An Innovative Layer on Surface Acoustic Wave Sensors Integrated in a Cascade Impactor to Optimize PM10 Detection for Air Pollution Monitoring ^†

Abstract

A cascade impactor coupled with Surface Acoustic Wave (SAW) sensors was developed in our laboratory to detect Particulate Matters with an aerodynamic diameter of less or equal to 2.5 and 10 µm (PM2.5 and PM10). To improve the collection of PM10, a layer was added to the SAW sensors, which was later on tested to check its effectiveness. These tests were carried out using polystyrene (PS) and silicon carbide (SiC) particles in order to have a representative sample of sizes. Results analysis was conducted quantitatively using a digital microscope.

1. Introduction

Air monitoring has been put on the frontline of the research field in the past decade, especially after the World Health Organization declared that air pollution lies behind more than 7 million premature deaths, notably due to the presence of Particulate Matters. PM10 and PM2.5 are particles with an aerodynamic diameter less than or equal to 10 and 2.5 µm, respectively, acquiring easy access to human lungs [1]. For this reason, a cascade impactor was developed in our laboratory consisting of stages where SAW sensors were mounted [2] and has been under continuous optimization, especially towards improving particle detection. This led us to develop an additional layer deposited on these SAW sensors. The layer works primarily by increasing the adhesion of PMs to their respective stages.

2. Materials and Methods

Experiments aimed to generate particles in a leak-proof chamber where a cascade impactor was placed. Two solutions were considered: a silicon carbide solution prepared out of 50 mg each of 5 and 7 µm SiC particles and a polystyrene solution made of 20 mg each of 3 and 8 µm of PS, both in 200 mL of distilled water. Tests with SiC particles were conducted under a PM2.5 concentration of 6 ± 1 µg/m³ and a PM10 concentration of 29 ± 1 µg/m³, while the tests with PS particles were conducted under a 6 ± 1 µg/m³ concentration of PM2.5 and a 15 ± 1 µg/m³ concentration of PM10. A Gilian^® pump, Gilair™ Plus, by SENSIDYNE Manufacturer (St. Petersburg, FL, USA) was then used to collect the generated particles inside the impactor at a 3 Lpm flowrate. The substrate used for the SAW sensors was lithium niobate and the sampling duration was 2 min for tests with SiC and 3 min for tests with PS for dimension and concentration reasons.

3. Discussion

Four tests were carried out with each solution, after which particle quantification over the sensor’s active zone was carried out. As seen in

Table 1. The number of PM10 particles collected on the PM2.5 and PM10 stages along with the efficiency of collection of PM10 particles on the PM10 stage using SiC and polystyrene particles.

Test	Sensor	% of PM10 on PM10 Stage
		SiC Particles	PS Particles
1	PM10 blank	47	57
	PM2.5 blank
2	PM10 with layer	61	48
	PM2.5 blank
3	PM10 blank	53	52
	PM2.5 with layer
4	PM10 with layer	73	43
	PM2.5 with layer

, a clear improvement in particle collection is noticed with the SiC particles going from a blank sensor placed on the PM10 stage (462 and 485 PM10) to a sensor with the new layer mounted on the same stage (857 and 754 PM10). Moreover, a lower number of PM10 particles is detected on the PM2.5 stage coupled with the layered PM10 stage (test 3 vs. 4). As for the tests with the PS solution, no improvement in the collection efficiency is noted. On the contrary, the PS seems to better adhere to the sensor’s surface where the new layer is absent (57% in test 1).

The effectiveness of the layer is proven to be selective. With SiC particles, the collection percentage of PM10 on their stage increased from 47% (test 1) to 61% (test 2) and from 53% (test 3) to 73% (test 4). This coherent increase in the collection percentages of PM10 endorses the improved particle adhesion to the sensor’s surface and the efficiency of the new layer with the SiC particles. However, the layer had no effect in improving particle collection with the polystyrene solution. This is largely related to the nature of polystyrene in accumulating charges, especially in high-velocity streams. Polystyrene does not seem to adhere to the developed layer. This conclusion underlines the nature-selective property of our new layer. Nevertheless, despite knowing that air particles act more like SiC ones, the utility of adding this layer is still evinced as a way to optimize our system’s performance. Optical characterizations also came out in agreement with our conclusions and will be presented at the conference. The layer will also be tested with an innovative Piezoelectric-on-Insulator (POI) substrate.

4. Patents

The cascade impactor with the SAW sensors was patented in 2017 (FR3050031A1) [2].

Another one is in the process of being carried out to patent the efficacy of this new layer.