Abstract: Background, aim, and scope Anthropogenic combustion of fossil fuel has emitted large quantities of CO2 to the atmosphere, which is closely related to global warming. More than 70% of global fossil fuel CO2 (CO2ff) emissions are concentrated in urban areas. Therefore, quantifying CO2ff, especially in the urban areas, is essential for us to formulate emission reduction strategies and understand earth’s carbon cycle. CO2ff contains no 14C because the half-life of 14C (5730 years) is much less than the age of fossil fuels. The difference between the 14C compositions of CO2 derived from fossil fuels and from modern CO2 sources is huge and this contrast makes 14C a unique tracer for quantifying CO2ff. Materials and methodsIn this study, we used self-designed active absorption system with a molecular sieve to continuously collect atmospheric CO2 samples from January 2016 to January 2017 in Xi’an. This active absorption system is based on the pressure gradient produced by the water flowing out of a glass bottle drop by drop. The samples were collected at a flow rate of 5 mL/min. When sampling finished, the molecular sieve device was inserted into the tube furnace and connected to the vacuum system with a metal joint to desorb and purify CO2 by cryogenic trapping. The purified CO2 was converted into graphite using the Zn-Fe method, in which Zn power is used as a reductant and Fe as a catalyst. The graphite was then pressed into an aluminum holder for 14C measurements using 3 MV accelerator mass spectrometer (AMS) in Xi’an, China, with a precision of 2‰ for 14C measurement. The atmospheric CO2 concentrations were measured using a Picarro G2131-i CO2 Isotopic Analyzer.ResultsFrom January 2016 to January 2017, the atmospheric Δ14C in Xi’an fluctuated significantly, ranging from (-1.00±2.84)‰ to (-187.25±3.62)‰; and the average is (-63.20±17.35)‰, which had a significant decline compared with the average value (-41.3±27.4)‰ during 2012—2013. The concentrations of CO2ff showed significantly seasonal variations, and varied from (6.91±1.94) μmol∙mol−1 (July 2016) to (105.60±3.09) μmol∙mol−1 (January 2017). The average concentration in winter ((53.22±11.78) μmol∙mol−1) was significantly higher than that in summer ((15.16±6.63) μmol∙mol−1).Discussion Based on the homology between CO2ff and air pollutants, we analyzed the relationships between CO2ff and air pollutants to figure out the main sources of CO2ff. The concentrations of CO2ff shared a similar trend over study period with air pollutants. However, the relationships between CO2ff and SO2/NO2 were different in different seasons. In spring and summer (from mid-March to August 2016), the concentrations of CO2ff showed stronge relationship with SO2 (R2=0.47, p<0.01), and had a weak correlation with NO2 (R2=0.28, p=0.02). In autumn and winter (from September 2016 to January 2017), the concentrations of CO2ff were strongly correlated with NO2 and SO2, but the correlation with NO2 was more significant (R2=0.73, p<0.01), and the correlation with SO2 was relatively weak (R2=0.46, p<0.01). This may be due to the different atmospheric diffusion conditions affected the contribution of industrial coal consumpution, which is maily from high-altitude point source (power plants), and the contribution of vehicles emissions to the CO2ff at sampling site. In spring and summer, atmospheric diffusion condition was conducive for vertical mixing and horizontal delivery of air pollutants, CO2ff at sampling site may be mainly affected by coal consumption emissions; in the autumn and winter, due to the adverse diffusion conditions, such as calm winds, the contribution of vehicle exhaust to CO2ff may significantly increase. Conclusions AMS-14C is an effective way to quantify CO2 emitted by fossil fuel consumption in cities. From January 2016 to January 2017, CO2ff concentrations showed significantly seasonal variations in Xi’an, with lowest values in summer and highest values in winter. By the relationships between CO2ff and air pollutants, this study qualitatively analyzed the main sources of CO2ff in different seasons. Due to the different atmospheric diffusion conditions, in spring and summer, CO2ff at the sampling site may be mainly affected by industrial coal combustion, while in autumn and winter, mainly by vehicles emissions. Recommendations and perspectives This study helps us to understand the seasonal variation characteristics of atmospheric CO2ff . And the homology between CO2ff and air pollutants will provide us a new idea for source analysis of CO2ff. Moreover, it’s important for us to select an appropriate sampling site in order to accurately analyse the impact of specific emission sources on CO2ff.
Keywords: AMS-14C tracing; molecular sieve; atmospheric Δ14C; fossil fuel CO2; seasonal variation