Intercomparison of open-path trace gas measurements with two dual-frequency-comb spectrometers

Abstract. We present the first quantitative intercomparison between two open-path dual-comb spectroscopy (DCS) instruments which were operated across adjacent 2 km open-air paths over a 2-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6023 to 6376 cm⁻¹ (1568 to 1660 nm), corresponding to a 355 cm⁻¹ bandwidth, at 0.0067 cm⁻¹ sample spacing. The measured absorption spectra agree with each other to within 5 × 10⁻⁴ in absorbance without any external calibration of either instrument. The absorption spectra are fit to retrieve path-integrated concentrations for carbon dioxide (CO₂), methane (CH₄), water (H₂O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14 % (0.57 ppm) for CO₂, 0.35 % (7 ppb) for CH₄, and 0.40 % (36 ppm) for H₂O at  ∼  30 s integration time over the 2-week measurement campaign, which included 24 °C outdoor temperature variations and periods of strong atmospheric turbulence. This agreement is at least an order of magnitude better than conventional active-source open-path instrument intercomparisons and is particularly relevant to future regional flux measurements as it allows accurate comparisons of open-path DCS data across locations and time. We additionally compare the open-path DCS retrievals to a World Meteorological Organization (WMO)-calibrated cavity ring-down point sensor located along the path with good agreement. Short-term and long-term differences between the open-path DCS and point sensor are attributed, respectively, to spatial sampling discrepancies and to inaccuracies in the current spectral database used to fit the DCS data. Finally, the 2-week measurement campaign yields diurnal cycles of CO₂ and CH₄ that are consistent with the presence of local sources of CO₂ and absence of local sources of CH₄.