UAV-based method for measuring CO₂ emissions in forest ecosystems

Abstract. This study addresses the difficulty of accurately quantifying CO₂ emissions in forest ecosystems due to spatial heterogeneity, complex terrain, and the combined effects of horizontal and vertical transport. A UAV-based platform, capable of three-dimensional CO₂ emission observation, was developed, evaluated and applied to a forest ecosystem. The system integrates a high-precision closed-path CO₂ analyzer with a calibrated ultrasonic anemometer, and employs complementary box-pattern and profile-pattern flight strategies within a mass-balance framework to simultaneously resolve horizontal transport and vertical exchange. Validation against a long-term eddy-covariance (EC) flux tower shows that vertical CO₂ fluxes derived using the gradient method agree well with EC observations across seasons (R² ≈ 0.76–0.77). Box-pattern flights further reveal pronounced diurnal variation: lateral advection dominates in the early morning, whereas vertical uptake prevails under well-mixed midday conditions. Sensitivity analyses across scales of 50 m, 100 m, 150 m indicate that CO₂ emission intensity is sensitive to control-volume dimensions and shows spatial heterogeneity above the forest. Uncertainty assessment suggests that the total relative uncertainty of UAV-derived emissions is typically ~15 %, with wind-field calibration as the main error source. Overall, this UAV approach provides a flexible and reliable complement for analyzing near-surface three-dimensional CO₂ transport over complex and heterogeneous forests, helping to overcome the limited spatial coverage of traditional flux towers and remote sensing.