Intercomparison of Open-Path Trace Gas Measurements with Two Dual Frequency Comb Spectrometers

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 two-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6021 to 6388 cm^-1 (1565 to 1661 nm), corresponding to a 367 cm^-1 bandwidth, at 0.0067 cm^-1 sample spacing. The measured absorption spectra agree with each other to within 5×10^-4 without any external calibration of either instrument. The absorption spectra are fit to retrieve 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 over the two-week measurement campaign, which included 23 °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 WMO-calibrated cavity ringdown point sensor located along the path with good agreement. Short-term and long-term differences between the two systems are attributed, respectively, to spatial sampling discrepancies and to inaccuracies in the current spectral database used to fit the DCS data. Finally, the two-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.