The River Runner: a low-cost sensor prototype for continuous dissolved greenhouse gas measurements
<p>Freshwater ecosystems are sources of the two most relevant greenhouse gases (GHGs): CO<span class="inline-formula"><sub>2</sub></span> and CH<span class="inline-formula"><sub>4</sub></span>. Understanding the importance of...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2024-04-01
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| Series: | Journal of Sensors and Sensor Systems |
| Online Access: | https://jsss.copernicus.org/articles/13/41/2024/jsss-13-41-2024.pdf |
| Summary: | <p>Freshwater ecosystems are sources of the two most relevant greenhouse gases (GHGs): CO<span class="inline-formula"><sub>2</sub></span> and CH<span class="inline-formula"><sub>4</sub></span>. Understanding the importance of freshwater ecosystems in the global carbon cycle and their role in global warming trends requires the accurate quantification of gas fluxes from the water phase to the atmosphere. These fluxes depend on the gas exchange velocity and the concentration gradient between the phases, which both cause high spatio-temporal variability in fluxes. On a global scale, the estimation of fluxes is limited by the lack of cheap and accurate methods to measure dissolved gas concentrations. Low-cost sensors, as an alternative to expensive gas analysers, are available; however, to date, the in situ performance of such sensors has been poorly examined. Here, we present an inexpensive data-logging sensor prototype that provides continuous measurements of dissolved CO<span class="inline-formula"><sub>2</sub></span> and CH<span class="inline-formula"><sub>4</sub></span> in submerged environments. Gas measurements are done in a confined gas space, which is rapidly equilibrated with the water phase through a single-layer polytetrafluoroethylene (PTFE) membrane, by a miniature non-dispersive infrared (NDIR) sensor for CO<span class="inline-formula"><sub>2</sub></span> (Sunrise sensor, Senseair, Sweden) and a cheap metal oxide sensor for CH<span class="inline-formula"><sub>4</sub></span> (TGS2611-E, Figaro Engineering Inc., Japan). Pressure, temperature and humidity are measured to correct raw sensor readings. For freshwater, the dissolved gas concentration is directly obtained from the measured molar fraction and temperature and pressure readings. In air, we measured the molar fraction of CO<span class="inline-formula"><sub>2</sub></span> in a range from 400 to 10 000 <span class="inline-formula">ppm</span> and the molar fraction of CH<span class="inline-formula"><sub>4</sub></span> in a range from 2 to 50 <span class="inline-formula">ppm</span> with an accuracy of <span class="inline-formula">±</span> 58 and <span class="inline-formula">±</span> 3 <span class="inline-formula">ppm</span> respectively. We successfully used our prototype to measure diurnal variations in dissolved CO<span class="inline-formula"><sub>2</sub></span> in a natural stream. We further calibrated the CH<span class="inline-formula"><sub>4</sub></span> sensor for in situ use at concentrations ranging from 0.01 to 0.3 <span class="inline-formula">µ</span>mol L<span class="inline-formula"><sup>−1</sup></span>. Underwater, we were able to measure the molar fraction of CH<span class="inline-formula"><sub>4</sub></span> in the prototype head with an accuracy of <span class="inline-formula">±</span> 13 <span class="inline-formula">ppm</span> in the range from 2 to 172 <span class="inline-formula">ppm</span>. The underwater measurement error of CH<span class="inline-formula"><sub>4</sub></span> is always higher than for the same concentration range in air, and CH<span class="inline-formula"><sub>4</sub></span> is highly overestimated below 10 <span class="inline-formula">ppm</span>. At low CH<span class="inline-formula"><sub>4</sub></span>, humidity was the most important influence on the TGS2611-E sensor output in air, whereas temperature became the predominant factor underwater. We describe the response behaviour of low-cost sensors in submerged environments and report calibration methods to correct for temperature and humidity influence on the sensor signal if used underwater. Furthermore, we provide do-it-yourself instructions to build a sensor for submerged continuous measurements of dissolved CO<span class="inline-formula"><sub>2</sub></span> and CH<span class="inline-formula"><sub>4</sub></span>. Our prototype does not rely on an external power source, and we anticipate that such robust low-cost sensors will be useful for future studies of GHG emissions from freshwater environments.</p> |
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| ISSN: | 2194-8771 2194-878X |