Sensing technologies
The national networks monitor and measure particulate matter by methods with prescribed accuracy and precision, using high-end monitors and sophisticated quality assurance and quality control systems. This ensures comparability of measurements across geographical areas and meteorological and climatic conditions, but seldom provides high temporal and spatial resolution of air quality information necessary for personalized information. Lately, miniaturization and improved information and communication technologies resulted in the availability of low-cost sensors, and the use of lower-cost sensor systems is increasing every day. On the market there are available low-cost PM sensors (that cost in range 10-100 EUR) and sensors kits (assembled sensor systems that cost up to 600 EUR and even more, or do-it-yourself kits), from different respectable producers. The main characteristics of available sensor systems were compared in Jovašević-Stojanović et al. (2015) and (Rai et al. 2017).
LCS-PM are suitable for deployment in large numbers in terms of the cost of a single unit, but questions remain regarding the quality of data produced. There is no doubt that LCS-PM sensors may increase spatial and temporal monitoring resolution and cover large variety of application in a cost-effective manner, but there are doubts regarding their reliability, accuracy and precision.
Majority of low-cost PM sensors models that are on the market work on the light scattering method. This type of PM sensor is cheap and simple, has low power requirements, and quick response time (Wang et al., 2015). A light source illuminates the particles and the scattered light from the particles is measured by a photometer. The amount of light scattered is roughly proportional to PNC concentration with diameters greater than ~0.3 μm, while particles with smaller diameter cannot be detected by this method as they do not scatter.
The most important simplifications that lead to reducing the particle sizing and counting capabilities of the LCS-PM in comparison with standardized devices that work on same principle are:
- absence of a sheath flow to keep the aerosol sampled in a confined beam,
- the using of a fan to draw the sample flow through the sensor
Studies show that reliability of data collected with LCS-PM in case of lower concentrations is more questionable (Kelly et al., 2017). For wide use of each of LCS-PM device we need to know:
- Operational characteristics such as a possibility of failure caused by aging due to accumulation of particles inside the measuring chamber
- Meteorological conditions during the LCS-PM operation
- Intramodel variability, i.e. reproducibility between units of the same type of LCS-PM
- Relationship between reference instrument and LCS-PM (linear responses are preferable due to simplicity of calibration procedure)
- LCS-PM response in ambient conditions with high relative humidity, as the response of a sensor declines under the influence of very high air humidity
The first generation of LCS-PM was able to detect the amount of opacity (the amount of light not transmitted by the sample), while newly developed sensors detect PM1, PM2.5 and PM10. Specifications of some sensors state that they may measure particles in different particle sizes. However, there is a lack of and disparity in the scientific literature on performance assessment and calibration procedures for PM low-cost sensors. That makes it difficult to evaluate the quality of data obtained in different studies and to make comparisons between them (Rai et al, 2017) , Jovašević-Stojanović et al. (2015). In addition, test results for the same sensor may be very different between the tests performed in the laboratory, in indoor ambient air or in the outdoor field conditions.