Development of an in situ analysis system for methane dissolved in seawater based on cavity ringdown spectroscopy

Abstract

This paper reports the development of a compact in situ real-time concentration analysis system for methane dissolved in seawater by using a continuous-wave cavity ringdown spectroscopy (CRDS) technique. The miniaturized design of the system, including optical resonance cavity and control and data acquisition-analysis electronics, has a cylindrical dimension of 550 mm in length and 100 mm in diameter. Ringdown signal generation, data acquisition and storage, current driver, and temperature controller of the diode laser are all integrated in the miniaturized system circuits, with an electrical power consumption of less than 12 W. Fitting algorithms of the ringdown signal and spectral line are implemented in a digital signal processor, which is the main control chip of the system circuit. The detection sensitivity for methane concentration can reach 0.4 ppbv with an approximate averaging time of 240 s (or 4 min). Comparing the system’s measurement of ambient air against a high-quality commercial CRDS instrument has demonstrated a good agreement in results. In addition, as a “proof of concept” for measuring dissolved methane, the developed instrument was tested in an actual underwater environment. The results showed the potential of this miniaturized portable instrument for in situ gas sensing applications.

A review on the methane emission detection during offshore natural gas hydrate production

“…When the light incident from the optically dense medium to the optically sparse medium, if the incident angle was greater than the critical Angle, the phenomenon of total reflection will be generated. The light wave generated along the parallel direction of the critical surface is the fading wave, and its amplitude decreases exponentially with the distance from the critical surface (Yuan et al, 2020). The sensor based on fading wave has the advantages of high sensitivity, simple design and small size, which is conducive to the realization of sensor miniaturization.…”

Cavity Ring-Down Spectroscopy: Recent Technological Advancements, Techniques, and Applications

“…179 The prototype provided high-sensitive CH 4 measurements with a precision in between 10 and 30 ppb under actual flight conditions. In a recent study by Yuan et al, 180 a compact CRDS system for real-time in situ measurement of dissolved CH 4 in seawater was demonstrated and subsequently field tested in an actual underwater environment with a detection sensitivity of 0.4 ppm with 4 min of data averaging. The measurement accuracy of the prototype CRDS system was further validated in 191 and D 2 O 14 and their isotopologues.…”

A Novel High‐Resolution In Situ Tool for Studying Carbon Biogeochemical Processes in Aquatic Systems: The Lake Aiguebelette Case Study

“…Because of the manual nature of these measurements, only a few or even just one sample is often taken in systems, particularly during multi‐lake surveys (e.g., Rasilo et al., 2015). Recently, however, equilibrator systems have been used to extract dissolved gas from water (either in situ or on site) which is then directed either to a laser‐based optical spectrometer (Gerardo‐Nieto et al., 2019; Gonzalez‐valencia et al., 2014; Grilli et al., 2020; Wankel et al., 2013; Yuan et al., 2020) or to a compact mass spectrometer (Bärenbold et al., 2020; Brennwald et al., 2016; Short et al., 2006) for highly resolved measurements. Note that this is a not extensive list of studies.…”