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

Abstract

Abstract. We present the first quantitative intercomparison between two open-path dual-comb spectroscopy (DCS) instruments which were operated across adjacent 2 km openair 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 (CO 2 ), methane (CH 4 ), water (H 2 O), and deuterated water (HDO). The retrieved dry mole fractions agree to 0.14 % (0.57 ppm) for CO 2 , 0.35 % (7 ppb) for CH 4 , and 0.40 % (36 ppm) for H 2 O 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. Shortterm 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 CO 2 and CH 4 that are consistent with the presence of local sources of CO 2 and absence of local sources of CH 4 .

Long open-path measurements of greenhouse gases in air using near-infrared Fourier transform spectroscopy

“…Table 2 and Fig. 13 show Allan deviations (ADs, the square root of Allan Variance, Werle et al, 1993) for open-path and in situ CO 2 , CH 4 and O 2 measurements and the open pathin situ differences. The ADs in Table 2 and Fig.…”

Ground-Based Remote Sensing and Imaging of Volcanic Gases and Quantitative Determination of Multi-Species Emission Fluxes

“…Future developments for active IR spectroscopy could use Dual frequency comb spectroscopy (e.g., [42]), which-although it works differently-has some similarities to a combination of TDLS plus FT spectrometer.…”

Bootstrap inversion technique for atmospheric trace gas source detection and quantification using long open-path laser measurements

“…First, n 2 min dispersion calculations gathered over longer timescales (n is between 30 and 63 for field data tests shown here) are aggregated for use in a single inversion, which is accepted practice (Scire et al, 2000). Second, both sources and receptors are close to the surface, which may help to mitigate crosswind-integrated concentration fluctuation intensity (Weil et al, 2012). Third, a sensitivity test in which we adjust the horizontal dispersion coefficient, σ y , for shorter time averaging, using the methods of Gifford (1976), shows negligible changes in the results (Supplement).…”