Portable Particulate Matter (PM) Sensor for Air Pollution Monitoring Zhao Zhan, Sk Hasan Hafizul Haque, Roshini Jayavel, Xingguo Xiong

Department of Electrical Engineering, University of Bridgeport, Bridgeport, CT 06604

Abstract Atomospheric particulate matter (PM) are tiny airborne pollutants floating in the air. They come from construction materials, dust, smoking, cooking, automobile exhaust, charcoal power plant, etc. Particulate matter pollutants are potential threat to human health. Long term exposure to high concentration of PM2.5 leads to mesothelioma, lung cancer, bronchitis, heart attacks and many other diseases. Particulate matter sensors have been developed to measure the PM concentration and monitor the air quality. Traditional PM detectors are heavy, bulky and expensive. In this research, we aim at developing a portable PM detector based on MEMS (Microelectromechanical Systems) technology. Due to MEMS technology, it has the advantages of small size, low weight, low cost and high sensitivity. People can easily carry it to monitor the air quality anywhere they go. This can protect users from potential exposure to the polluted air in-door or during travel. The PM sensor was designed and its function is verified with COMSOL simulation.

Introduction

Conclusion and Future Work In this poster, the design and simulation of a PM2.5 sensor based on MEMS technology is proposed. Due to MEMS technology, it has the advantages of small size, low cost, low weight and high sensitivity. The basic structure design and working principle of the PM2.5 sensor are discussed. Pre-filter is used to separate the PM2.5 from air sample, and ionization is used for the detection. The PM2.5 sensor is simulated with COMSOL, but the device is not fabricated yet. The proposed PM2.5 sensor can be used for personal monitor of air pollution at home, work place or outdoors. In this future, we will further improve the design and eventually make a device prototype for real applications.

Atomospheric particulate matter (PM) are tiny airborne pollutants floating in the air. They come from construction materials, dust, smoking, cooking, automobile exhaust, charcoal power plant, etc. PM with diameter less than 10µm is called PM10 (the inhalable particulate matter), and PM with diameter less than 2.5µm is called PM2.5 (fine PM). As shown in Figure 1, typical diameter of human hair is about 70µm. PM10 is just one fifth of diameter of human hair. PM2.5 is about ¼ of PM10 and they are too small to be visible by human eyes. Particulate matter pollutants are potential threat to human health (see Fig 2). Particulate matters with diameter of 2.5µm or less are called PM2.5 and they are especially harmful for human respiratory system. They can penetrate into deeper part of lungs and human body does not have a mechanism to repel them out. Long term exposure to high concentration of PM2.5 leads to mesothelioma, lung cancer, bronchitis, heart attacks and many other diseases. As a result, a PM sensor which can be used to monitor the air quality is necessary. Furthermore, air pollution is becoming a serious problem in many developing countries due to their increased activity in developing the economy. Some large cities are often surrounded by heavy fog due to air pollution (see Fig. 3). Although PM2.5 concentration can be announced by city government, the actual PM2.5 concentration varies from location to location, depending on the condition of nearby pollution source. As a result, low-cost portable PM sensors are in high demand. Such PM sensors allow people to carry it everywhere they go and use it to know the PM concentration in surroundings. This can help them to make decision as on whether they should wear protector masks or simply stay away from the pollution source. In this research, we aim at developing a low-cost, portable PM sensor based on MEMS (Microelectromechanical Systems) technology. The overall PM sensor is designed to be in credit card size powered by batteries. It utilizes ionization technique to sense the PM concentration air. It is designed for low-cost personal usage.

Figure 8. Velocity distribution of air flow

Figure 10. Particles charge in diameter

The block diagram of PM2.5 sensor is shown in Fig. 4. A fan is used to suck the air from air inlet. The air will then pass through a filter with diameter of filtering holes of 2.5µm. Particulate matter with diameter more than 2.5µm will be filtered out, so that only PM with diameter of 2.5µm or less will pass through the filter. Once the PM2.5 passes through the filter and the fan, it will then be ionized to carry one unit of positive electric charge, which is then accelerated by electrical field and moved toward the negative electrode in the other end of the measurement unit. By measuring the voltage increase across the electrodes with a voltage meter and considering the air flow rate caused by the fan, we can calculate the PM2.5 concentration in the air sample. The air is then pumped out from the system via outlet to allow a constant air flow through the sensor. The PM2.5 sensor also should have self-clean capability. It should be able to clean itself to be prepared for continuous operation, so that the device is reusable. The COMSOL model of the PM2.5 sensor is shown in Fig. 5. The filter design is shown in Fig. 6.

An inlet filter is used to filter out the large size PM with diameter more than 2.5 µm. Only PM with diameter of 2.5µm or less are allowed to pass through the filter and enter the detection zone. An example inlet filter through efficiency curve for different particulate size is shown in Fig. 7. This ensure only PM2.5 is detected. Fig. 8 shows COMSO simulation of air velocity passing through the filter.

Figure 7. Size- selection curve of air filter for PM2.5 sensor

PM Sensor Design

Fig. 5. COMSOL 3D model of PM2.5 sensor

Fig 10. Electrostatic force on ionized PM vs acceleration voltage

Figure 1. Size comparison of hair, PM2.5, PM10

Figure 9. Velocity distribution along cross-section of air chamber

Assume metal oxide is used as the film for ionization electrode, acceleration electrical filed E=1V/m. The relation between particles in diameter and the number of particle is shown in Fig. 10. Note that the number of particle is integer. Since PM passing through the filter has diameter of 2.5µm or less, most of the PM particles get 0~3 unit charges after ionization.

Results and Discussion

Figure 11. The pipe corona onset voltage vs discharge line in radius The pipe corona onset voltage vs discharge line in radius is shown in Fig. 11. We can see that it’s approximately a simple linear relationship. For our work, nonuniform film such as metal film (SiO, SixNy) are used for the ionization electrode.

Figure 3. Foggy air pollution in big cities

Figure 2. Health threat of PM

Figure 4. Block diagram of PM2.5 sensor

Fig 6. Air filter design with diameter of holes of 2.5 µm

Fig. 9 shows velocity distribution along cross-section of air after passing through filter. We can see that it’s a typical Laminar flow. Fig. 10 shows quadratic relationship between electrostatic force on ionized PM and acceleration voltage.