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.