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Abstract
Abstract. Simultaneous measurement of C2H6 and CH4 concentrations, and of the δ13C-CH4 isotope ratio is demonstrated using a cavity-enhanced absorption spectroscopy technique in the mid-IR region. The spectrometer is compact and has been designed for field operation. It relies on optical-feedback-assisted injection of 3.3 µm radiation from an interband cascade laser (ICL) into a V-shaped high-finesse optical cavity. A minimum absorption coefficient of 2.8×10-9 cm−1 is obtained in a single scan (0.1 s) over 0.7 cm−1. Precisions of 3 ppbv, 11 ppbv, and 0.08 ‰ for C2H6, CH4, and δ13C-CH4, respectively, are achieved after 400 s of integration time. Laboratory calibrations and tests of performance are reported here. They show the potential for the spectrometer to be embedded in a sensor probe for in situ measurements in ocean waters, which could have important applications for the understanding of the source and fate of hydrocarbons from the seabed and in the water column.
Highlights
Methane (CH4) is the second most abundant anthropogenic greenhouse gas after carbon dioxide (CO2), but with a 25 times higher global warming potential
While biogenic gas has a high concentration of CH4 with respect to heavier HCs and a more negative δ13C-CH4, thermogenic gas is characterized by a lower ratio of CH4/C2H6 and a less negative δ13C-CH4 signature (−50 ‰ to −40 ‰)
In this work we report a novel application for the simultaneous measurement of C2H6, CH4, and δ13C-CH4, using an interband cascade laser (ICL) radiation source in the mid-IR region that could be applied to the in situ measurement of these compounds in seawater
Summary
Methane (CH4) is the second most abundant anthropogenic greenhouse gas after carbon dioxide (CO2), but with a 25 times higher global warming potential. It is important to document the origin and fate of methane and ethane present below the seabed and dissolved in the water column, for process understanding and for future climate and ocean acidity projections. Standard techniques for dissolved gas measurements usually rely on discrete sampling of the water column using Niskin bottles, followed by laboratory analysis This method has the advantage of being simple and easy to set up, but it suffers from possible artifacts of the measurements due to outgassing of sample during the ascent ( for deep water samples). In this work we report a novel application for the simultaneous measurement of C2H6, CH4, and δ13C-CH4, using an ICL radiation source in the mid-IR region that could be applied to the in situ measurement of these compounds in seawater
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