Integration and calibration of NDIR CO 2 low-cost sensors, and their operation in a sensor network covering Switzerland

Abstract. More than 300 LP8 CO2 sensors were integrated into sensor units and evaluated for the purpose of long-term operation in the Carbosense CO2 sensor network in Switzerland. Prior to deployment, all sensors were calibrated in a pressure and climate chamber, and in ambient conditions co-located with a reference instrument. To investigate their long-term performance and to test different data processing strategies, 18 sensors were deployed at five locations equipped with a reference instrument after calibration. Their accuracy during 19 to 25 months deployment was between 8 to 12 ppm. This level of accuracy requires careful sensor calibration prior to deployment, continuous monitoring of the sensors, efficient data filtering, and a procedure to correct drifts and jumps in the sensor signal during operation. High relative humidity (> ∼85 %) impairs the LP8 measurements, and corresponding data filtering results in a significant loss during humid conditions. The LP8 sensors are not suitable for the detection of small regional gradients and long-term trends. However, with careful data processing, the sensors are able to resolve CO2 changes and differences with a magnitude larger than about 20 ppm. Thereby, the sensor can resolve the site-specific CO2 signal at most locations in Switzerland. A low power network (LPN) using LoRaWAN allowed reliable data transmission with low energy consumption, and proved to be a key element of the Carbosense low-cost sensor network.