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 80.8ppm 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 (CO2ff) contributed 61.8±10.6% and 57.4±9.7% of the excess CO2 (CO2ex) in urban and suburban areas respectively. Combining with the result of estimated δ13C value of fossil fuel (δ13Cff=-24‰), which suggest coal burning was the dominant source of fossil fuel emissions. In contrast, the proportions of CO2ff in CO2ex 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 (δ13Cs) 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.