Atmospheric CO₂ and ¹⁴CO₂ observations at the northern foot of the Qinling Mountains in China: Temporal characteristics and source quantification

A two-year (March 2021 to February 2023) continuous atmospheric CO₂ and a one-year regular atmospheric ¹⁴CO₂ measurement records were measured at the northern foot of the Qinling Mountains in Xi'an, China, aiming to study the temporal characteristics of atmospheric CO₂ and the contributions from the sources of fossil fuel CO₂ (CO_2ff) and biological CO₂ (CO_2bio) fluxes. The two-year mean CO₂ mole fraction was 442.2 ± 16.3 ppm, with a yearly increase of 4.7 ppm (i.e., 1.1 %) during the two-year observations. Seasonal CO₂ mole fractions were the highest in winter (452.1 ± 17.7 ppm) and the lowest in summer (433.5 ± 13.3 ppm), with the monthly CO₂ levels peaking in January and troughing in June. Diurnal CO₂ levels peaked at dawn (05:00–07:00) in spring, summer and autumn, and at 10:00 in winter. ¹⁴C analysis revealed that the excess CO₂ (CO_2ex, atmospheric CO₂ minus background CO₂) at this site was mainly from CO_2ff emissions (67.0 ± 26.8 %), and CO_2ff mole fractions were the highest in winter (20.6 ± 17.7 ppm). Local CO enhancement above the background mole fraction (ΔCO) was significantly (r = 0.74, p < 0.05) positively correlated with CO_2ff in a one-year measurement, and ΔCO:CO_2ff showed a ratio of 23 ± 6 ppb/ppm during summer and winter sampling days, much lower than previous measurements and suggesting an improvement in combustion efficiency over the last decade. CO_2bio mole fractions also peaked in winter (14.2 ± 9.6 ppm), apparently due to biomass combustion and the lower and more stable wintertime atmospheric boundary layer. The negative CO_2bio values in summer indicated that terrestrial vegetation of the Qinling Mountains had the potential to uptake atmospheric CO₂ during the corresponding sampling days. This site is most sensitive to local emissions from Xi'an and to short distance transportation from the southern Qinling Mountains through the valleys.