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From October 2010 to November 2011, the urban atmospheric CO2 concentration in Guangzhou ranged from 550 to 460 ppm, with mean monthly concentration fluctuating between 530 and 470 ppm. A lower concentration was observed in summer and autumn, while a higher concentration occurred in spring and winter. The urban atmospheric CO2 δ13C value varied between -9.00 and -13.10‹, with mean monthly value fluctuating between -9.60 and -11.80‹. There was no significant relationship between the CO2 concentration and δ13C value, reflecting the influence from the fossil-fuelderived CO2 on the urban atmospheric CO2. The urban atmospheric CO2 Δ14C value fluctuated dramatically from 29.1 ± 2.5‹ to -85.2 ± 3.1‹, with a mean annual value of -16.4 ± 3.0‹. A similar seasonal variation of Δ14C value with the concentrations was observed: the higher Δ14C values mainly appeared in summer and autumn (July to September), with a mean value of about -5.2 ± 2.9‹, while lower Δ14C values occurred in spring and winter (December to April), about -27.1 ± 3.2‹ average. Based on the atmospheric Δ14C values, the calculated fossil-fuel-derived CO2 concentrations range between 1 and 58 ppm, with the mean annual concentration around 24 ppm. Similarly, a lower fossil-fuel-derived CO2 concentration appeared in summer and autumn (July to September) with a mean value of ~17 ppm, while the higher fossil-fuel-derived CO2 concentration occurred in spring and winter (December to April) with an average value of ~29 ppm. A comparison of the CO2 concentrations before and after the Guangzhou Asian Games (in November 2010) and the Spring Festival of 2011 confirmed that human activities can greatly decrease the fossil-fuel-derived CO2 emissions to the urban atmosphere in Guangzhou. [ABSTRACT FROM AUTHOR]