Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer

Grilli, Roberto; Darchambeau, François; Chappellaz, Jérôme; Mugisha, Ange; Triest, Jack; Umutoni, Augusta

We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol Linline-formula−1 to mmol Linline-formula−1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to inline-formula±22 % (inline-formula2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to inline-formula±0.112 mbar of inline-formulaCH4 in water or inline-formula±160 nmol Linline-formula−1 at 25 inline-formulaC and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of inline-formulaCH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol Linline-formula−1. Deeper down, dissolved inline-formulaCH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of inline-formulaCH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of inline-formulaCH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved inline-formulaCO2 and total dissolved gas pressure, for exploring settings with very high concentrations of inline-formulaCH4 such as the bottom waters of Lake Kivu.

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Grilli, Roberto / Darchambeau, François / Chappellaz, Jérôme / et al: Continuous in situ measurement of dissolved methane in Lake Kivu using a membrane inlet laser spectrometer. 2020. Copernicus Publications.

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