"IBUKI" (GOSAT) Satellite's Historically Unprecedented Observation of Anthropogenic CO2 Emissions of Mega-City Regions from Space

Carbon dioxide (CO₂) is a greenhouse gas the atmospheric concentration of which has increased by 40% when compared to pre-industrial times, as a result of anthropogenic activities. Fossil fuel consumption in large fossil fuel power plants and mega-cities are assumed to be the main sources of anthropogenic CO₂ emissions. The CO₂ emission rates in these areas should be accurately evaluated to monitor greenhouse gas emissions.

A validation study has shown that the greenhouse gases observation sensor, “TANSO-FTS” onboard “IBUKI” observes column-averaged CO₂ concentrations (hereinafter, “CO₂ concentrations”) to a precision (random error) of 0.5% (approximately 2 ppm) in comparison to the data acquired from ground-based observations. This study investigated the detectability of enhanced CO₂ concentrations due to anthropogenic emissions, by satellite observation.

CO₂ concentrations acquired by “IBUKI” comprise not only those pertaining to anthropogenic emissions but also encompass photosynthetic uptake and respiration-release by plants; emissions due to forest fires; and sink/source by ocean. These processes should be considered when interpreting the information on anthropogenic CO₂ emissions from the CO₂ concentrations acquired by “IBUKI”. This study estimated CO₂ concentrations due to anthropogenic activities from observational data by “IBUKI” by the method described below.

Firstly, the spatiotemporal distribution of CO₂ concentrations originating from anthropogenic CO₂ emissions are estimated using the data for anthropogenic CO₂ emission rates from fossil fuel consumption (emission inventory) - a value based on the data for nighttime lights on the earth’s surface as observed by satellite, as well as the information from the database for fossil fuel power plants and an atmospheric tracer transport model. Subsequently, on the basis of this estimate, the observational data for CO₂ concentrations acquired by “IBUKI” are classified into two categories: data including contamination by anthropogenic CO₂ emissions (top panel in Figure 1) and those not including this contamination. The individual observational data sets including anthropogenic contamination are subtracted using averaged values for the data not including contamination, and values for CO₂ concentrations from forest fires and plant activities are then subtracted from the data. These calculations produce the figures for anthropogenic CO₂ concentrations using observational data from “IBUKI” (bottom panel in Figure 1).

Figure 1 shows global distribution of anthropogenic CO₂ concentrations estimated from observational data from “IBUKI”. Data including contamination by anthropogenic CO₂ emissions is mainly concentrated on the Northern Hemisphere, especially in mega-city regions (top panel in Figure 1).

Table 1 A list of regions indicating high concentrations of anthropogenic CO₂ emissions derived from “IBUKI” in Figure 1.

Regions with higher CO₂ concentrations from anthropogenic activities are shown in the bottom of Figure 1 and Figure 2 and summarized in Table 1. The regions in Table 1 can be identified as those with dense populations or industrial zones with fossil fuel power plants and developments of oil and gas fields.

Figure 3 shows the relationships between anthropogenic CO₂ concentrations derived from the emission inventory and the observational data acquired by “IBUKI” in four regions (Japan, East Asia, North America, and the globe), as shown in the bottom of Figure 1. Key aspects relating to Figure 3 are as follows:

These results suggest that “IBUKI” has the potential to detect enhanced CO₂ concentrations resulting from fossil fuel consumption in mega-cities.

The present estimates demonstrated the potential utility of observations by “IBUKI” and future satellites to monitor anthropogenic CO₂ emissions. The methodology to improve both the frequency and net volume of observational data by satellites will be further investigated towards an enhancement of the accuracy of estimates of anthropogenic CO₂ concentrations. Additionally, further studies and evaluations, for example, ground-based observation in the surroundings of large emission sources, will be implemented to refine estimates of anthropogenic emission rates from CO₂ concentration measurements acquired by satellites.

The mixing ratio of CO₂ molecules relative to that of dry air, from the earth's surface to the upper extremities of the atmosphere.

Inventories of CO₂ emissions from fossil fuel consumption and forest fires, a biospheric model, and an atmospheric tracer transport model were used in this study to estimate anthropogenic CO₂ concentrations by inventory.

ODIAC (Open-sources Data Inventory for Anthropogenic CO₂) was used in this study as inventory for CO₂ emissions by fossil fuel consumption. Oda and Maksyutov, Atmos. Chem. Phys., 11, 543-556, 2011.

CARMA (Carbon Monitoring for Actions) is used in ODIAC as the database for fossil fuel power plants.

GFAS (Global Fire Assimilation System) was used in this study as inventory for CO₂ emissions by forest fires. Kaiser et al., Biogeosciences, 9, 527-554, 2011.