Quantifying agricultural N₂O and CH₄ emissions in the Netherlands using an airborne eddy covariance system

Abstract. This study reports on the first successful deployment of a new airborne eddy covariance (EC) setup to better characterize and quantify non-CO₂ greenhouse gas emissions from agriculture. The system was deployed aboard the DLR research aircraft Cessna Caravan to quantify growing-season emissions of methane (CH₄) and nitrous oxide (N₂O) in Friesland, an agricultural region in the Netherlands, in early summer 2023. The EC system consists of a commercial quantum cascade laser spectrometer, specifically adapted for airborne observations and providing 10 Hz data of N₂O and CH₄, and the meteorological measurement suite METPOD, delivering data of the vertical wind, horizontal winds, water vapor and temperature. Our measurements are a novelty for N₂O, since they are the first implementation of quantifying agricultural emissions with airborne EC, combining the advantages of regional-scale coverage, while maintaining high spatial resolution and hence are well suited to capture the spatial complexity of this dominant emission sector. The system provides fluxes with minimal low- and high-frequency distortions, low detection limits, and total uncertainties (30−100 %) comparable to other airborne methods, despite the complexity of agricultural emissions. During measurements in Friesland, we identified clear N₂O emission hotspots and hot-moments, with peak fluxes of 0.34 µg m⁻² s⁻¹ on the regional-scale after intensive precipitation following a relatively dry period. Single small-scale hotspot emissions were as high as 1 µg m⁻² s⁻¹. In contrast, CH₄ fluxes showed less temporal variations around a mean flux of 1.62 µg m⁻² s⁻¹ throughout the three-week campaign. N₂O emissions were relatively high compared to other agricultural regions worldwide, and preliminary comparisons with EDGAR v8.0 and the Dutch emission inventory Emissieregistratie suggest substantial underestimation of growing-season N₂O emissions in current inventories and the lack of an appropriate annual cycle. Our results further document the urgent need for independent verification of reported N₂O and CH₄ emissions from agriculture, which is the most dominant anthropogenic sector of non-CO₂ greenhouse gas emissions and is expected to become even more dominant in the future, with an increasing world population and food demand.