Measurements of Particulate Matter Mass Concentration using Quartz Crystal Microbalance and Electrostatic Particle Concentrator

Abstract

Respirable particulate matter (e.g, PM2.5 and PM10) is one of the most critical global pollutants, causing adverse effects on human health, and various instruments to measure its mass or number concentration have been developed. Of these, quartz crystal microbalance (QCM) based instruments have received much attention owing to high sensitivity, rapid response, low cost as well as portability for on-site measurements. However, there are still significant drawbacks: bounce due to poor adhesion in the inertial impaction based instruments, need for frequent cleanings of the crystal electrodes, and nonuniform distribution of collected particles, especially in the piezoelectric electrostatic precipitators, which still requires further improvement. In this study, an instrument capable of measuring the mass concentration of particulate matter and of avoiding the drawbacks of previous instruments is proposed. The instrument consists of two major components: an electrosatic particle concentrator (EPC) and a QCM. Airborne red fluorescent polymer microspheres, a representative of PM, were generated using a three-jet Collison nebulizer. A 5MHz QCM crystal was fixed inside the EPC, and the crystal electrode working as a collection electrode was wired to a voltage of -10kV, under which charged particles were attached onto the quartz crystal electrode by electrostatic force at Q = 1.2 L/min for 20 minutes. This was followed by the recording of QCM frequency change for further calculations. The experimental results showed that the resonant frequency shift of the crystal turned downward linearly (R2 = 0.9997) with the PM mass loading. Moreover, the frequency response for 30 minutes reveals no particle bouncing. Fluorescent images showed very uniform distribution of captured particles on the quartz crystal electrode. This instrument can be further improved by using higher resonance frequency QCM crystal so as to improve mass concentration sensitivity.