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

We have developed a mid‐infrared continuous‐wave quantum cascade laser direct‐absorption spectrometer (QCLS) capable of high frequency (≥1 Hz) measurements of 12 CH 4 and 13 CH 4 isotopologues of methane (CH 4 ) 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 CH 4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH 4 fluxes of 6.5 ± 0.7 mg CH 4 m −2 hr −1 over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH 4 fluxes (6.6 ± 0.8 mgCH 4 m −2 hr −1 ) over the same period. Mean composition of emitted CH 4 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 CH 4 fluxes at this site, were on average 1.2 ‰ enriched in 13 C compared to diffusive fluxes. CH 4 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 CH 4 modeling analysis.