A Novel Particulate Matter 2 . 5 Sensor Based on Surface Acoustic Wave Technology

Design, fabrication and experiments of a miniature particulate matter (PM) 2.5 sensor based on the surface acoustic wave (SAW) technology were proposed. The sensor contains a virtual impactor (VI) for particle separation, a thermophoretic precipitator (TP) for PM2.5 capture and a SAW sensor chip for PM2.5 mass detection. The separation performance of the VI was evaluated by using the finite element method (FEM) model and the PM2.5 deposition characteristic in the TP was obtained by analyzing the thermophoretic theory. Employing the coupling-of-modes (COM) model, a low loss and high-quality SAW resonator was designed. By virtue of the micro electro mechanical system (MEMS) technology and semiconductor technology, the SAW based PM2.5 sensor detecting probe was fabricated. Then, combining a dual-port SAW oscillator and an air sampler, the experimental platform was set up. Exposing the PM2.5 sensor to the polystyrene latex (PSL) particles in a chamber, the sensor performance was evaluated. The results show that by detecting the PSL particles with a certain diameter of 2 μm, the response of the SAW based PM2.5 sensor is linear, and in accordance with the response of the light scattering based PM2.5 monitor. The developed SAW based PM2.5 sensor has great potential for the application of airborne particle detection.


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 µm, 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 β-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 µg/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.
trend and urgent requirement of the PM2.5 monitoring methods, a miniature PM2.5 sensor based on the surface acoustic wave (SAW) technology was developed.
The schematic of the SAW based PM2.5 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 PM2.5 is transferred to the plate-to-plate TP.By the thermophoresis effect, the mass loading from the deposited PM2.5 results in the SAW velocity shift, and accordingly, the change of the oscillation frequency is utilized for PM2.5 mass detection.
In this paper, the system design and evaluation of the SAW based PM2.5 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 PM2.5 sensor was evaluated by detecting the monodispersed polystyrene latex (PSL) particles.

Design and Simulation
The exploded view of the proposed detecting probe is shown in Figure 2. The detecting probe consists of an upper shell (1); a lower shell (2); a slit nozzle VI (3); a hot source (4) and a SAW sensor chip (5).An air inlet ( 6) and an air outlet (7) are at both ends of the lower shell.
The SAW sensor chip is embedded in the lower shell and aligned with the hot source in the upper shell, forming a plate-to-plate TP in the channel (9).When the airborne particles are inhaled into the VI, the airflow is divided into two.The large particles follow into the middle channel (10) owing to their large inertia.Accordingly, because of small inertia, the small particles, namely PM2.5, follow into the left channel (8) and right channel (9) equally.When the PM2.5 flows through the TP in the channel (9), particles are captured on the surface of the SAW sensor chip by thermophoretic force.Meanwhile, the particles in the left and middle channels are expelled from the outlet together.In the following section, the design and simulation of the slit nozzle VI, the plate-to-plate TP and the SAW sensor chip are presented.

Design and Simulation
The exploded view of the proposed detecting probe is shown in Figure 2. The detecting probe consists of an upper shell (1); a lower shell (2); a slit nozzle VI (3); a hot source (4) and a SAW sensor chip (5).An air inlet ( 6) and an air outlet (7) are at both ends of the lower shell.
The SAW sensor chip is embedded in the lower shell and aligned with the hot source in the upper shell, forming a plate-to-plate TP in the channel (9).When the airborne particles are inhaled into the VI, the airflow is divided into two.The large particles follow into the middle channel (10) owing to their large inertia.Accordingly, because of small inertia, the small particles, namely PM 2.5 , follow into the left channel (8) and right channel (9) equally.When the PM 2.5 flows through the TP in the channel (9), particles are captured on the surface of the SAW sensor chip by thermophoretic force.Meanwhile, the particles in the left and middle channels are expelled from the outlet together.In the following section, the design and simulation of the slit nozzle VI, the plate-to-plate TP and the SAW sensor chip are presented.

Particle Separation
The sampling principle of the VI is based on the different inertia of particles of different diameters [15,16].The VI is composed of two coaxial nozzles, shown in Figure 3.The upper nozzle is the acceleration nozzle, and the lower nozzle is the collection probe.The geometric parameters are the width of inlet W, the angle of inlet θ0, the width of acceleration nozzle W0, the length of acceleration nozzle L0, the nozzle-to-probe distance S, the width of collection probe W1 and the height of microchannel h (perpendicular to the cross-sectional view).When the air flows into the nozzle, the high-speed airflow is divided into two.One part of it, occupying 90% of the total flow, changes direction with an angle of 90 degrees and flows into the side passage.This part is called major flow.Another part of airflow, occupying 10% of the total flow, enters the collection probe directly.This part is called minor flow.Meanwhile, particles of small diameter have small inertia and are easy to move with the major flow to the side passage.On the other hand, large particles have large inertia and are easy to break away from the deflecting airflow, entering the collection probe.The inertia based separation property of particles is characterized by a parameter, namely the Stokes number (Stk) [17].If the particles with a certain diameter that 50% of which flow into the side passage and another 50% of which flow into the collection probe, the certain diameter is called the cut-off diameter (D50).The corresponding Stokes number is Stk50.The D50 and Stk50 satisfy the Stokes equation (Equation ( 1)), which is influenced by the fluid properties and the VI

Particle Separation
The sampling principle of the VI is based on the different inertia of particles of different diameters [15,16].The VI is composed of two coaxial nozzles, shown in Figure 3.The upper nozzle is the acceleration nozzle, and the lower nozzle is the collection probe.The geometric parameters are the width of inlet W, the angle of inlet θ 0 , the width of acceleration nozzle W 0 , the length of acceleration nozzle L 0 , the nozzle-to-probe distance S, the width of collection probe W 1 and the height of microchannel h (perpendicular to the cross-sectional view).

Particle Separation
The sampling principle of the VI is based on the different inertia of particles of different diameters [15,16].The VI is composed of two coaxial nozzles, shown in Figure 3.The upper nozzle is the acceleration nozzle, and the lower nozzle is the collection probe.The geometric parameters are the width of inlet W, the angle of inlet θ0, the width of acceleration nozzle W0, the length of acceleration nozzle L0, the nozzle-to-probe distance S, the width of collection probe W1 and the height of microchannel h (perpendicular to the cross-sectional view).When the air flows into the nozzle, the high-speed airflow is divided into two.One part of it, occupying 90% of the total flow, changes direction with an angle of 90 degrees and flows into the side passage.This part is called major flow.Another part of airflow, occupying 10% of the total flow, enters the collection probe directly.This part is called minor flow.Meanwhile, particles of small diameter have small inertia and are easy to move with the major flow to the side passage.On the other hand, large particles have large inertia and are easy to break away from the deflecting airflow, entering the collection probe.The inertia based separation property of particles is characterized by a parameter, namely the Stokes number (Stk) [17].If the particles with a certain diameter that 50% of which flow into the side passage and another 50% of which flow into the collection probe, the certain diameter is called the cut-off diameter (D50).The corresponding Stokes number is Stk50.The D50 and Stk50 satisfy the Stokes equation (Equation ( 1)), which is influenced by the fluid properties and the VI When the air flows into the nozzle, the high-speed airflow is divided into two.One part of it, occupying 90% of the total flow, changes direction with an angle of 90 degrees and flows into the side passage.This part is called major flow.Another part of airflow, occupying 10% of the total flow, enters the collection probe directly.This part is called minor flow.Meanwhile, particles of small diameter have small inertia and are easy to move with the major flow to the side passage.On the other hand, large particles have large inertia and are easy to break away from the deflecting airflow, entering the collection probe.The inertia based separation property of particles is characterized by a parameter, namely the Stokes number (Stk) [17].If the particles with a certain diameter that 50% of which flow into the side passage and another 50% of which flow into the collection probe, the certain diameter is called the cut-off diameter (D 50 ).The corresponding Stokes number is Stk 50 .The D 50 and Stk 50 satisfy the Stokes equation (Equation ( 1)), which is influenced by the fluid properties and the VI geometric parameters.In Equation (1), µ is the dynamic viscosity of the fluid, ρ s is the particle density, Q 0 is the total flow and C is the Cunningham slip correction.
First, the value of Stk 50 was assumed to be 0.55 and the D 50 was set to 2.5 µm.According to Equation (1), the total flow Q 0 = 13.5 mL/min, the height h = 200 µm and the width W 0 = 290 µm were determined.Then, the other geometric parameters were obtained in succession referring to [18].Lastly, the proposed design of the microchannel was further adjusted by using the FEM software COMSOL to deduce the collection efficiency.The collection efficiency is the ratio of the particles collected in the minor flow to the particles supplied to the inlet.By using the Laminar Flow and the Particle Tracing for Fluid Flow packages in COMSOL, the velocity field distribution and the collection efficiency curve of the VI were obtained, shown in Figures 4 and 5 respectively.The maximum velocity is 7.16 m/s and the cut-off diameter is 2.5 µm.
Appl.Sci.2018, 8, 82 4 of 13 geometric parameters.In Equation (1), μ is the dynamic viscosity of the fluid, ρs is the particle density, Q0 is the total flow and C is the Cunningham slip correction.
First, the value of Stk50 was assumed to be 0.55 and the D50 was set to 2.5 μm.According to Equation (1), the total flow Q0 = 13.5 mL/min, the height h = 200 μm and the width W0 = 290 μm were determined.Then, the other geometric parameters were obtained in succession referring to [18].Lastly, the proposed design of the microchannel was further adjusted by using the FEM software COMSOL to deduce the collection efficiency.The collection efficiency is the ratio of the particles collected in the minor flow to the particles supplied to the inlet.By using the Laminar Flow and the Particle Tracing for Fluid Flow packages in COMSOL, the velocity field distribution and the collection efficiency curve of the VI were obtained, shown in Figures 4 and 5 respectively.The maximum velocity is 7.16 m/s and the cut-off diameter is 2.5 μm.

PM2.5 Deposition
The upper hot source and the lower SAW sensor chip embedded in the air current microchannel form the plate-to-plate TP.As shown in Figure 6, the gas molecules coming from the hot plate have a larger kinetic energy than those coming from the cold plate [19].Thus, the greater momentum received from the hot side causes a net force (thermophoretic force Fth) in the direction of decreasing temperature, leading to the overall movement of particles to the cooler side.The velocity resulting from the thermophoretic force is called thermophoretic velocity Vth.geometric parameters.In Equation ( 1), μ is the dynamic viscosity of the fluid, ρs is the particle density, Q0 is the total flow and C is the Cunningham slip correction.
First, the value of Stk50 was assumed to be 0.55 and the D50 was set to 2.5 μm.According to Equation (1), the total flow Q0 = 13.5 mL/min, the height h = 200 μm and the width W0 = 290 μm were determined.Then, the other geometric parameters were obtained in succession referring to [18].Lastly, the proposed design of the microchannel was further adjusted by using the FEM software COMSOL to deduce the collection efficiency.The collection efficiency is the ratio of the particles collected in the minor flow to the particles supplied to the inlet.By using the Laminar Flow and the Particle Tracing for Fluid Flow packages in COMSOL, the velocity field distribution and the collection efficiency curve of the VI were obtained, shown in Figures 4 and 5 respectively.The maximum velocity is 7.16 m/s and the cut-off diameter is 2.5 μm.

PM2.5 Deposition
The upper hot source and the lower SAW sensor chip embedded in the air current microchannel form the plate-to-plate TP.As shown in Figure 6, the gas molecules coming from the hot plate have a larger kinetic energy than those coming from the cold plate [19].Thus, the greater momentum received from the hot side causes a net force (thermophoretic force Fth) in the direction of decreasing temperature, leading to the overall movement of particles to the cooler side.The velocity resulting from the thermophoretic force is called thermophoretic velocity Vth.

PM 2.5 Deposition
The upper hot source and the lower SAW sensor chip embedded in the air current microchannel form the plate-to-plate TP.As shown in Figure 6, the gas molecules coming from the hot plate have a larger kinetic energy than those coming from the cold plate [19].Thus, the greater momentum received from the hot side causes a net force (thermophoretic force F th ) in the direction of decreasing temperature, leading to the overall movement of particles to the cooler side.The velocity resulting from the thermophoretic force is called thermophoretic velocity V th .In the microscale TP, the particle deposition efficiency η can be written as [21]: which is determined by the collection area of the cold plate (Scold), the thermophoretic velocity (Vth) and the inlet flow rate (Qin).ν is the kinematic viscosity of the fluid, T   is the temperature gradient, T is the particle temperature and Kth is the thermophoretic coefficient.Here, the Kth formula given by Talbot is adopted [22].In Figure 7, the dimensionless Kth is plotted as a function of R in a semilogarithmic coordinate, where the area in red dashed line represents the PM2.5 area.With the decrease of particle radius, the Kth increases monotonously, and accordingly, the deposition efficiency will increase.Thus, when the deposition efficiency for particles of a certain diameter of 2.5 μm is set to 100%, the TP dimensions can be deduced and drawn in Figure 8.The corresponding operating conditions were the temperature of the hot plate equaling 393.15K, the temperature of the cold plate equaling 303.15K and the Qin equaling 6 mL/min.Besides, considering the surface size of a SAW sensor chip, the dimensions of the thermophoretic precipitator were 200 μm × 3.6 mm × 6.3 mm.In the microscale TP, the particle deposition efficiency η can be written as [21]: which is determined by the collection area of the cold plate (S cold ), the thermophoretic velocity (V th ) and the inlet flow rate (Q in ).ν is the kinematic viscosity of the fluid, ∇T ∞ is the temperature gradient, T is the particle temperature and K th is the thermophoretic coefficient.Here, the K th formula given by Talbot is adopted [22].In Figure 7, the dimensionless K th is plotted as a function of R in a semi-logarithmic coordinate, where the area in red dashed line represents the PM 2.5 area.With the decrease of particle radius, the K th increases monotonously, and accordingly, the deposition efficiency will increase.Thus, when the deposition efficiency for particles of a certain diameter of 2.5 µm is set to 100%, the TP dimensions can be deduced and drawn in Figure 8.The corresponding operating conditions were the temperature of the hot plate equaling 393.15K, the temperature of the cold plate equaling 303.15K and the Q in equaling 6 mL/min.Besides, considering the surface size of a SAW sensor chip, the dimensions of the thermophoretic precipitator were 200 µm × 3.6 mm × 6.3 mm.In the microscale TP, the particle deposition efficiency η can be written as [21]: which is determined by the collection area of the cold plate (Scold), the thermophoretic velocity (Vth) and the inlet flow rate (Qin).ν is the kinematic viscosity of the fluid, T   is the temperature gradient, T is the particle temperature and Kth is the thermophoretic coefficient.Here, the Kth formula given by Talbot is adopted [22].In Figure 7, the dimensionless Kth is plotted as a function of R in a semilogarithmic coordinate, where the area in red dashed line represents the PM2.5 area.With the decrease of particle radius, the Kth increases monotonously, and accordingly, the deposition efficiency will increase.Thus, when the deposition efficiency for particles of a certain diameter of 2.5 μm is set to 100%, the TP dimensions can be deduced and drawn in Figure 8.The corresponding operating conditions were the temperature of the hot plate equaling 393.15K, the temperature of the cold plate equaling 303.15K and the Qin equaling 6 mL/min.Besides, considering the surface size of a SAW sensor chip, the dimensions of the thermophoretic precipitator were 200 μm × 3.6 mm × 6.3 mm.In the microscale TP, the particle deposition efficiency η can be written as [21]: which is determined by the collection area of the cold plate (Scold), the thermophoretic velocity (Vth) and the inlet flow rate (Qin).ν is the kinematic viscosity of the fluid, T   is the temperature gradient, T is the particle temperature and Kth is the thermophoretic coefficient.Here, the Kth formula given by Talbot is adopted [22].In Figure 7, the dimensionless Kth is plotted as a function of R in a semilogarithmic coordinate, where the area in red dashed line represents the PM2.5 area.With the decrease of particle radius, the Kth increases monotonously, and accordingly, the deposition efficiency will increase.Thus, when the deposition efficiency for particles of a certain diameter of 2.5 μm is set to 100%, the TP dimensions can be deduced and drawn in Figure 8.The corresponding operating conditions were the temperature of the hot plate equaling 393.15K, the temperature of the cold plate equaling 303.15K and the Qin equaling 6 mL/min.Besides, considering the surface size of a SAW sensor chip, the dimensions of the thermophoretic precipitator were 200 μm × 3.6 mm × 6.3 mm.

PM 2.5 Mass Detection
When the VI and TP on the front end are working at the steady flow and temperature fields, the SAW propagation along the piezoelectric substrate is perturbed by the PM 2.5 mass loading, causing a linear velocity shift of SAW.This characteristic is used to detect the particulate mass concentration.The corresponding response mechanism was analyzed theoretically by solving the piezoelectric medium equations of motion and the surface effective permittivity in [23], indicating the threshold detection limit of the PM 2.5 sensor using a SAW sensor chip was 0.17 ng.
To obtain the sufficient deposition area, a SAW sensor chip configuring a three-transducerstructure resonator was adopted.The schematic of the resonator structure is shown in Figure 9, which consists of three interdigital transducers (IDTs) and two identical shorted grating reflectors [24].

PM2.5 Mass Detection
When the VI and TP on the front end are working at the steady flow and temperature fields, the SAW propagation along the piezoelectric substrate is perturbed by the PM2.5 mass loading, causing a linear velocity shift of SAW.This characteristic is used to detect the particulate mass concentration.The corresponding response mechanism was analyzed theoretically by solving the piezoelectric medium equations of motion and the surface effective permittivity in [23], indicating the threshold detection limit of the PM2.5 sensor using a SAW sensor chip was 0.17 ng.
To obtain the sufficient deposition area, a SAW sensor chip configuring a three-transducerstructure resonator was adopted.The schematic of the resonator structure is shown in Figure 9, which consists of three interdigital transducers (IDTs) and two identical shorted grating reflectors [24].Prior to the fabrication of the SAW sensor chip, the design parameters of the SAW resonator are determined by using the COM model [25].The aim is to obtain low insertion loss and high-quality value of the resonator.By using the cascading mixed P-matrixes of the IDTs, reflectors and gaps, where the acoustic ports are cascaded and the electrical ports are in parallel, the frequency response S12 is obtained as [26]: In Equation (3), Y01 and Y02 are the input and output impedance respectively, Y11, Y12, Y21 and Y22 are the admittance matrix elements.
Considering the area of the collection plate in TP, the center frequency of the resonator was set to 311.6 MHz, where the period of electrode λ0 is 10 μm.The electrode width is λ0/4.The electrode thickness is 1600 Å.The aluminum (Al) fingers and the ST-X quartz piezoelectric substrate were used.The acoustic aperture is 150λ0.The numbers of electrodes of IDT1, IDT2, IDT3 and adjacent gratings are 45, 90, 45 and 400, respectively.The gaps between the IDTs and reflectors are all 1.25λ0, and the distances between the IDTs are both 22.25λ0 [27].The frequency response of the SAW resonator was deduced and shown in Figure 10.The insertion loss was 5.2 dB and the quality value was 4500.Prior to the fabrication of the SAW sensor chip, the design parameters of the SAW resonator are determined by using the COM model [25].The aim is to obtain low insertion loss and high-quality value of the resonator.By using the cascading mixed P-matrixes of the IDTs, reflectors and gaps, where the acoustic ports are cascaded and the electrical ports are in parallel, the frequency response S 12 is obtained as [26]: Considering the area of the collection plate in TP, the center frequency of the resonator was set to 311.6 MHz, where the period of electrode λ 0 is 10 µm.The electrode width is λ 0 /4.The electrode thickness is 1600 Å.The aluminum (Al) fingers and the ST-X quartz piezoelectric substrate were used.The acoustic aperture is 150λ 0 .The numbers of electrodes of IDT 1 , IDT 2 , IDT 3 and adjacent gratings are 45, 90, 45 and 400, respectively.The gaps between the IDTs and reflectors are all 1.25λ 0 , and the distances between the IDTs are both 22.25λ 0 [27].The frequency response of the SAW resonator was deduced and shown in Figure 10.The insertion loss was 5.2 dB and the quality value was 4500.

PM2.5 Mass Detection
When the VI and TP on the front end are working at the steady flow and temperature fields, the SAW propagation along the piezoelectric substrate is perturbed by the PM2.5 mass loading, causing a linear velocity shift of SAW.This characteristic is used to detect the particulate mass concentration.The corresponding response mechanism was analyzed theoretically by solving the piezoelectric medium equations of motion and the surface effective permittivity in [23], indicating the threshold detection limit of the PM2.5 sensor using a SAW sensor chip was 0.17 ng.
To obtain the sufficient deposition area, a SAW sensor chip configuring a three-transducerstructure resonator was adopted.The schematic of the resonator structure is shown in Figure 9, which consists of three interdigital transducers (IDTs) and two identical shorted grating reflectors [24].Prior to the fabrication of the SAW sensor chip, the design parameters of the SAW resonator are determined by using the COM model [25].The aim is to obtain low insertion loss and high-quality value of the resonator.By using the cascading mixed P-matrixes of the IDTs, reflectors and gaps, where the acoustic ports are cascaded and the electrical ports are in parallel, the frequency response S12 is obtained as [26]: In Equation (3), Y01 and Y02 are the input and output impedance respectively, Y11, Y12, Y21 and Y22 are the admittance matrix elements.
Considering the area of the collection plate in TP, the center frequency of the resonator was set to 311.6 MHz, where the period of electrode λ0 is 10 μm.The electrode width is λ0/4.The electrode thickness is 1600 Å.The aluminum (Al) fingers and the ST-X quartz piezoelectric substrate were used.The acoustic aperture is 150λ0.The numbers of electrodes of IDT1, IDT2, IDT3 and adjacent gratings are 45, 90, 45 and 400, respectively.The gaps between the IDTs and reflectors are all 1.25λ0, and the distances between the IDTs are both 22.25λ0 [27].The frequency response of the SAW resonator was deduced and shown in Figure 10.The insertion loss was 5.2 dB and the quality value was 4500.

Sensor Detecting Probe
The air current microchannel in Figure 2 is composed of an upper shell (1) and a lower shell (2).For the upper shell, a 1 µm Al was deposited on a silicon substrate by using an electron beam evaporator.Then, the photoresist (PR) was spin-coated, exposed and patterned.By using the inductively coupled plasma (ICP) technology, the window was etched off.For the lower shell, the fabrication process was similar to the upper one.First, a 1 µm Al was deposited on a silicon substrate and the PR was patterned.Then, the air inlet, outlet and window were etched off.Second, we repeated the first step and etched the wafer to a depth of 200 µm by using the ICP.Lastly, the two silicon wafers were bonded together into an individual die.The fabricated air current microchannel is shown in Figure 11, where (a) is the upper shell and (b) is the lower shell.

Sensor Detecting Probe
The air current microchannel in Figure 2 is composed of an upper shell (1) and a lower shell (2).For the upper shell, a 1 μm Al was deposited on a silicon substrate by using an electron beam evaporator.Then, the photoresist (PR) was spin-coated, exposed and patterned.By using the inductively coupled plasma (ICP) technology, the window was etched off.For the lower shell, the fabrication process was similar to the upper one.First, a 1 μm Al was deposited on a silicon substrate and the PR was patterned.Then, the air inlet, outlet and window were etched off.Second, we repeated the first step and etched the wafer to a depth of 200 μm by using the ICP.Lastly, the two silicon wafers were bonded together into an individual die.The fabricated air current microchannel is shown in Figure 11, where (a) is the upper shell and (b) is the lower shell.To realize a stable and constant temperature gradient, two Peltier elements were utilized.One was placed in the hole of the upper shell, and the other was pasted on the back of the lower shell.When a direct current (DC) current was applied to a Peltier element, a heat flux was created, causing one side of the Peltier element to become warmer and the other side colder [28].The difference in temperature was proportional to the current applied.By using two Peltier elements in parallel, the desired temperature gradient of 4.5 × 10 5 K/m was realized in the microchannel.
To fabricate the SAW device, a 1600 Å Al was deposited on the ST-X quartz substrate using an electron beam evaporator.Then, the PR was spin-coated, exposed and patterned for the IDTs and the adjacent shorted grating reflectors.Lastly, the Al was wet-etched and the PR was dissolved in acetone.The fabricated resonator is shown in Figure 12a.Measured by a network analyzer (E5071B, Keysight, Palo Alto, CA, USA), the frequency response S12 of the SAW resonator is shown in Figure 12b.The center frequency was 311.625 MHz, the insertion loss was 5.0 dB and the quality value was 4371.To realize a stable and constant temperature gradient, two Peltier elements were utilized.One was placed in the hole of the upper shell, and the other was pasted on the back of the lower shell.When a direct current (DC) current was applied to a Peltier element, a heat flux was created, causing one side of the Peltier element to become warmer and the other side colder [28].The difference in temperature was proportional to the current applied.By using two Peltier elements in parallel, the desired temperature gradient of 4.5 × 10 5 K/m was realized in the microchannel.
To fabricate the SAW device, a 1600 Å Al was deposited on the ST-X quartz substrate using an electron beam evaporator.Then, the PR was spin-coated, exposed and patterned for the IDTs and the adjacent shorted grating reflectors.Lastly, the Al was wet-etched and the PR was dissolved in acetone.The fabricated resonator is shown in Figure 12a.Measured by a network analyzer (E5071B, Keysight, Palo Alto, CA, USA), the frequency response S 12 of the SAW resonator is shown in Figure 12b.The center frequency was 311.625 MHz, the insertion loss was 5.0 dB and the quality value was 4371.

Sensor Detecting Probe
The air current microchannel in Figure 2 is composed of an upper shell (1) and a lower shell (2).For the upper shell, a 1 μm Al was deposited on a silicon substrate by using an electron beam evaporator.Then, the photoresist (PR) was spin-coated, exposed and patterned.By using the inductively coupled plasma (ICP) technology, the window was etched off.For the lower shell, the fabrication process was similar to the upper one.First, a 1 μm Al was deposited on a silicon substrate and the PR was patterned.Then, the air inlet, outlet and window were etched off.Second, we repeated the first step and etched the wafer to a depth of 200 μm by using the ICP.Lastly, the two silicon wafers were bonded together into an individual die.The fabricated air current microchannel is shown in Figure 11, where (a) is the upper shell and (b) is the lower shell.To realize a stable and constant temperature gradient, two Peltier elements were utilized.One was placed in the hole of the upper shell, and the other was pasted on the back of the lower shell.When a direct current (DC) current was applied to a Peltier element, a heat flux was created, causing one side of the Peltier element to become warmer and the other side colder [28].The difference in temperature was proportional to the current applied.By using two Peltier elements in parallel, the desired temperature gradient of 4.5 × 10 5 K/m was realized in the microchannel.
To fabricate the SAW device, a 1600 Å Al was deposited on the ST-X quartz substrate using an electron beam evaporator.Then, the PR was spin-coated, exposed and patterned for the IDTs and the adjacent shorted grating reflectors.Lastly, the Al was wet-etched and the PR was dissolved in acetone.The fabricated resonator is shown in Figure 12a.Measured by a network analyzer (E5071B, Keysight, Palo Alto, CA, USA), the frequency response S12 of the SAW resonator is shown in Figure 12b.The center frequency was 311.625 MHz, the insertion loss was 5.0 dB and the quality value was 4371.Finally, the Peltier element and the SAW resonator were placed into the air current microchannel together to form the final detecting probe.The hot source was over against the surface of the SAW sensor chip.To guarantee sealing of the air channel, a silicone sealant was used to adhere around the cracks.Exposing the bonded probe to the air for about 24 h, the sealant was solidified continuously from within.By optimizing the fabrication process, a detecting probe was successfully prepared.Figure 13 gives a fabricated exploded detecting probe prototype, where the SAW sensor chip is inserted in the channel.The dimensions of the detecting probe were 50 mm × 15 mm × 3 mm.Finally, the Peltier element and the SAW resonator were placed into the air current microchannel together to form the final detecting probe.The hot source was over against the surface of the SAW sensor chip.To guarantee sealing of the air channel, a silicone sealant was used to adhere around the cracks.Exposing the bonded probe to the air for about 24 h, the sealant was solidified continuously from within.By optimizing the fabrication process, a detecting probe was successfully prepared.Figure 13 gives a fabricated exploded detecting probe prototype, where the SAW sensor chip is inserted in the channel.The dimensions of the detecting probe were 50 mm × 15 mm × 3 mm.

SAW Oscillator
The fabricated sensor detecting probe was used as the control element of an oscillator and was connected to the oscillation circuit through a RF cable.The circuit was made of an amplifier, a phase shifter and so on.Then, the output of the amplifier was mixed in the mixer to obtain a differential frequency comparing with the referenced SAW oscillator loop, shown in Figure 14.The differential frequency was picked by a programmable frequency counter and plotted by a computer in real-time.Figure 15 shows the measured medium-term frequency stability, and excellent frequency stability of 18 Hz per hour was observed.

SAW Oscillator
The fabricated sensor detecting probe was used as the control element of an oscillator and was connected to the oscillation circuit through a RF cable.The circuit was made of an amplifier, a phase shifter and so on.Then, the output of the amplifier was mixed in the mixer to obtain a differential frequency comparing with the referenced SAW oscillator loop, shown in Figure 14.Finally, the Peltier element and the SAW resonator were placed into the air current microchannel together to form the final detecting probe.The hot source was over against the surface of the SAW sensor chip.To guarantee sealing of the air channel, a silicone sealant was used to adhere around the cracks.Exposing the bonded probe to the air for about 24 h, the sealant was solidified continuously from within.By optimizing the fabrication process, a detecting probe was successfully prepared.Figure 13 gives a fabricated exploded detecting probe prototype, where the SAW sensor chip is inserted in the channel.The dimensions of the detecting probe were 50 mm × 15 mm × 3 mm.

SAW Oscillator
The fabricated sensor detecting probe was used as the control element of an oscillator and was connected to the oscillation circuit through a RF cable.The circuit was made of an amplifier, a phase shifter and so on.Then, the output of the amplifier was mixed in the mixer to obtain a differential frequency comparing with the referenced SAW oscillator loop, shown in Figure 14.The differential frequency was picked by a programmable frequency counter and plotted by a computer in real-time.Figure 15 shows the measured medium-term frequency stability, and excellent frequency stability of 18 Hz per hour was observed.The differential frequency was picked by a programmable frequency counter and plotted by a computer in real-time.Figure 15 shows the measured medium-term frequency stability, and excellent frequency stability of 18 Hz per hour was observed.
Finally, the Peltier element and the SAW resonator were placed into the air current microchannel together to form the final detecting probe.The hot source was over against the surface of the SAW sensor chip.To guarantee sealing of the air channel, a silicone sealant was used to adhere around the cracks.Exposing the bonded probe to the air for about 24 h, the sealant was solidified continuously from within.By optimizing the fabrication process, a detecting probe was successfully prepared.Figure 13 gives a fabricated exploded detecting probe prototype, where the SAW sensor chip is inserted in the channel.The dimensions of the detecting probe were 50 mm × 15 mm × 3 mm.

SAW Oscillator
The fabricated sensor detecting probe was used as the control element of an oscillator and was connected to the oscillation circuit through a RF cable.The circuit was made of an amplifier, a phase shifter and so on.Then, the output of the amplifier was mixed in the mixer to obtain a differential frequency comparing with the referenced SAW oscillator loop, shown in Figure 14.The differential frequency was picked by a programmable frequency counter and plotted by a computer in real-time.Figure 15 shows the measured medium-term frequency stability, and excellent frequency stability of 18 Hz per hour was observed.

Experimental Setup
Figures 16 and 17 exhibit the schematic and photograph of the experimental setup.The emitted PM was introduced into the particle chamber in the front end.In the air sampling unit, an air sampler (QCS 3000, Yancheng Galaxy Technology Co., Ltd., Yancheng, Jiangsu province, China) with a mass flow controller (D07-19B, Beijing Sevenstar Electronics Co., Ltd., Beijing, China) were used to precise control the sampling flow.A diffusion dryer was used in front of the mass flow controller to absorb the water and the residual particles.In the data processing unit, the output resonant frequency change of the SAW sensor chip was measured by a frequency counter (TF930, Aim and Thurlby Thandar Instruments, Huntingdon, Cambridgeshire, UK) and recorded in a personal computer by a custom-developed program.
Before sampling the particles, the air sampler was kept off until a stable temperature gradient was formed.When the Peltier element reached the given temperature during the heat up phase, the air sampler was turned on to sample the pure nitrogen gas to clean the channel.After about 10-15 min, when the SAW oscillator was stable and the concentration of particles in the chamber was steady, the sampling inlet was switched to the particle chamber.During the sampling, the flow rate of the mass flow controller was 13.5 mL/min and the temperature difference was 90 K.The continuous change of differential frequency was recorded to evaluate the mass concentration of PM 2.5 .

Experimental Setup
Figures 16 and 17 exhibit the schematic and photograph of the experimental setup.The emitted PM was introduced into the particle chamber in the front end.In the air sampling unit, an air sampler (QCS 3000, Yancheng Galaxy Technology Co., Ltd., Yancheng, Jiangsu province, China) with a mass flow controller (D07-19B, Beijing Sevenstar Electronics Co., Ltd., Beijing, China) were used to precise control the sampling flow.A diffusion dryer was used in front of the mass flow controller to absorb the water and the residual particles.In the data processing unit, the output resonant frequency change of the SAW sensor chip was measured by a frequency counter (TF930, Aim and Thurlby Thandar Instruments, Huntingdon, Cambridgeshire, UK) and recorded in a personal computer by a custom-developed program.
Before sampling the particles, the air sampler was kept off until a stable temperature gradient was formed.When the Peltier element reached the given temperature during the heat up phase, the air sampler was turned on to sample the pure nitrogen gas to clean the channel.After about 10-15 min, when the SAW oscillator was stable and the concentration of particles in the chamber was steady, the sampling inlet was switched to the particle chamber.During the sampling, the flow rate of the mass flow controller was 13.5 mL/min and the temperature difference was 90 K.The continuous change of differential frequency was recorded to evaluate the mass concentration of PM2.5.

Results and Discussions
In the experiment, the monodispersed PSL particles with the diameter of 2 μm were adopted.Meanwhile, a light scattering aerosol monitor was parallel connected to the particle chamber to provide a reference.Although the light scattering method was proven inaccurate in practical application [12], in terms of the particles of known material and size, the light scattering method was accurate after calibration.In the experiments, an atomizer was used to disperse PSL particles and then mixed up uniformly by fans in the chamber.Figure 18 shows the frequency output of the SAW based PM2.5 sensor for PSL particle tests.The horizontal line areas are the intervals between sampling and the oblique line areas are the three sampling processes.

Experimental Setup
Figures 16 and 17 exhibit the schematic and photograph of the experimental setup.The emitted PM was introduced into the particle chamber in the front end.In the air sampling unit, an air sampler (QCS 3000, Yancheng Galaxy Technology Co., Ltd., Yancheng, Jiangsu province, China) with a mass flow controller (D07-19B, Beijing Sevenstar Electronics Co., Ltd., Beijing, China) were used to precise control the sampling flow.A diffusion dryer was used in front of the mass flow controller to absorb the water and the residual particles.In the data processing unit, the output resonant frequency change of the SAW sensor chip was measured by a frequency counter (TF930, Aim and Thurlby Thandar Instruments, Huntingdon, Cambridgeshire, UK) and recorded in a personal computer by a custom-developed program.
Before sampling the particles, the air sampler was kept off until a stable temperature gradient was formed.When the Peltier element reached the given temperature during the heat up phase, the air sampler was turned on to sample the pure nitrogen gas to clean the channel.After about 10-15 min, when the SAW oscillator was stable and the concentration of particles in the chamber was steady, the sampling inlet was switched to the particle chamber.During the sampling, the flow rate of the mass flow controller was 13.5 mL/min and the temperature difference was 90 K.The continuous change of differential frequency was recorded to evaluate the mass concentration of PM2.5.

Results and Discussions
In the experiment, the monodispersed PSL particles with the diameter of 2 μm were adopted.Meanwhile, a light scattering aerosol monitor was parallel connected to the particle chamber to provide a reference.Although the light scattering method was proven inaccurate in practical application [12], in terms of the particles of known material and size, the light scattering method was accurate after calibration.In the experiments, an atomizer was used to disperse PSL particles and then mixed up uniformly by fans in the chamber.Figure 18 shows the frequency output of the SAW based PM2.5 sensor for PSL particle tests.The horizontal line areas are the intervals between sampling and the oblique line areas are the three sampling processes.

Results and Discussions
In the experiment, the monodispersed PSL particles with the diameter of 2 µm were adopted.Meanwhile, a light scattering aerosol monitor was parallel connected to the particle chamber to provide a reference.Although the light scattering method was proven inaccurate in practical application [12], in terms of the particles of known material and size, the light scattering method was accurate after calibration.In the experiments, an atomizer was used to disperse PSL particles and then mixed up uniformly by fans in the chamber.Figure 18 shows the frequency output of the SAW based PM 2.5 sensor for PSL particle tests.The horizontal line areas are the intervals between sampling and the oblique line areas are the three sampling processes.
To handle the results, the slope of the oblique line is used to represent the SAW sensor response.Figure 19 gives the performance comparison between the light scattering monitor and the SAW based PM 2.5 sensor for measuring PSL particles.The horizontal axis presents the sampling time.The left vertical axis presents the particle mass concentration (unit: µg/m 3 ) measured by the light scattering monitor, and the right vertical axis presents the response of the SAW based PM 2.5 sensor (unit: Hz/min).Furthermore, the curve of the SAW sensor responses changing with the particle mass concentration is shown in Figure 20.The slope of the linear fitting equation is 93.96 (Hz/min)/(µg/m 3 ).The results show that by detecting the PSL particles with a certain diameter of 2 µm, the response of the SAW based PM 2.5 sensor is linear, and in accordance with the response of the light scattering based PM 2.5 monitor.To get a better view of the sensor response, an optimal data processing approach needs to be developed.To handle the results, the slope of the oblique line is used to represent the SAW sensor response.Figure 19 gives the performance comparison between the light scattering monitor and the SAW based PM2.5 sensor for measuring PSL particles.The horizontal axis presents the sampling time.The left vertical axis presents the particle mass concentration (unit: μg/m 3 ) measured by the light scattering monitor, and the right vertical axis presents the response of the SAW based PM2.5 sensor (unit: Hz/min).Furthermore, the curve of the SAW sensor responses changing with the particle mass concentration is shown in Figure 20.The slope of the linear fitting equation is 93.96 (Hz/min)/(μg/m 3 ).The results show that by detecting the PSL particles with a certain diameter of 2 μm, the response of the SAW based PM2.5 sensor is linear, and in accordance with the response of the light scattering based PM2.5 monitor.To get a better view of the sensor response, an optimal data processing approach needs to be developed.When more particles accumulate on the surface of SAW devices, the sensitivity of the SAW sensor will be worse.Therefore, to disperse the particles on the sensor, an "inverse temperature  To handle the results, the slope of the oblique line is used to represent the SAW sensor response.Figure 19 gives the performance comparison between the light scattering monitor and the SAW based PM2.5 sensor for measuring PSL particles.The horizontal axis presents the sampling time.The left vertical axis presents the particle mass concentration (unit: μg/m 3 ) measured by the light scattering monitor, and the right vertical axis presents the response of the SAW based PM2.5 sensor (unit: Hz/min).Furthermore, the curve of the SAW sensor responses changing with the particle mass concentration is shown in Figure 20.The slope of the linear fitting equation is 93.96 (Hz/min)/(μg/m 3 ).The results show that by detecting the PSL particles with a certain diameter of 2 μm, the response of the SAW based PM2.5 sensor is linear, and in accordance with the response of the light scattering based PM2.5 monitor.To get a better view of the sensor response, an optimal data processing approach needs to be developed.When more particles accumulate on the surface of SAW devices, the sensitivity of the SAW sensor will be worse.Therefore, to disperse the particles on the sensor, an "inverse temperature  To handle the results, the slope of the oblique line is used to represent the SAW sensor response.Figure 19 gives the performance comparison between the light scattering monitor and the SAW based PM2.5 sensor for measuring PSL particles.The horizontal axis presents the sampling time.The left vertical axis presents the particle mass concentration (unit: μg/m 3 ) measured by the light scattering monitor, and the right vertical axis presents the response of the SAW based PM2.5 sensor (unit: Hz/min).Furthermore, the curve of the SAW sensor responses changing with the particle mass concentration is shown in Figure 20.The slope of the linear fitting equation is 93.96 (Hz/min)/(μg/m 3 ).The results show that by detecting the PSL particles with a certain diameter of 2 μm, the response of the SAW based PM2.5 sensor is linear, and in accordance with the response of the light scattering based PM2.5 monitor.To get a better view of the sensor response, an optimal data processing approach needs to be developed.When more particles accumulate on the surface of SAW devices, the sensitivity of the SAW sensor will be worse.Therefore, to disperse the particles on the sensor, an "inverse temperature

Figure 1 .
Figure 1.Schematic of the surface acoustic wave (SAW) based PM 2.5 sensor.

Figure 2 .
Figure 2. Exploded view of the detecting probe.

Figure 3 .
Figure 3. Cross-sectional view of the virtual impactor.

Figure 2 .
Figure 2. Exploded view of the detecting probe.

Figure 2 .
Figure 2. Exploded view of the detecting probe.

Figure 3 .
Figure 3. Cross-sectional view of the virtual impactor.

Figure 3 .
Figure 3. Cross-sectional view of the virtual impactor.

Figure 4 .
Figure 4. Velocity field distribution of the virtual impactor.

Figure 5 .
Figure 5. Collection efficiency curve of the virtual impactor.

Figure 4 .
Figure 4. Velocity field distribution of the virtual impactor.

Figure 4 .
Figure 4. Velocity field distribution of the virtual impactor.

Figure 5 .
Figure 5. Collection efficiency curve of the virtual impactor.

Figure 5 .
Figure 5. Collection efficiency curve of the virtual impactor.

Figure 7 .
Figure 7. Thermophoretic coefficient Kth with different particle radius R in a semi-logarithmic coordinate.

Figure 7 .
Figure 7. Thermophoretic coefficient Kth with different particle radius R in a semi-logarithmic coordinate.

Figure 7 . 13 Figure 6 .
Figure 7. Thermophoretic coefficient K th with different particle radius R in a semi-logarithmic coordinate.

Figure 7 .
Figure 7. Thermophoretic coefficient Kth with different particle radius R in a semi-logarithmic coordinate.

Figure 10 .
Figure 10.Frequency response of the SAW resonator.
) In Equation (3), Y 01 and Y 02 are the input and output impedance respectively, Y 11 , Y 12 , Y 21 and Y 22 are the admittance matrix elements.

Figure 10 .
Figure 10.Frequency response of the SAW resonator.Figure 10.Frequency response of the SAW resonator.

Figure 10 .
Figure 10.Frequency response of the SAW resonator.Figure 10.Frequency response of the SAW resonator.

Figure 11 .
Figure 11.Photograph of a fabricated air current microchannel: (a) the upper shell; (b) the lower shell.

Figure 11 .
Figure 11.Photograph of a fabricated air current microchannel: (a) the upper shell; (b) the lower shell.

Figure 11 .
Figure 11.Photograph of a fabricated air current microchannel: (a) the upper shell; (b) the lower shell.

Figure 13 .
Figure 13.Photograph of a fabricated exploded sensor detecting probe: (a) the upper shell; (b) the lower shell.

Figure 14 .
Figure 14.Photograph of a fabricated dual-port SAW oscillator.

Figure 15 .
Figure 15.Measured medium-term frequency stability of the SAW oscillator.

Figure 13 .
Figure 13.Photograph of a fabricated exploded sensor detecting probe: (a) the upper shell; (b) the lower shell.

Figure 13 .
Figure 13.Photograph of a fabricated exploded sensor detecting probe: (a) the upper shell; (b) the lower shell.

Figure 14 .
Figure 14.Photograph of a fabricated dual-port SAW oscillator.

Figure 15 .
Figure 15.Measured medium-term frequency stability of the SAW oscillator.

Figure 14 .
Figure 14.Photograph of a fabricated dual-port SAW oscillator.

Figure 13 .
Figure 13.Photograph of a fabricated exploded sensor detecting probe: (a) the upper shell; (b) the lower shell.

Figure 14 .
Figure 14.Photograph of a fabricated dual-port SAW oscillator.

Figure 15 .
Figure 15.Measured medium-term frequency stability of the SAW oscillator.Figure 15.Measured medium-term frequency stability of the SAW oscillator.

Figure 15 .
Figure 15.Measured medium-term frequency stability of the SAW oscillator.Figure 15.Measured medium-term frequency stability of the SAW oscillator.

Figure 16 .
Figure 16.Experimental setup for evaluating the characteristics of the SAW based PM2.5 sensor.

Figure 17 .
Figure 17.Photograph of the experimental setup.

Figure 16 .
Figure 16.Experimental setup for evaluating the characteristics of the SAW based PM 2.5 sensor.

Figure 16 .
Figure 16.Experimental setup for evaluating the characteristics of the SAW based PM2.5 sensor.

Figure 17 .
Figure 17.Photograph of the experimental setup.

Figure 17 .
Figure 17.Photograph of the experimental setup.

Figure 18 .
Figure 18.Frequency output of the SAW based PM2.5 sensor for polystyrene latex (PSL) particle tests.

Figure 19 .
Figure 19.Sensor response comparison between the light scattering monitor and the SAW based PM2.5 sensor for PSL particles sampling.

Figure 20 .
Figure 20.SAW based PM2.5 sensor response changes with the particle mass concentration.

Figure 18 .
Figure 18.Frequency output of the SAW based PM 2.5 sensor for polystyrene latex (PSL) particle tests.

Figure 18 .
Figure 18.Frequency output of the SAW based PM2.5 sensor for polystyrene latex (PSL) particle tests.

Figure 19 .
Figure 19.Sensor response comparison between the light scattering monitor and the SAW based PM2.5 sensor for PSL particles sampling.

Figure 20 .
Figure 20.SAW based PM2.5 sensor response changes with the particle mass concentration.

Figure 19 .
Figure 19.Sensor response comparison between the light scattering monitor and the SAW based PM 2.5 sensor for PSL particles sampling.

Figure 18 .
Figure 18.Frequency output of the SAW based PM2.5 sensor for polystyrene latex (PSL) particle tests.

Figure 19 .
Figure 19.Sensor response comparison between the light scattering monitor and the SAW based PM2.5 sensor for PSL particles sampling.

Figure 20 .
Figure 20.SAW based PM2.5 sensor response changes with the particle mass concentration.

Figure 20 .
Figure 20.SAW based PM 2.5 sensor response changes with the particle mass concentration.