Emission characteristics of atmospheric carbon dioxide in Xi'an, China based on the measurements of CO₂ concentration, △¹⁴C and δ¹³C

Given that cities contributed most of China's CO₂ emissions, understanding the emission characteristics of urban atmospheric CO₂ is critical for regulating CO₂ emissions. Regular observations of atmospheric CO₂ concentration, △¹⁴C and δ¹³C values were performed at four different sites in Xi'an, China in 2016 to illustrate the temporal and spatial variations of CO₂ emissions and recognize their sources and sinks in urban carbon cycles. We found seasonal variations in CO₂ concentration and δ¹³C values, the peak to peak amplitude of which was 80.8 ppm for CO₂ concentration and 4.0‰ for its δ¹³C. With regard to the spatial variations, the urban CO₂ “dome” effect was the most pronounced during the winter season. The use of △¹⁴C combines with δ¹³C measurements aid in understanding the emission patterns. The results show that in the winter season, emissions from fossil fuel derived CO₂ (CO_2ff) contributed 61.8 ± 10.6% and 57.4 ± 9.7% of the excess CO₂ (CO_2ex) in urban and suburban areas respectively. Combining with the result of estimated δ¹³C value of fossil fuel (δ¹³C_ff = − 24‰), which suggest coal burning was the dominant source of fossil fuel emissions. In contrast, the proportions of CO_2ff in CO_2ex varied more in the summer season than that in the winter season, ranging from 42.3% to > 100% with the average contributions of 82.5 ± 23.8% and 90.0 ± 24.8%. Given the estimation of δ¹³C value of local sources (δ¹³C_s) was − 21.9‰ indicates that the intensively biogenic activities, such as soil respiration and corn growth have significantly impacted urban carbon cycles, and occasionally played a role of carbon sink.