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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. Plain Language Summary: Quantifying Carbon dioxide (CO2) sources from cities is crucial for formulating policies and evaluating targets because cities have become the key region and basic unit for regulating emissions. Here, we take Xi'an and Beijing, two Chinese megacities, as the study case to conduct source attribution of atmospheric CO2 based on the powerful tracer ability of dual carbon isotopes (14C and 13C). We estimated the emission contributions of fossil source CO2 (CO2ff) and the fractions of different fossil fuels in CO2ff. We also performed independent evaluation for emission inventory data by the direct comparisons between the fractions of different fossil fuels obtained from "top‐down" and "bottom‐up" methods. This study can provide a new perspective for the verification of urban carbon emission data, and is of great significance to further improve the reliability of carbon budgets for Chinese cities. Key Points: The Carbon dioxide (CO2) emission patterns regarding the source contributions are completely different between Beijing and Xi'anBiogenic CO2 is also an important source for megacities and need to be considered in urban carbon budgetsWe confirm the potential of direct verification between top‐down and bottom‐up methods from a new perspective [ABSTRACT FROM AUTHOR]

• Beijing haze is strongly related to enhanced atmospheric oxidizing capacity. • Two regimes separated by [O 3 ] of 80 μg m−3: AOC-sufficient and -deficient regimes. • Increasing oxidation capacity considerably weakens the emission mitigation effort. Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM 2.5) concentrations have not decreased, and particularly, ozone (O 3) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O 3 and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution. The WRF-Chem model simulations show elevated O 3 concentrations and high fractions of oxygenated secondary aerosols (OSA) in PM 2.5 (0.53–0.73) during the severe pollution period. During daytime there exhibits an AOC-sufficient regime with the persistently high OSA fraction and an AOC-deficient regime with varied OSA fractions, separated by the O 3 level of 80 μg m−3. Our results suggest that increasing AOC can considerably weaken the emission mitigation effort by enhancing the secondary aerosol formation [ABSTRACT FROM AUTHOR]