Emission characteristics of atmospheric carbon dioxide in Xi'an, China based on the measurements of CO 2 concentration, △ 14 C and δ 13 C.

Given that cities contributed most of China's CO ₂ emissions, understanding the emission characteristics of urban atmospheric CO ₂ is critical for regulating CO ₂ emissions. Regular observations of atmospheric CO ₂ concentration, △ ¹⁴ C and δ ¹³ C values were performed at four different sites in Xi'an, China in 2016 to illustrate the temporal and spatial variations of CO ₂ emissions and recognize their sources and sinks in urban carbon cycles. We found seasonal variations in CO ₂ concentration and δ ¹³ C values, the peak to peak amplitude of which was 80.8ppm for CO ₂ concentration and 4.0‰ for its δ ¹³ C. With regard to the spatial variations, the urban CO ₂ "dome" effect was the most pronounced during the winter season. The use of △ ¹⁴ C combines with δ ¹³ C measurements aid in understanding the emission patterns. The results show that in the winter season, emissions from fossil fuel derived CO ₂ (CO _2ff ) contributed 61.8±10.6% and 57.4±9.7% of the excess CO ₂ (CO _2ex ) in urban and suburban areas respectively. Combining with the result of estimated δ ¹³ C value of fossil fuel (δ ¹³ C _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 δ ¹³ C value of local sources (δ ¹³ C _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.
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