Understanding Urban Carbon Enhancement at the Megacity Using Satellite XCO2 Data: A Case Study of Seoul, South Korea

Assessment of urban carbon emission is a critical issue to understand the global carbon cycle. This study evaluates urban enhancement of CO₂ over Seoul Capital Area (SCA), which is one of the apparent Megacities across the world, by using column integrated CO₂ concentration (XCO₂) from Orbiting Carbon Observatory-2 (OCO-2) from 2014 to 2018. Overall, observed XCO₂ concentration over the SCA shows apparent seasonal variations with higher concentrations in winter time XCO₂ (404.97 ± 3.78 ppmv) than that of summer (401.54 ± 4.86 ppmv). Analyzing with Solar-Induced Fluorescence (SIF), which is a direct measure of vegetation photosynthetic activity from OCO-2, at the same pixel with XCO₂, we find seasonal variations in XCO₂ has a clear opposite features of SIF, suggesting local vegetation activity to diminish summer time atmospheric CO₂ concentrations in SCA. To estimate urban enhancement of XCO₂, we compared XCO₂ concentration over SCA with nearby background area of Jirisan National Park (JNP). Comparison of XCO₂ between two areas are divided with wind speed and direction to eliminate the transport effect in XCO₂. Dominant urban enhancement in the SCA are only observed in the day with wind speed less than 4 m/s, indicating wind speed constrain to define urban enhancement from satellite. On that case, SCA's XCO₂ has a higher than those in JNP from 1.71 to 2.21 ppm on average. At the 95^th percentile level, SCA has higher concentrations of XCO₂ by up to 2.86 ppmv. Finally, to utilize our estimation in urban enhancement, we compare urban enhancement of XCO₂ from OCO-2 with Greenhouse Gases Observing Satellite (GOSAT) and CarbonTracker 2017 (CT2017). In different with OCO-2, both GOSAT and CT2017 haven't shown urban enhancement of XCO₂ at the SCA, implying importance of high-resolution measurements in urban CO₂ detection at the region with difficulty in defining urban-rural boundary. Consequently, our results indicate high-resolution satellite data show prominent urban CO₂ characteristics over the regions with limited ground observations. This study suggests increasing number of urban measurements with city-target mode from new satellite such as OCO-3 and GOSAT-2 will help us to see more details in urban carbon emissions.