Atmospheric Fossil Fuel CO₂ Traced by Δ¹⁴C in Beijing and Xiamen, China: Temporal Variations, Inland/Coastal Differences and Influencing Factors

One year of atmospheric Δ¹⁴CO₂ were observed in 2014 in the inland city of Beijing and coastal city of Xiamen, China, to trace temporal CO_2ff variations and to determine the factors influencing them. The average CO_2ff concentrations at the sampling sites in Beijing and Xiamen were 39.7 ± 36.1 ppm and 13.6 ± 12.3 ppm, respectively. These contributed 75.2 ± 14.6% and 59.1 ± 26.8% to their respective annual ΔCO₂ offsets over background CO₂ concentrations. Significantly (p < 0.05) high CO_2ff values were observed in winter in Beijing. We did not find any significant differences in CO_2ff values between weekdays and weekends. Diurnal CO_2ff variations were plainly evident, with high values between midnight and 4:00, and during morning and afternoon rush hours. The sampling site in the inland city of Beijing displayed much higher CO_2ff inputs and overall temporal variations than the site in the coastal city of Xiamen. The variations of CO_2ff at both sites were controlled by a combination of emission sources, topography, and atmospheric dispersion. In particular, diurnal observations at the urban site in Beijing showed that CO_2ff was easily accumulated under the southeast wind conditions.