Solid State Gas Sensor Systems: Deployment for an Extensive Air Quality Monitoring in Urban Environment
Authors: Fioravanti Ambra , Marani Pietro , Lettieri Stefano , Salvatore Marcella , Maddalena Pasqualino , Carotta Maria Cristina 
Published: July 2021

Introduction

Air pollution is a global threat having huge impact on human health and ecosystems [1]. In urban areas, the air quality can be considered as resulting from the overlapping of three different pollution components: regional background, urban background and hot spot events. Therefore, the continuous monitoring of these components is of critical importance. Indeed, the ability to separate the three components is the key to define reduction actions of atmospheric pollution linked to the related sources and for this reason, really effective.
This work demonstrates that it is possible to determine, with reliability, sensitivity and selectivity comparable to those of the analytical instruments, the pollutants’ concentration in the atmosphere using specific advanced thick film gas sensors. They are based on nanostructured metal oxides (MOX sensors) which modify their surface properties when interacting with reducing or oxidizing gases [2].
Nowadays, monitoring is usually performed in few fixed stations and through mobile stations (where the conventional equipment is installed on vehicles) for temporary measurement campaigns or occasional environmental impact estimations. The equipment used in these stations is often complex, expensive and difficult to deploy, requiring controlled environmental conditions and frequent maintenance and calibrations [1].
Differently, systems based on MOX gas sensors constitute an advantageous solution because of their low-cost fabrication, maintenance, and installation, low power consumption, small dimensions and of easy deployment for a wide range of situations for example along heavy traffic roads, or in historic centers where fixed monitoring stations cannot be used due to their large size.

Experimental

The thick film gas sensors were fabricated through screen-printing technology. The sensing layers are based on specific functional materials synthesized in form of crystalline nanometric powders via wet chemistry processes [2]. A careful coordination of all different processes involved in the gas sensors realization, such as functional material preparation, deposition of sensing layers, substrates and electronics were done to optimize the sensors’ precision and accuracy.
The arrays of thick film gas sensors were assembled in small remotely controlled units and used for on-site tests. A monitoring unit (see figure A) consists in a small box to lodge the sensors, the electronic circuitry related to each sensor and a main electronic control unit provided with serial connection through which the data are transferred to a GSM module which sends them to an input data server, on request.
With the aim to validate the values of gas concentrations obtained through the array of MOX sensors, few monitoring units have been exposed to the real atmosphere beside fixed-site monitoring stations for air quality control. The sensor responses were studied in comparison with the pollutant concentrations as measured by the analytical instruments. In particular, IR and UV spectrophotometers for carbon monoxide and ozone, chemiluminescence analyzer for NOx and gas chromatograph for BTEX.

Results and Conclusions

The developed monitoring units based on different advanced thick film gas sensors have been successfully prepared with optimal level of reliability, stability, reproducibility and used to detect gaseous pollutants in the atmosphere with the same trend observed by the conventional analytical techniques for long monitoring periods, in different seasons and areas.
As example, a laboratory experiment devoted to test the stability and repeatability of sensors is reported in the figure B. In the image, the figure B refers to the alternate signal of two sensors, nominally identical, to 0.1 and 0.8 ppm of benzene. The responses, defined as ratio between the conductance in presence of benzene and the conductance in air, were 1.2 to 0.1 ppm and 2.1 to 0.8 ppm of benzene for both sensors. A second experiment is shown in figure C: the ozone concentration measured in real atmosphere through a WO3 sensor was compared with the measurements carried out by means of a conventional analyzer (UV spectrophotometer). The data are expressed in both cases as hourly average obtaining a quite good agreement. The graph is relating to one month of monitoring, while, in the inset, a zoom on few days is shown.
The innovative system, based on a specific MOX sensors array, is able to detect and quantify atmospheric pollutants constituting an advantageous solution for a wide range of situations, as for example along the roads of city centers or along heavy traffic roads, or in historic centers where fixed monitoring stations cannot be used due to their large size.

References

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[2] M.C. Carotta, A. Fioravanti, S. Gherardi, C. Malagù, M. Sacerdoti, G. Ghiotti, S.Morandi, (Ti,Sn) solid solutions as functional materials for gas sensing, Sens.Actuators B: Chem. 194 (2014) 195–205, doi:10.1016/j.snb.2013.12.021.
[3] S. Morandi, A. Amodio, A. Fioravanti, A. Giacomino, M. Mazzocchi, M. Sacerdoti, M.C. Carotta, A.N. Skouloudis, Operational functionalities of air-quality W-Sn metal-oxide sensors correlating semiconductor defect levels and surface potential barriers, Sci. Total Environ. 706 (2020) 135731, doi:10.1016/j.scitotenv.2019.135731.

Figure 1