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

Abstract

Biogenic greenhouse gas emissions, e.g., of methane ( span classCombining double low line inline-formula CH4 span) and carbon dioxide ( span classCombining double low line inline-formula CO2 span) 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 span classCombining double low line inline-formula CH4 span and span classCombining double low line inline-formula CO2 span, 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 span classCombining double low line inline-formula CH4 span and span classCombining double low line inline-formula CO2 span concentrations at the water surface and in the water column. FaRAGE can achieve an exceptionally short response time ( span classCombining double low line inline-formula it i95 i iCombining double low line12 span thinsp;s when including the response time of the gas analyzer) while retaining an equilibration ratio of 62.6 thinsp; and a measurement accuracy of 0.5 thinsp; for span classCombining double low line inline-formula CH4 span. A similar performance was observed for dissolved span classCombining double low line inline-formula CO2 span ( span classCombining double low line inline-formula it i95 i iCombining double low line10 span thinsp;s, equilibration ratio 67.1 thinsp;). An equilibration ratio as high as 91.8 thinsp; can be reached at the cost of a slightly increased response time (16 thinsp;s). The FaRAGE is capable of continuously measuring dissolved span classCombining double low line inline-formula CO2 span and span classCombining double low line inline-formula CH4 span concentrations in the nM-to-sub mM (10 span classCombining double low line inline-formula -9 pan-10 span classCombining double low line inline-formula -3 span thinsp;mol thinsp;L span classCombining double low line inline-formula -1 span) range with a detection limit of sub-nM (10 span classCombining double low line inline-formula -10 span thinsp;mol thinsp;L span classCombining double low line inline-formula -1 span), 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. p.