State of Art: outdoor air quality monitoring using low-cost sensor

Air quality monitoring is becoming increasingly critical nowadays. The city of Paris, for instance, operates 13 measuring sites (AIRPARIF, 2018), whereas in London, air quality is monitored at around 100 locations (Greater London Authority, 2018). Although these numbers are seemingly high, they correspond to a spatial coverage of the order of one station over a few tens of square kilometers. This limitation can be lifted by employing low-cost sensors (LCSs) in dense observational networks that can enable quick and effective identification of pollution sources and determination of concentration gradients over specific areas. In Germany, e.g. in Munich, there exist only five reference stations, which are insufficient for accurately depicting a real-time pollution map for the entire city. A research study from TU Munich[Zhu et al], illustrates the importance of combining different measurement techniques to capture spatial and temporal patterns within a city and derive gas concentration values that are representative for the air most people breathe in. Recently, affordable air quality monitors have surfaced as a viable option to enhance the precision of monitoring. The utilization of inexpensive air quality monitors, however, poses numerous challenges[Panda, Sagarika, et al]: They experience cross-sensitivities to various ambient contaminants; they are influenced by external factors, including traffic, meteorological variations, and human actions; and their precision diminishes over time.

Regarding the low-cost sensor level, operating temperature and RH are two of the most important factors affecting the performance of electrochemistry sensors, mainly because they employ aqueous electrolytes containing sulfuric acid for the transfer of charges between the measuring and the reference electrodes. The concentration of sulfuric acid in the electrolyte is 5 M, which requires 60 % RH at 20 ∘C in ambient air to be in equilibrium (Alphasense, 2013). Exposure of the sensors to RH levels less than 60 % can in principle lead to evaporation of the solvent (i.e. water), whereas the opposite can happen under RH conditions above 60 %. The maker says that the Alphasense electrochemical devices can give accurate readings even when they are exposed to RH levels as low as 15% or as high as 90% at room temperature, as long as they are given enough time to get used to the new conditions, which should be a few days[Roubina Papaconstantinou et al.]. When electrochemical monitors are exposed to RH levels below 60% for a long time, they should be re-calibrated to work with the lower RH levels. However, since atmospheric RH is very changeable and can shift a lot in just a few hours, it is not possible to use RH-dependent calibration.

Our solid polymer electrolyte three-electrode gas sensors have gained attention due to their compact design, low power consumption, and mechanical stability. Unlike liquid electrolyte sensors, they do not suffer from leakage issues, making them suitable for portable and wearable applications. Additionally, they typically have a longer lifespan of 3–5 years due to the absence of liquid evaporation. One of the key advantages of solid polymer sensors is their baseline stability, as they experience less zero-point drift than liquid-based sensors.

Solid polymer electrochemical sensors are cost-effective for large-scale production and ensure consistent quality through automation. Their compact size, low power consumption and broad temperature range allow for a seamless integration into a wide range of applications. The sensors can perform measurements in the ppb range as they are noise-free and have low drift. The solid core eliminates the risk of leakage or drying out and ensures consistent measurements with high accuracy even in high humidity environments, contributing to their long lifespan[Ecsense].

One major challenge in gas sensing is ensuring the sensor's proper functionality, especially at low signal levels. With our integrated electronics and smart algorithm, our sensor modules address this with its Lifetime Detection Mode, which continuously monitors sensor performance and provides early warnings for maintenance or end-of-life, ensuring reliability in critical applications. 

Solid Polymer Electrochemical Technology offers flexibility in design and size since the core is dry and contains no liquid electrolytes. Traditionally, electrochemical sensors are used to detect toxic gases in industrial settings. The concept of a dry electrochemical cell based on a solid polymer electrolyte challenges not only the design restrictions of the gas sensor, but also the traditional applications for electrochemical cells. This revolutionary technology enables new and innovative mechanical designs for the finished cell[Ecsense].

Reference

Papaconstantinou, R., Demosthenous, M., Bezantakos, S., Hadjigeorgiou, N., Costi, M., Stylianou, M., ... & Biskos, G. (2023). Field evaluation of low-cost electrochemical air quality gas sensors under extreme temperature and relative humidity conditions. Atmospheric Measurement Techniques, 16(12), 3313-3329.

Zhu, Y., Chen, J., Bi, X., Kuhlmann, G., Chan, K. L., Dietrich, F., Brunner, D., Ye, S., and Wenig, M.: Spatial and temporal representativeness of point measurements for nitrogen dioxide pollution levels in cities, Atmos. Chem. Phys., 20, 13241–13251, https://doi.org/10.5194/acp-20-13241-2020, 2020. 

AIRPARIF: https://commercialisation.esa.int/wp-content/uploads/2020/11/Air-quality-monitoring-in-the-City-of-Paris_Basthiste.pdf (last access: 8 March 2022), 2018. 

Greater London Authority: Guide for monitoring air quality in London, https://www.london.gov.uk/sites/default/files/air_quality_monitoring_guidance_january_2018.pdf (last access: 23 November 2022), 2018. 

Alphasense: Humidity Extremes: Drying Out and Water Absorption, Alphasense Application Note AAN106, https://www.alphasense.com/wp-content/uploads/2013/07/AAN_106.pdf, 2013. 

Alphasense: Correcting for background currents in four electrode toxic gas sensors, Alphasense Application Note AAN803-05, 2019. 

Liang, Y., Wu, C., Jiang, S., Li, Y. J., Wu, D., Li, M., Cheng, P., Yang, W., Cheng, C., Li, L., Deng, T., Sun, J. Y., He, G., Liu, B., Yao, T., Wu, M., and Zhou, Z.: Field comparison of EC gas sensor data correction algorithms for ambient air measurements, Sensor. Actuat. B-Chem., 327, 128897, https://doi.org/10.1016/j.snb.2020.128897, 2021. 

Solid Polymer Electrochemical Gas Sensor Technology https://ecsense.com/solid-polymer-electrochemical-gas-sensor-technology/ 

Seesaard, T., Kamjornkittikoon, K., & Wongchoosuk, C. (2024). A comprehensive review on advancements in sensors for air pollution applications. Science of The Total Environment, 175696.

Panda, S., Mehlawat, S., Dhariwal, N., Kumar, A., & Sanger, A. (2024). Comprehensive review on gas sensors: Unveiling recent developments and addressing challenges. Materials Science and Engineering: B, 308, 117616.