A Fast-Response Automated Gas Equilibrator (FaRAGE) for continuous in situ measurement of CH4 and CO2 dissolved in water

Biogenic greenhouse gas emissions, e.g., of methane (CH4) and carbon dioxide (CO2) from inland waters, contribute substantially to global warming. In aquatic systems, dissolved greenhouse gases are highly heterogeneous in both space and time. To better understand the biological and physical processes that affect sources and sinks of both CH4 and CO2, their dissolved concentrations need to be measured with high spatial and temporal resolution. To achieve this goal, we developed the Fast-Response Automated Gas Equilibrator (FaRAGE) for real-time in situ measurement of dissolved CH4 and CO2 concentrations 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 62.6 % and a measurement accuracy of 0.5 % for CH4. A similar performance was observed for dissolved CO2 (t95 % = 10 s, equilibration ratio 67.1 %). 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 CO2 and CH4 concentrations in the nM-to-sub mM (10−9–10−3 mol L−1) range with a detection limit of subnM (10−10 mol L−1), when coupling with a cavity ring-down greenhouse gas analyzer (Picarro GasScouter). FaRAGE allows for the possibility of mapping dissolved concentration in a “quasi” three-dimensional manner in lakes and provides an inexpensive alternative to other commercial gas equilibrators. It is simple to operate and suitable for continuous monitoring with a strong tolerance for suspended particles. While the FaRAGE is developed for inland waters, it can be also applied to ocean waters by tuning the gas–water mixing ratio. The FaRAGE is easily adapted to suit other gas analyzers expanding the range of potential applications, including nitrous oxide and isotopic composition of the gases.


S1. Details of parts, gas analyzers and costs
To make the FaRAGE field deployable, parts were tightly packed into an aluminum box 22 with a built-in power supply. The electric parts were separated from other parts containing 23 water in the box by using a plastic board. Ports were well labelled on the right-handed side so 24 that even somebody new to the system can work with it. To help interested readers rebuild the 25 device, the two key components (gas-water mixing unit and gas-water separation unit) were 26 shown in the detailed technical drawings (Fig. S1). The suppliers and costs for these parts 27 were listed in Table S1. A total of 3,560 € was calculated for building the complete device 28 excluding the costs for the power supply. As the expensive RBR temperature logger is not a 29 necessity since we happen to have it in storage, a cheaper temperature logger can always be 30 used. For example, a fast HOBO temperature logger (HOBO U12 with a Temperature probe 31 TMC1-HD) is available for < 200 €. The total cost can be cut down significantly to < 3,000 €.

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The FaRAGE is capable of coupling with different greenhouse gas analyzers, depending 33 on the research question and instrument availability. Three most widely used field-deployable 34 gas analyzers were compared in Table S2 to provide a reference for readers when choosing a 35 gas analyzer. They are GasScouter G4301 (Picarro, USA), Ultraportable Greenhouse Gas 36 Analyzer (Model 915-0011, LosGatos Research, USA) and Picarro G2132-i isotope analyzer 37 (Picarro, USA). We noticed Picarro 2201-i has been more often used, but our Picarro G2132-i 38 is an equivalent instrument except that the module for isotopic CO2 is not installed. The 39 former two instruments measure CH4, CO2 and H2O and the last one additionally measures 40 stable isotopic CH4. As shown in Table 2, clearly GasScouter G4301 is most suitable for field 41 measurement of dissolved CH4 concentrations due to its extremely high mobility. The built-in 42 battery pack can support 8 h continuous measurements and the ability to amount GPS antenna 43 offers the advantage in doing spatially-resolved measurements. The Picarro G2132-i isotope 44 analyzer is most immobile because of it is heavy and relative high power consumption in S3 addition to its particularly long time to warm up (30 min). However, Picarro G2132-i 46 measures stable isotopic CH4, while the other two instruments cannot. Care must be taken and a proper boat with stable power supply is needed in order to use Picarro G2132-i as a coupling 48 unit for the FaRAGE.   Note: 1) GasScouter G4301 does not use a vacuum pump to maintain a stable cavity pressure and the gas flow rate should be stable but slightly 56 above/below the recommended value.

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2) All gas analyzers are sensitive to liquid-phase water, therefore a hydrophobic filter is normally placed before the gas intake to protect 58 instrument from being flooded.

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3) According to Picarro, interference can occur for concentrations of H2O and CO2 well above normal ambient levels, as well as other 60 organics, ammonia, ethane, ethylene, or sulfur containing compounds.

S2. Re-evaluation of response time of gas analyzers 62
While response time for each gas analyzer has been provided by its manufacturer (Table   63 S2), a large difference was found when they were re-evaluated (Fig. S2). Picarro GasScouter 64 has the fastest response to concentration increase, in comparison to four-fold and eight-fold 65 slower response for portable Los Gatos and Picarro G2132-i, respectively. All three gas 66 analyzers were seen longer response time when concentration changed from high to low. The

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Picarro GasScouter still has the best performance compared to the other two.