Vertical measurements of atmospheric CO₂ and ¹⁴CO₂ at the northern foot of the Qinling Mountains in China

The CO₂ and ¹⁴CO₂ levels in air samples from the northern foot of the Qinling Mountains (Xi'an, China) were determined. In 2021, a hexacopter unmanned aerial vehicle sampled air at different heights, from near-ground to 2000 m. The objectives of this study were to determine vertical characteristics of CO₂ and ¹⁴CO₂, the sources of different-height CO₂, and the influence of air mass transport. The CO₂ concentrations mainly exhibited a slight decreasing trend with increasing height during summer observations, which was in contrast to the increasing trend that was followed by a subsequent gradual decreasing trend during early winter observations, with peak CO₂ levels (443.4 ± 0.4–475.7 ± 0.5 ppm) at 100–500 m. The variation in vertical concentrations from 20 to 1000 m in early winter observations (21.6 ± 19.3 ppm) was greater than that in summer observations (14.6 ± 14.3 ppm), and the maximum vertical variation from 20 to ∼2000 m reached 61.1 ppm. Combining Δ¹⁴C and δ¹³C vertical measurements, the results showed that fossil fuel CO₂ (CO_2ff, 56.1 ± 15.2 %), which mainly come from coal combustion (81.2 ± 3.4 %), was the main contributor to CO₂ levels in excess of the background level (CO_2ex) during early winter observations. In contrast, biological CO₂ (CO_2bio) dominated CO_2ex in summer observations. The vertical distributions of CO_2ff in early winter observations and CO_2bio in summer observations were consistent with those of CO₂ during early winter and summer observations, respectively. The strong correlation between winter CO_2bio and ΔCO (r = 0.81, p < 0.01) indicated that biomass burning was the main contributor to CO_2bio during early winter observations. Approximately half of the air masses originated from the Guanzhong Basin during observations. The results provide insights into the vertical distribution of different-sources of atmospheric CO₂ in scientific support of formulating carbon emission-reduction strategies.