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

• Main Authors: S. Xiao, L. Liu, W. Wang, A. Lorke, J. Woodhouse, H.-P. Grossart
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-07-01
• Series: Hydrology and Earth System Sciences
• Online Access: https://hess.copernicus.org/articles/24/3871/2020/hess-24-3871-2020.pdf

<p>Biogenic greenhouse gas emissions, e.g., of methane (<span class="inline-formula">CH₄</span>) and carbon dioxide (<span class="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 class="inline-formula">CH₄</span> and <span class="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 class="inline-formula">CH₄</span> and <span class="inline-formula">CO₂</span> concentrations at the water surface and in the water column. FaRAGE can achieve an exceptionally short response time (<span class="inline-formula"><i>t</i>_{95 <i>%</i>}=12</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 class="inline-formula">CH₄</span>. A similar performance was observed for dissolved <span class="inline-formula">CO₂</span> (<span class="inline-formula"><i>t</i>_{95 <i>%</i>}=10</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 class="inline-formula">CO₂</span> and <span class="inline-formula">CH₄</span> concentrations in the nM-to-sub mM (10<span class="inline-formula">⁻⁹</span>–10<span class="inline-formula">⁻³</span>&thinsp;mol&thinsp;L<span class="inline-formula">⁻¹</span>) range with a detection limit of sub-nM (10<span class="inline-formula">⁻¹⁰</span>&thinsp;mol&thinsp;L<span class="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>