Evaluation of low-cost sensors for air pollution monitoring : effect of gaseous interfering compounds and meteorological conditions

In this report the performances of low-cost sensors for air pollution monitoring are evaluated in order to give guidance to users on which parameters to take into account when performing field calibration of these sensors prior to using them to monitor air pollution. In particular, the effects of gaseous interfering compounds and meteorological conditions on four low cost sensors selected to be mounted on the AirSensEUR platform are characterised. The selected sensors are of the electrochemical type, as they are less power consuming and they have been shown, in previous studies, to give fastest response times and to suffer less from gaseous interference than metal-oxide sensors. Ten set of four sensors for ozone (Membrapor O3/M-5), nitrogen dioxide (Alphasense NO2-B43F), carbon monoxide (Membrapor CO/MF-200) and nitrogen monoxide (Alphasense NO-B4) were evaluated under controlled conditions in a laboratory exposure chamber. The tests allow the evaluation of the interference from gaseous compounds together with the effect of relative humidity, temperature and pressure variations. In general, each sensor was found to be highly linear when measuring its target gaseous species. Concerning gaseous interference, only the ozone sensors showed a high interference (> 75%) to nitrogen dioxide. The ozone filter of the NO2-B43F appears to be effective. The sensitivity of the CO and NO sensors was sufficient to be able to detect concentration levels expected at ambient gaseous concentrations. However, the interference of NO to the CO/MF-200 was found relevant at high NO and low CO values. The cross-sensitivity (CO and NO on O3 and NO2 sensors and vice-versa) was found low or not significant for each type of sensor. The meteorological evaluation showed that the four types of sensor behave similarly concerning the temperature interference. In fact both sensors showed a quadratic response with the increase of the temperature. Relative humidity was only found relevant for the two Alphasense sensors (NO2-B43F and NO-B4) with linear sensitivity associated with a clear hysteresis effect. Ambient pressure, however, was found to be relevant only for the two Membrapor sensors (O3/M-5 and CO/MF-200). Finally, the good reproducibility between sensors for the majority of effects including sensitivity to gas concentration and to meteorological variables with relative standard deviations of less than 10 % suggests that satisfactory calibration of sensors could be achieved without the need of a full characterisation of each sensor. By using calibration coefficients equal to the averages of the effects given in this report, a reasonable calibration function could be established. This result is promising, supporting future increased use of sensors for low cost for air pollution monitoring both by expert institutes and citizen science projects.