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

Given that cities contributed most of China's CO emissions, understanding the emission characteristics of urban atmospheric CO is critical for regulating CO emissions. Regular observations of atmospheric CO concentration, △C and δC values were performed at four different sites in Xi'an, China in 2016 to illustrate the temporal and spatial variations of CO emissions and recognize their sources and sinks in urban carbon cycles. We found seasonal variations in CO concentration and δC values, the peak to peak amplitude of which was 80.8ppm for CO concentration and 4.0‰ for its δC. With regard to the spatial variations, the urban CO "dome" effect was the most pronounced during the winter season. The use of △C combines with δC measurements aid in understanding the emission patterns. The results show that in the winter season, emissions from fossil fuel derived CO (CO) contributed 61.8±10.6% and 57.4±9.7% of the excess CO (CO) in urban and suburban areas respectively. Combining with the result of estimated δC value of fossil fuel (δC=-24‰), which suggest coal burning was the dominant source of fossil fuel emissions. In contrast, the proportions of CO in CO 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 δC value of local sources (δC) 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.