Spatial Distribution of Atmospheric N<sub>2</sub>O Around a Full-scale Carrousel Reactor

Nitrous oxide (N2O) is a potent greenhouse gas emitted from bioreactors in sewage treatment plants. Quantifying emissions from uncovered reactors remains challenging because of methodological and operational limitations. This study aimed to characterize the three-dimensional spatial distribution of atmospheric N2O concentrations around a full-scale, uncovered Carrousel reactor under various conditions, as a step toward reliable non-invasive quantification methods. Atmospheric gas samples were collected using combined manual and drone-based techniques at multiple heights and locations. The agitator functioned as the primary emission source of high N2O concentrations, and atmospheric N2O levels tended to be elevated near the reactor surface. The emitted N2O was dispersed into the atmosphere under the influence of wind, creating asymmetric and complex vertical patterns. As distance from the emission source increased, concentrations decreased and quantification became more difficult, whereas elevated concentrations were observed near the source even under low wind. These findings demonstrate that atmospheric N2O analysis is a promising non-invasive approach for quantification and may serve as a robust foundation for emission monitoring. Additional data—especially under diverse environmental conditions—are needed to refine and generalize this methodology for practical application.