Validation of open-path dual-comb spectroscopy against an O₂ background

Dual-comb spectroscopy measures greenhouse gas concentrations over kilometers of open air with high precision. However, the accuracy of these outdoor spectra is challenging to disentangle from the absorption model and the fluctuating, heterogenous concentrations over these paths. Relative to greenhouse gases, O₂ concentrations are well-known and evenly mixed throughout the atmosphere. Assuming a constant O₂ background, we can use O₂ concentration measurements to evaluate the consistency of open-path dual-comb spectroscopy with laboratory-derived absorption models. To this end, we construct a dual-comb spectrometer spanning 1240 nm to 1700nm, which measures O₂ absorption features in addition to CO₂ and CH₄. O₂ concentration measurements across a 560 m round-trip outdoor path reach 0.1% precision in 10 minutes. Over seven days of shifting meteorology and spectrometer conditions, the measured O₂ has -0.07% mean bias, and 90% of the measurements are within 0.4% of the expected hemisphere-average concentration. The excursions of up to 0.4% seem to track outdoor temperature and humidity, suggesting that accuracy may be limited by the O₂ absorption model or by water interference. This simultaneous O₂, CO₂, and CH₄ spectrometer will be useful for measuring accurate CO₂ mole fractions over vertical or many-kilometer open-air paths, where the air density varies.