UAV-Borne Two-Dimensional Differential Optical Absorption Spectroscopy for Observing the Spatial Distribution of Near-Surface Trace Gases

Near-surface trace gases exhibit significant spatial heterogeneity, whereas existing observational techniques are unable to simultaneously resolve their horizontal distributions at multiple altitudes with high spatial resolution. To address this limitation, this study developed a lightweight unmanned aerial vehicle (UAV)-borne two-dimensional differential optical absorption spectroscopy (2D-DOAS) system that integrates multi-altitude hovering with multi-azimuth spectral scanning for high-resolution characterization of near-surface NO2, SO2, HCHO, and O4-related optical parameters. Spectral retrievals were performed using the QDOAS software, and the developed system was first validated through synchronous observations with a commercial ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument. The NO2 differential slant column density (DSCD) retrievals from the two systems showed strong agreement, with correlation coefficients greater than 0.90 at all six elevation angles, demonstrating the reliability of the developed system. Following validation, a 20-day field campaign was conducted in Huaibei using multi-altitude hovering observations at 30–110 m above ground level combined with synchronous measurements in 24 azimuth directions at 15° intervals. The observations revealed pronounced horizontal and vertical variability in trace-gas distributions, with enhanced DSCDs associated with local industrial emissions, prevailing winds, and atmospheric transport. These results demonstrate that the proposed UAV-borne 2D-DOAS system provides a reliable and flexible tool for high-resolution monitoring of near-surface trace gases, identification of spatial patterns associated with potential emission sources, and investigation of atmospheric transport processes.