Mass fluxes and isofluxes of methane (CH4) at a New Hampshire fen measured by a continuous wave quantum cascade laser spectrometer

• Published in: Journal of Geophysical Research 2012-05, Vol.117 (D10), p.D10301-n/a
• Main Authors: Santoni, Gregory W., Lee, Ben H., Goodrich, Jordan P., Varner, Ruth K., Crill, Patrick M., McManus, J. Barry, Nelson, David D., Zahniser, Mark S., Wofsy, Steven C.
• Format: Article
• Language: English
• Subjects: Atmospheric sciences; Earth sciences; Earth, ocean, space; eddy-covariance; Exact sciences and technology; Geophysics; isoflux; Methane; Methanogenesis; Scientific apparatus & instruments; spectrometer; Troposphere; wetland
• ISSN: 0148-0227; 2169-897X; 2156-2202; 2156-2202; 2169-8996
• DOI: 10.1029/2011JD016960

We have developed a mid‐infrared continuous‐wave quantum cascade laser direct‐absorption spectrometer (QCLS) capable of high frequency (≥1 Hz) measurements of 12CH4 and 13CH4 isotopologues of methane (CH4) with in situ 1‐s RMS precision of 1.5 ‰ and Allan‐minimum precision of 0.2 ‰. We deployed this QCLS in a well‐studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 ± 0.7 mg CH4 m−2 hr−1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 ± 0.8 mgCH4 m−2 hr−1) over the same period. Mean composition of emitted CH4 calculated using EC isoflux methods was −71 ± 8 ‰ (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of −64.5 ± 0.8 ‰. Ebullitive fluxes, representing ∼10% of total CH4 fluxes at this site, were on average 1.2 ‰ enriched in 13C compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process‐based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis. Key Points QCL spectrometer is capable of 1‐sec RMS del13C_CH4 precision of 1.5 permil Chamber and EC measurements yield consistent source isotopic composition CH4 isofluxes can improve process‐based understanding of methanogenesis