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

The CO and CO 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 and CO, the sources of different-height CO, and the influence of air mass transport. The CO 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 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 ΔC and δC vertical measurements, the results showed that fossil fuel CO (CO, 56.1 ± 15.2 %), which mainly come from coal combustion (81.2 ± 3.4 %), was the main contributor to CO levels in excess of the background level (CO) during early winter observations. In contrast, biological CO (CO) dominated CO in summer observations. The vertical distributions of CO in early winter observations and CO in summer observations were consistent with those of CO during early winter and summer observations, respectively. The strong correlation between winter CO and ΔCO (r = 0.81, p < 0.01) indicated that biomass burning was the main contributor to CO 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 in scientific support of formulating carbon emission-reduction strategies.