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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 9 | Copyright
Atmos. Meas. Tech., 10, 3295-3311, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 11 Sep 2017

Research article | 11 Sep 2017

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

Eleanor M. Waxman1, Kevin C. Cossel1, Gar-Wing Truong1, Fabrizio R. Giorgetta1, William C. Swann1, Sean Coburn2, Robert J. Wright2, Gregory B. Rieker2, Ian Coddington1, and Nathan R. Newbury1 Eleanor M. Waxman et al.
  • 1Physical Measurement Laboratory, National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, USA
  • 2Precision Laser Diagnostics Laboratory, University of Colorado Boulder, Boulder, CO 80309, USA

Abstract. We present the first quantitative intercomparison between two open-path dual-comb spectroscopy (DCS) instruments which were operated across adjacent 2km open-air paths over a 2-week period. We used DCS to measure the atmospheric absorption spectrum in the near infrared from 6023 to 6376cm−1 (1568 to 1660nm), corresponding to a 355cm−1 bandwidth, at 0.0067cm−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% (0.57ppm) for CO2, 0.35% (7ppb) for CH4, and 0.40% (36ppm) for H2O at  ∼ 30s 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.

Publications Copernicus
Short summary
In this paper, we compare greenhouse gas measurements from two novel open-path instruments to determine how similarly they measure the same gas concentration. We do this over an outdoor path, so the two instruments measured the same outdoor air. We found that the two instruments agree extremely well, so in the future if the instruments were at two different locations we could reliably compare their measurements to determine differences between the two places.
In this paper, we compare greenhouse gas measurements from two novel open-path instruments to...