Low-cost methane (CH4) sensors for use in in flux chambers

The lack of reliable low-cost greenhouse gas flux measurement approaches limit our ability quantify regulation and verify mitigation efforts at the local level.   Methane (CH4), one of the most important greenhouse gases, is particularly dependent on local measurements because levels are regulated by a complex combination of sources, sinks and environmental conditions. There are still major gaps in the global methane budget and the reasons for the irregular development over time remains unclear. Facilitation of local flux measurements in all parts of the world therefore seem important to constrain large-scale assessments. As the high cost of gas analysers is a limiting factor for flux measurements, we here present how low-cost CH4 sensors can be used outside their specified range to yield reasonably accurate chamber-based flux measurements. By using a two-step calibration approach, testing multiple alternatives on how to model interference from temperature and humidity, an R2 ≥ 0.99 was achieved over a CH4 concentration range of 2 – 700 ppm under variable temperature and relative humidity. We also demonstrate ways to reach such calibration results without complicated calibration experiments and instead using in the order of 20 in situ reference measurements at different environmental conditions. Finally we, constructed and described a make-it-yourself Arduino based logger with the tested sensors for CH₄, temperature, humidity and carbon dioxide (CO₂) intended for flux chamber use with a material cost of approximately 200 Euro. We hope that this can contribute to more widespread greenhouse gas flux measurements in many environments and countries.