Source Attribution of Atmospheric CO₂ Using ¹⁴C and ¹³C as Tracers in Two Chinese Megacities During Winter

Identifying the sources of atmospheric Carbon dioxide (CO₂) is an important prerequisite for developing effective mitigation strategies. Here we conducted regular observations of the atmospheric CO₂ mixing ratio and its carbon isotope compositions (i.e., Δ¹⁴C and δ¹³C) in Xi'an and Beijing during winter, to estimate source contributions of CO₂ emissions in Chinese megacities. The results showed that CO₂ emissions in both Xi'an and Beijing originated mainly from fossil-fuel sources, which contributed 65 ± 3% and 82 ± 2% of the total CO₂ enhancement, respectively, during the sampling period; the results also revealed a substantial biogenic CO₂ contribution during winter. We further separated the fossil-fuel sources into contributions from coal, oil and natural gas combustions. We found that coal combustion was the dominant anthropogenic source in Xi'an, accounting for 54 ± 4% of the total fossil-fuel emissions, and oil and natural gas contribute almost equally to the emissions. In contrast, emission from natural-gas combustion was the main fossil-fuel source in Beijing, accounting for more than half of the total fossil-fuel emissions, whereas, coal combustion contributed only 17 ± 10%. These top-down results are generally consistent with emission inventory when seasonal variations of emissions are considered; some differences between the two methods indicated that the inventory for Xi'an might be underestimating the emissions from oil consumption. This pilot study confirms the potential of direct verification between top-down and bottom-up methods from the perspective of source attribution.