Spatio-temporal variability of atmospheric CO₂ and its main causes: A case study in Xi'an city, China

Cities account for most anthropogenic carbon dioxide (CO₂) emissions, and thus play an important role in carbon mitigation. However, the variability of atmospheric CO₂ caused by the complex emission sources and topography within city remains a challenge in evaluating urban CO₂ emissions. Specifically, the mechanism of spatial and temporal patterns is still unclear at city scale. In this work, we explored the spatial and seasonal patterns of atmospheric CO₂ at the city scale in Xi'an and attempted to illustrate the mechanisms of these patterns based on the analysis of CO₂ sources and sinks. Using the observed radiocarbon (¹⁴C) in atmospheric CO₂ as a tracer, we determined the CO₂ emissions from fossil fuel (CO_2ff) and the biosphere (CO_2bio) in different seasons and city areas. The results showed that CO_2ff in urban areas is significantly (p < 0.05) higher than that in suburban areas, and explained 82% and 74% of the spatial differences in winter and summer, respectively. Based on the analysis of δ¹³C of CO₂ in the local sources, the depleted δ¹³C of urban site indicated greater influences from traffic or natural gas in the urban area than that in the suburban area. These findings suggest that the spatial differences of atmospheric CO₂ are dominated by different emission intensities of fossil fuels between urban and suburban areas. In contrast, the contributions of CO_2ff to the seasonal variations of atmospheric CO₂ were different between urban and suburban areas, that is CO_2ff contributed as much as CO_2bio to the seasonal variations of atmospheric CO₂ in suburban area, but the contribution increased with the increasing of human activities from suburban to urban area, and accounted for 57% of seasonal variations of atmospheric CO₂ in the urban area. In addition, we also found that under the influence of the special topography of Xi'an city, the frequent fluctuations of atmospheric CO₂ concentration in winter largely depend on the different wind conditions.