With the new method, calibration of aerosol sensors can be done in less than 15 minutes, while previously the same work could take up to two weeks. The new calibration method could be an important tool in the future, when the number of sensor type air quality measurements is expected to multiply.
Aerosol sensors are expected to have a more significant role in the future in air quality monitoring. Current monitoring methods are accurate and reliable, but due to the high instrument costs, spatial coverage of measurements remains small. Research has been conducted at the Finnish Meteorological Institute on the feasibility of spatial expansion of monitoring coverage using less expensive instrumentation, such as sensors. The increased spatial coverage would benefit air quality models developed at Finnish Meteorological Institute, which produce forecasts of pollution levels.
The instruments used in the new calibration method are old and well-defined; only the operating techniques have changed. The operating principle of a Vibrating Orifice Aerosol Generator (VOAG) relies on the disruption of liquid column using vibration. Once the disrupted column breaks into droplets, the solvent of droplets evaporates and the remaining non-evaporating substance, particle, remains. The size of the particle can be controlled by changing the ratio of evaporating solvent and the non-evaporating substance within the liquid; the higher the non-evaporating portion the bigger the particle, and vice versa. The non-evaporating substance can be e.g. salt or oil. The evaporating solvent is usually isopropanol. Traditionally, calibration of aerosol instruments has been conducted by using several different solutions of varying concentrations, and testing the response of the instrument individually for each solution. Instead of using multiple solutions, the new method uses only two solutions; strong and mild. When both of the solutions are fed one by one and uninterruptedly to the VOAG, a continuously changing size distribution of particles is generated. This way the method allows quicker calibration than what was previously possible.
The new calibration method was utilized in a research conducted at Finnish Meteorological Institute, which evaluated the performance of two aftermarket aerosol sensors in both laboratory and field experiments. The results indicated that the inexpensive sensors were promising even though some development has to be made. The experiments were conducted during the spring and summer of 2017 in Kumpula at the Finnish Meteorological Institute research laboratory and at the SMEARIII outdoor measurement site. The reasearch was funded by TEKES funded INKA-ILMA/EAKR project (Tekes nro: 4588/31/2015) and by the Regional innovations and experimentations funds AIKO, governed by the Helsinki Regional Council (project HAQT, AIKO014).
Researcher Joel Kuula, tel. 0447227718, email@example.com
Kuula, J., Mäkelä, T., Hillamo, R. & Timonen, H. (2017). Response characterization of an inexpensive aerosol sensor. Sensors, 17(12), 2915; doi: 10.3390/s17122915.
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