Vertical measurements of atmospheric CO 2 and 14 CO 2 at the northern foot of the Qinling Mountains in China.

The CO ₂ and ¹⁴ CO ₂ 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 CO ₂ and ¹⁴ CO ₂ , the sources of different-height CO ₂ , and the influence of air mass transport. The CO ₂ 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 CO ₂ 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 Δ ¹⁴ C and δ ¹³ C vertical measurements, the results showed that fossil fuel CO ₂ (CO _2ff , 56.1 ± 15.2 %), which mainly come from coal combustion (81.2 ± 3.4 %), was the main contributor to CO ₂ levels in excess of the background level (CO _2ex ) during early winter observations. In contrast, biological CO ₂ (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 CO ₂ 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 CO ₂ in scientific support of formulating carbon emission-reduction strategies.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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