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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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doi:10.5194/amt-2017-62
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
14 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
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 1Physical Measurement Laboratory, National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305
2Precision Laser Diagnostics Laboratory , University of Colorado Boulder, Boulder, CO 80309
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 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.

Citation: Waxman, E. M., Cossel, K. C., Truong, G.-W., Giorgetta, F. R., Swann, W. C., Coburn, S., Wright, R. J., Rieker, G. B., Coddington, I., and Newbury, N. R.: Intercomparison of Open-Path Trace Gas Measurements with Two Dual Frequency Comb Spectrometers, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2017-62, in review, 2017.
Eleanor M. Waxman et al.
Eleanor M. Waxman et al.
Eleanor M. Waxman et al.

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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...
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