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

• Published in: Atmospheric measurement techniques Vol. 10; no. 9; pp. 3295 - 3311
• Main Authors: Waxman, Eleanor M, Cossel, Kevin C, Truong, Gar-Wing, Giorgetta, Fabrizio R, Swann, William C, Coburn, Sean, Wright, Robert J, Rieker, Gregory B, Coddington, Ian, Newbury, Nathan R
• Format: Journal Article
• Language: English
• Published: Germany Copernicus GmbH 11.09.2017; Copernicus Publications, EGU; Copernicus Publications
• Subjects: Absorbance; Absorption; Absorption spectra; Aerodynamics; Agreements; Analytical methods; Atmospheric absorption; Atmospheric gases; Atmospheric turbulence; Calibration; Carbon; Carbon dioxide; Cavity ringdown; Deuteration; Diurnal; Greenhouse gases; Instruments; Intercomparison; Laboratories; Measurement; Methane; Near infrared radiation; Organizations; Sensors; Spectra; Spectrometers; Spectroscopy; Spectrum analysis; Teeth; Telescopes; Temperature variations; Trace gases; Turbulence
• ISSN: 1867-1381; 1867-8548; 1867-8548
• DOI: 10.5194/amt-10-3295-2017

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 (1565 to 1661 nm), corresponding to a 367 cm bandwidth, at 0.0067 cm sample spacing. The measured absorption spectra agree with each other to within 5×10 without any external calibration of either instrument. The absorption spectra are fit to retrieve concentrations for carbon dioxide (CO ), methane (CH ), water (H O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14% (0.57 ppm) for CO , 0.35% (7 ppb) for CH , and 0.40% (36 ppm) for H O 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 CO and CH that are consistent with the presence of local sources of CO and absence of local sources of CH .