Abstract
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 (CO2) datasets. These data are applicable for a top-down approach towards the verification of anthropogenic CO2 emissions from megacities and updating of the inventory. However, great uncertainties regarding natural CO2 flux estimates remain when back-casting CO2 emissions from concentration data, making accurate disaggregation of urban CO2 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 CO2 uptake and biogenic CO2 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 CO2 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 CO2 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 CO2 emissions of the Greater Tokyo Area increased sixfold, whereas the amplitude of the midday hourly integrated photosynthetic CO2 uptake was enhanced by nearly five times and could offset up to 79.04 ± 12.31% of the hourly integrated anthropogenic CO2 emissions in summer. The gridded hourly photosynthetic CO2 uptake and biogenic respiration estimates not only provide reference data for the estimation of total natural CO2 removal in our study area, but also supply prior input values for the disaggregation of anthropogenic CO2 emissions and biogenic CO2 fluxes when applying top-down approaches to update the megacity’s CO2 emissions inventory. The latter contribution allows unprecedented amounts of GOSAT and ground measurement data regarding CO2 concentration to be analyzed in inverse modeling of anthropogenic CO2 emissions from Tokyo and the Kantō Plain.
Highlights
To address climate change, greenhouse gas inventories including both source emissions and sink removals have been exceedingly important for policymaking and for scientific studies
Cities are the focal points of climate action for the following reasons: more than half of the human population resides in urban areas [4], where cities and their affiliated power plants, airports and other infrastructure constitute the largest human-induced CO2 emission sources, accounting for 50%–80% of the global total [3,5,6,7]; urban populations will continue growing to 68% of the global total by 2050 [4], with expected rapid rises occurring in Africa, Asia, and Latin America, presumably increasing CO2 emissions markedly [8,9]
PPFDt diffuse was calculated based on diffuse shortwave solar radiation (SRt diffuse ), whereas PPFDt direct was based on direct shortwave solar radiation (SRt direct ) using reference parameter settings in Ito and Oikawa [65]
Summary
Greenhouse gas inventories including both source emissions and sink removals have been exceedingly important for policymaking and for scientific studies. The primary greenhouse gas in the atmosphere affected by human activities is carbon dioxide (CO2 ), with a global averaged concentration of 407.8 ± 0.1 parts per million (ppm) in 2018 [1]. Scientific reports such as the Global Carbon Budget 2019 [2]. Estimated sources and sinks of CO2 globally. National greenhouse gas inventories have included annual totals and trends of CO2 emissions and removals by sector. Cities are the focal points of climate action for the following reasons: more than half of the human population resides in urban areas [4], where cities and their affiliated power plants, airports and other infrastructure constitute the largest human-induced CO2 emission sources, accounting for 50%–80% of the global total [3,5,6,7]; urban populations will continue growing to 68% of the global total by 2050 [4], with expected rapid rises occurring in Africa, Asia, and Latin America, presumably increasing CO2 emissions markedly [8,9]
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