Field evaluation of low-cost electrochemical air quality gas sensors under extreme temperature and relative humidity conditions
Authors: Papaconstantinou Roubina , Demosthenous Marios , Bezantakos Spyros , Hadjigeorgiou Neoclis , Costi Marinos , Stylianou Melina , Symeou Elli , Savvides Chrysanthos , Biskos George 
Published: June 2023

Abstract. Modern electrochemical gas sensors hold great potential for improving practices in air quality (AQ) monitoring as their low cost, ease
of operation and compact design can enable dense observational networks and
mobile measurements. Despite that, however, numerous studies have shown that
the performance of these sensors depends on a number of factors (e.g.
environmental conditions, sensor quality, maintenance and calibration), thereby adding significant uncertainties in the reported measurements
and large discrepancies from those recorded by reference-grade instruments.
In this work we investigate the performance of electrochemical sensors,
provided by two manufacturers (namely Alphasense and Winsen), for measuring
the concentrations of CO, NO2, O3 and SO2. To achieve that we carried out collocated yearlong measurements with reference-grade
instruments at a traffic AQ monitoring station in Nicosia, Cyprus, where
temperatures ranged from ca. 0 ∘C in the winter to almost 45 ∘C in the summer. The CO sensors exhibit the best performance
among all the ones we tested, having minimal mean relative error (MRE)
compared to reference instruments (ca. −5 %), although a significant
difference in their response was observed before and after the summer
period. At the other end of the spectrum, the SO2 sensors reported
concentration values that were at least 1 order of magnitude higher than
the respective reference measurements (with MREs being more than 1000 %
for Alphasense and almost 400 % for Winsen throughout the entire
measurement period), which can be justified by the fact that the
concentrations of SO2 at our measuring site were below their limit of
detection. In general, variabilities in the environmental conditions (i.e.
temperature and relative humidity) appear to significantly affect the
performance of the sensors. When compared with reference instruments, the CO and NO2 electrochemical sensors provide measurements that exhibit
increasing errors and decreasing correlations as temperature increases (from below 10 to above 30 ∘C) and RH decreases (from >75 %
to below 30 %). Interestingly, the performance of the sensors was affected irreversibly during the hot summer period, exhibiting different responses before and after that, resulting in a signal deterioration that was more than twice that reported by the manufacturers. With the exception of the Alphasense NO2 sensor, all low-cost sensors (LCSs) exhibited measurement uncertainties that were much higher, even at the beginning of our measurement period, compared to those required for qualifying the sensors for indicative air quality measurements according to the respective European Commission (EC) Directive. Overall, our results show that the response of all LCSs is strongly affected by the environmental conditions, warranting further investigations on how they are manufactured, calibrated and employed in the field.