# A Novel Particulate Matter 2.5 Sensor Based on Surface Acoustic Wave Technology
## 1 Introduction  
Atmospheric particulate matter (PM) is the sum of solid and liquid particles suspended in air. The complex mixture originates from a variety of sources such as salt, soot, metals and fly ash. PM\({}_{2.5}\), also known as fine particle, refers to particles with a nominal aerodynamic diameter less than or equal to 2.5 \(\upmu\)m, which have an adverse impact on air environment. Exposure to such air pollution may carry a risk of epidemic disease and the atmospheric visibility is degraded [1; 2; 3; 4]. To control the PM\({}_{2.5}\) pollution, the monitoring work, especially extensive area monitoring, is indispensable. At present, the main methods for PM\({}_{2.5}\) monitoring are the gravimetric method, the \(\upbeta\)-ray decay method and the tapered element oscillating microbalance (TEOM) method [5], which suffer from large volume and high price. On the other hand, for portable purposes, the light scattering method, the micro electro mechanical systems (MEMS) method and some novel detecting approaches [6; 7; 8; 9] are reported. Besides, to realize better sensitivity for mass sensors, high-frequency and high-Q aluminum nitride Lamb wave resonators were also developed recently [10; 11]. The light scattering method's output depends highly on particle composition and size, leading to low accuracy [12]. The MEMS method based on a film bulk acoustic resonator (FBAR) mass sensor has a sensitivity of 2 \(\upmu\)g/m\({}^{3}\) with a 10 min testing time [13]. The sensor's fabrication process is complex [14]. Thus, to satisfy the development trend and urgent requirement of the PM\({}_{2.5}\) monitoring methods, a miniature PM\({}_{2.5}\) sensor based on the surface acoustic wave (SAW) technology was developed.  
The schematic of the SAW based PM\({}_{2.5}\) sensor is shown in Figure 1. In the sensor, a virtual impactor (VI), a thermophoretic precipitator (TP) and a SAW sensor chip are integrated into an air current microchannel. When the airborne particles are separated by size in the VI, the PM\({}_{2.5}\) is transferred to the plate-to-plate TP. By the thermophoresis effect, the mass loading from the deposited PM\({}_{2.5}\) results in the SAW velocity shift, and accordingly, the change of the oscillation frequency is utilized for PM\({}_{2.5}\) mass detection.  
In this paper, the system design and evaluation of the SAW based PM\({}_{2.5}\) sensor were presented. To determine the optimal dimensions of the VI, a finite element method (FEM) model was established in the software COMSOL MULTIPHYSICS 4.4 (COMSOL Inc., Stockholm, Sweden, 2013) to analyze collection efficiency. Based on the thermophoretic deposition efficiency for a laminar flow profile, the dimensions of the TP were obtained. Also, the coupling-of-modes (COM) model, an efficient technique for SAW device simulation, was used to find optimal design parameters of the SAW sensor chip prior to fabrication. Accordingly, a dual-port 300 MHz SAW oscillator was implemented by using the sensor detecting probe as the feedback element. Combining the precise air sampling unit and the data processing unit, the performance of the developed SAW based PM\({}_{2.5}\) sensor was evaluated by detecting the monodispersed polystyrene latex (PSL) particles.