A comparative analysis of anthropogenic CO2 emissions at city level using OCO-2 observations: A global perspective

Authors: FU, PENG - University Of Illinois; XIE, YANHUA - University Of Wisconsin; MOORE, CAITLIN - University Of Illinois; MYINT, SOE - Arizona State University; Bernacchi, Carl

Submitted to: Earth's Future
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2019
Publication Date: 8/28/2019
Citation: Fu, P., Xie, Y., Moore, C.E., Myint, S.W., Bernacchi, C.J. 2019. A comparative analysis of anthropogenic CO2 emissions at city level using OCO-2 observations: A global perspective. Earth's Future. 7(9):1058-1070.

Interpretive Summary: People emit a significant amount of the greenhouse gas, carbon dioxide, to the atmosphere, with a significant fraction coming from urban areas. While measurements of atmospheric CO2 are made on a regular basis, the major drivers that influence how much CO2 is released in urban areas is not well understood. Factors that can impact how much carbon dioxide is released includes how big the urban environment is, the type of urban environment (for example, industrial or residential), and how many plants are located in the area. This research project used satellite-based measurements of carbon dioxide along with other satellite-based measurements related to number of plants, type of urban environment, and size of urban environment to determine whether factors influencing carbon dioxide emissions can be better determined. This research will help to understand the roles of plant in urban pollution, and provide better accounting of carbon dioxide from earth's cities.

Technical Abstract: Satellite observations of anthropogenic carbon dioxide (CO2) emissions within urban areas offer unique potential to understand carbon sources and sinks and evaluate carbon mitigation strategies. Despite availability of time series column-averaged dry air mole fraction of CO2 (XCO2) from Orbiting Carbon Observatory-2 (OCO2), temporal variations of XCO2 and their drivers in cities remain poorly understood due to inconsistent definitions of urban extent, diverse urban forms, and unresolved impacts of urban vegetation on carbon fluxes. Using the City Clustering Algorithm, this study delineated spatially consistent urban extent for 48 cities across the globe, based on seasonal and trend modeling of XCO2, vegetation abundance, and fossil fuel consumption. Urban morphological factors including urban size, compactness, and complexity were also estimated from the urban extent maps. Results revealed that OCO-2 XCO2 measurements from 2014 to 2018 exhibited statistically significant seasonal and trend variations for each city, supported with a R2 varying from 0.74 to 0.96 and a RMSE from 0.66 to 1.93 ppm. Correlation analysis suggested a weak association between XCO2 trends and fossil fuel CO2 emissions (FF_CO2) trends but a close relationship between yearly average XCO2 and FF_CO2 trends. Vegetation abundance exhibited a negative relationship with the XCO2 seasonality, though it only explained 21% of the variance. Finally, no statistically significant relationship between urban morphological factors and temporal XCO2 components was observed. However, urban morphological factors had a close relationship (either positive or negative) with the total amount of FF_CO2 aggregated over the study period. Thus, it was speculated that urban morphological factors exerted their influence on XCO2 through fossil fuel consumption. When only cities of high NDVI seasonality component were used, statistically significant correlation coefficients between urban morphological factors and winter/summer averaged XCO2 measurements were found. The variations of these correlation coefficients between leaf-on and leaf-off seasons stressed the important role of urban trees in mitigating carbon emissions in cities. Overall, this study highlighted the value of satellite-based measurements to understand and quantify temporal variations of carbon dioxide emissions in cities.