Design Study for an Airborne N₂O Lidar

Abstract. Nitrous oxide (N₂O) is the third most important greenhouse gas modified by human activities after carbon dioxide and methane. This study examines the feasibility of airborne differential absorption lidar to quantify N₂O emissions from agriculture, fossil fuel combustion, industry and biomass burning. The mid-infrared spectral region, where suitably strong N₂O absorption lines exist, challenges passive remote sensing by means of spectroscopy due to both low solar and terrestrial radiation. Lidar remote sensing is principally possible thanks to the laser as independent radiation source, but has not yet been realized due to technological challenges. Mid-infrared N₂O absorption bands suitable for remote sensing are investigated. Simulations show that a spectral trough position between two strong N₂O lines in the 4.5 µm band is the best option. A second option exists in the 3.9 µm band at the cost of higher laser frequency stability constraints and less measurement sensitivity. Both options fulfill the N₂O measurement requirements for agricultural areal or point source emission quantification (0.5 % measurement precision, 500 m spatial resolution) with technically realizable and affordable transmitter (100 mW average laser power) and receiver (20 cm telescope) characteristics for integrated-path differential absorption lidar that measures the column concentration beneath the aircraft. While a satellite implementation is still out of reach owing to the lack of space-proof technology, the development of an airborne N₂O lidar is feasible, yet would benefit from progress in infrared laser transmitter and low-noise detection technology.