RiverRunner: a low-cost sensor prototype for continuous dissolved GHG measurements

The role of freshwater ecosystems in the global carbon budget has yet to be accurately quantified. Substantial uncertainties remain in estimation of greenhouse gas (GHG) fluxes to the atmosphere due to heterogeneity, temporal variability and small scale of many systems. Additionally, methods to measure dissolved gases involve expensive equipment and/or are time consuming, making fine scale resolution challenging. We here present a self-made low-cost (~ 250 €) sensor unit which can measure carbon dioxide (CO2) and methane (CH4) in the water phase, allowing inexpensive continuous in-situ logging of GHG concentrations with little manpower.

The electronic hardware of the sensor unit is integrated into a polypropylene tubing with two parts: The sensor body is completely waterproof and houses electrical hardware and battery. The sensor head houses the gas sensors and is separated from the water phase by a semipermeable PTFE membrane that is hydrophobic but permeable to gases, thereby allowing the gaseous phase in the sensor head to equilibrate with the water phase.

For CO2, we use a miniature non-dispersive infrared sensor; data from the factory-calibrated sensor can be read via I2C serial communication. For CH4, we use a semiconductor gas quality sensor from the Figaro sensor family. Originally developed for explosion warning systems, these sensors were shown to detect CH4 near ambient concentration. Incorporated into a voltage divider, sensor output voltage can be measured and translated into CH4 concentration. Electrical resistance of this sensor varies in presence of combustible gases but also with temperature and humidity. Additional sensors provide pressure, temperature and relative humidity; and mathematical models fitted to calibration data allow to adjust for reference output voltage at background concentration levels, thereby allowing measurement of CH4 concentration. As a microprocessor, we use an Arduino mini board in combination with a real-time clock, a voltage regulator and a micro SD-card module. The microprocessor is programmed using Arduino´s integrated development environment. Data is stored on the internal SD card and powered by two Li-Ion 18650 batteries connected in series. The sensor is able to measure continuously for 24 hours.

Our low-cost, yet accurate-enough sensor can help to address the major bottlenecks in better quantification of GHG fluxes: continuous measurements to capture natural temporal variability, as well as spatially replicated measurements to map carbon sources and sinks across heterogeneous ecosystems with little investment costs.