimation of total natural CO<sub>2</sub> removal in our study area, but also supply prior input values for the disaggregation of anthropogenic CO<sub>2</sub> emissions and biogenic CO<sub>2</sub> fluxes when applying top-down approaches to update the megacity’s CO<sub>2</sub> emissions inventory. The latter contribution allows unprecedented amounts of GOSAT and ground measurement data regarding CO<sub>2</sub> concentration to be analyzed in inverse modeling of anthropogenic CO<sub>2</sub> emissions from Tokyo and the Kantō Plain.
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