A Fast-response automated gas equilibrator (FaRAGE) for continuous in situ measurement of methane dissolved in water

Abstract

Abstract. Biogenic methane (CH4) emissions from inland waters contribute substantially to global warming. In aquatic systems, CH4 dissolved in freshwater lakes and reservoirs is highly heterogeneous both in space and time. To better understand the biological and physical processes that affect sources and sinks of CH4 in lakes and reservoirs, dissolved CH4 needs to be measured with a highest temporal resolution. To achieve this goal, we developed the Fast-Response Automated Gas Equilibrator (FaRAGE) for real-time in situ measurement of dissolved CH4 concentration at the water surface and in the water column. FaRAGE can achieve an exceptionally short response time (t95 % = 12 s when including the response time of the gas analyzer) while retaining an equilibration ratio of 63 % and a measurement accuracy of 0.5 %. An equilibration ratio as high as 91.8 % can be reached at the cost of a slightly increased response time (16 s). The FaRAGE is capable of continuously measuring dissolved CH4 concentrations in the nM-to-mM (10−9–10−3 mol L−1) range with a detection limit of sub-nM (10−10 mol L−1), when coupled with a cavity ring-down greenhouse gas analyzer (Picarro GasScouter). It enables the possibility of mapping dissolved CH4 concentration in a quasi three-dimensional manner in lakes. The FaRAGE is simple to operate, inexpensive, and suitable for continuous monitoring with a strong tolerance to suspended particles. The easy adaptability to other gas analyzers such as Ultra-portable Los Gatos and stable isotopic gas analyzer (Picarro G2132-i) also provides the potential for many further applications, e.g. measuring dissolved 13δC-CH4 and CO2.

Field-Portable dissolved gas sensing and perspectives in aqueous microplastic detection

“…This section presents the design of each of these components. The gas extractor design is based on the FaRAGE (Fast-Response Automated Gas Equilibrator) system presented in Xiao et al (2020) [68], redesigned and built here using low-cost equipment, totaling just under < $350 cost (Table Both the gas extraction system and sensor payload are powered by a portable power system. The gas paths are drawn in black, the water paths are drawn in blue, and the power paths are drawn in red.…”

“…The LC-GEMS uses the same implementation because this part of the FaRAGE method is low-cost and simple by design. [68] and the extraction approach used in the LC-GEMS gas extraction stage. The passive pressure regulation in the water-air mixing chamber allows the air and water flow balance to be slightly imprecise without endangering the sensor payload.…”

“…Low cost COTS sensors, like the K30 CO 2 sensors, do not match the precision and robustness of lab grade spectrometers and gas analyzers, like those used in other in situ dissolved gas measurement systems [50,65,68]. To correct for the relatively large (10s of ppm) root mean square errors (RMSEs) of these sensors, a multivariate linear regression model was used to correct for the effects of H, T, and P on the sensor reading, as previously presented for K30 CO 2 sensors in Martin et al (2017) [77].…”

“…Figure 2-2: The gas extraction system used in the LC-GEMS. The middle labeled portion (gas-water mixing state, gas-water mixing tubing, gas-water seperation stage, and the membrane and desiccant drying elements) are of the FaRAGE system as presented by Xiao et al (2020)[68] .…”

“…2.2.2 State of the art in dissolved CO 2 sampling and analysis 95% = 10 seconds for CO 2 ) when integrated with commercial gas analyzers[68].These emergent techniques show promise in making continuous, in situ gas extraction for gas sensors more accessible and portable. However, the techniques used by Nicholson et al (2018), Crawford et al (2015), and Xiao et al (2020) all make use…”