Airborne performance assessment of the DLR MIRO MGA³ quantum cascade laser spectrometer for fast N₂O measurements

Abstract. Nitrous oxide (N₂O) is the most dominant precursor of ozone-depleting substances and the third most important anthropogenic greenhouse gas, with agriculture contributing the largest share of emissions (56 %). Over the past two decades, airborne in-situ measurements have become increasingly important for studying emission and transport of N₂O in the atmosphere, also driven by the development of more precise and simultaneously faster Quantum Cascade Laser (QCL)-based spectrometers. However, many QCL-based spectrometers exhibit sensitivities to environmental and flight-related parameters that can vary rapidly (≤ 1 s), such as static or cabin pressure and aircraft roll and pitch angles. The impact of changing ambient water vapor is particularly critical due to both dilution and quantum-mechanical effects. Because the variability of N₂O in the lower troposphere is often very small (<1 ppb) relative to its high background (∼ 338 ppb), even minor variations in these parameters can significantly affect data quality and must be corrected. Although many instruments can resolve such small concentration changes, their typical temporal resolution of 1 Hz may limit the application of advanced measurement techniques such as eddy covariance on fast-moving aircraft. Here, we present and evaluate a new instrument setup for precise (< 0.2 ppb) and high-frequency (10 Hz) airborne in-situ N₂O measurements in altitudes up to 4500 m based on a MIRO MGA³ QCL spectrometer (MIRO Analytical AG). The instrument was successfully deployed during two airborne science missions, namely onboard the unpressurized DLR Cessna during the Greenhouse Gas Monitoring (GHGMon) campaign in 2023 in the Netherlands, as well as onboard the NASA DC8 during the Satellite Investigation of the Asian Air Quality (ASIA-AQ) campaign in 2024. Specifically, we evaluate and compare different water vapor correction approaches using ASIA-AQ data sampled within the tropical boundary layer over South Korea, the Philippines, Thailand and Taiwan, which partly were characterized by specifically high ambient humidity (up to 30000 ppm H₂O). The water vapor correction methods include an empirical approach that relies on both native and corrected MIRO in-flight water vapor measurements, as well as an approach by an updated version of the MIRO specific fitting software. Comparison with N₂O measurements from a well-established instrument onboard the NASA DC8 shows agreement within combined measurement uncertainties for all tested water vapor correction approaches, albeit special caution is needed for humidities larger than 15000 ppm. The new water vapor corrected data shows a 42 % better precision than with original default settings of the instrument. We further show that the instrument setup is insensitive to flight parameter changes such as roll and pitch angle of the aircraft, and allows for stable measurements even under challenging conditions such as in the turbulent boundary layer. This instrument setup enables improved characterization of N₂O emissions and sources from the agricultural sector which is, in particular, relevant for tropical regions with strong agricultural activity and high humidity, where observational data remain scarce.