CO2 measurements in ambient air: testing of SenseAir K96 low-cost sensor

An AirSensEUR sensor system equipped with a prototype CO2 low-cost sensor (LCS), model K96 Senseair, has been tested at the Joint Research Centre (JRC) observatory station between December 2021 and September 2022 in order to evaluate whether its expanded measurement uncertainty could meet a Data Quality Objective (DQO) of 10 ppm. Firstly, the K96 LCS has been integrated into the JRC AirSensEUR sensor system, for which a measuring chamber with temperature control capacity set at 25 °C was designed. Additionally, a reference Picarro G2401 has been placed on the roof of the JRC observatory station at less than 50 cm away from the AirSensEUR sensor system. The Picarro measurements were used as reference CO2 data for comparing with K96 sensor data and for sensor calibration once the Picarro was adjusted for calibration drift. We calibrated the wet K96 LCS data against wet CO2 Picarro reference data. Linear models were found unreliable as residuals of the model were strongly associated with water vapour and temperature in the measuring chamber, and with air pressure to a lower extent. A multiple linear model was finally used for calibration using water vapour and air pressure in the measuring chamber as covariates. Applying this calibration, the expanded uncertainty of hourly LCS measurements remained below the DQO between February and early June 2022 when the reference CO2 was between 380 and 550 ppm as well as the temperature in the measuring was maintained around 25 °C. This result is promising, as the measurement uncertainty of K96 LCS was found three times smaller than the lowest measurement uncertainty claimed by the other manufacturers of CO2 LCSs available in the market. However, from June 2022, the newly designed AirSensEUR measuring chamber was unable to maintain the set temperature of 25 °C due to the high air temperature recorded in summer 2022. As expected, the increased temperature triggered a drift of the K96 LCS measurements resulting in the measurement uncertainty of 25 ppm being higher than the DQO. In the future, the software-operated control of temperature in the measuring chamber of AirSensEUR as well as better sun shielding shall be further developed and implemented to take the full benefit of the promising CO2 measurement capacity of the K96 sensor when the set temperature is more precisely maintained.