Particulate matter (PM) is one of the most critical air pollutants, and various instruments have been developed to measure PM mass concentration. Of these, quartz crystal microbalance (QCM) based instruments have received much attention. However, these instruments are subject to significant drawbacks: particle bounce due to poor adhesion, need for frequent cleanings of the crystal electrode, and non-uniform distribution of collected particles. In this study, we present an electrostatic particle concentrator (EPC)-based QCM (qEPC) instrument capable of measuring the mass concentration of PM 2.5 (PM smaller than 2.5 µm), while avoiding the drawbacks. Experimental measurements showed high collection efficiencies (∼99% at 1.2 liters/min), highly uniform particle distributions for long sampling periods (up to 120 min at 50 µg/m 3 ), and high mass concentration sensitivity [0.068(Hz/min)/(µg/m 3 )]. The enhanced uniformity of particle deposition profiles and mass concentration sensitivity were made possible by the unique flow and electrical design of the qEPC instrument.

No abstract

Fine particulate matter (PM2.5) is a major environmental health risk. Several instruments based on the quartz crystal microbalance (QCM) have been developed for PM2.5 measurement because of their accurate, sensitive, real-time, and low-cost mass measurements. However, prolonged or non-uniform deposition on the quartz crystal can cause nonlinear responses between frequency shifts and mass deposition, and its frequent manual cleaning with wet sponges is required. These disable long-term measurements of the instruments, thus limiting their applications in remote areas. Herein, we present a new PM2.5 instrument called qEPC-Snow. This instrument consists of a QCM crystal embedded in an electrostatic particle concentrator (EPC) for collection and sensing of PM2.5 and a carbon dioxide aerosol (snow) jet unit for residue-free, rapid, effective, and non-destructive cleaning of the crystal. Laboratory tests were conducted with aerosolized 100nm and 2-µm polystyrene latex microspheres as PM2.5 representatives to evaluate (i) frequency responses and (ii) mass sensitiveness of qEPC-Snow, (iii) particle removal efficiencies, and (iv) reuse of the used crystals. Experimental results demonstrated high removal efficiencies (approximately 99.9% for both particle sizes) and statistical similarity between the initial and cleaned QCM crystals in the frequency shift-mass deposition relationship, thereby enabling measurement for more than one month without demounting the crystals. The mass sensitivity was 57.34 (Hz/µg) with R 2 = 0.9904, corresponding to 0.05667 [(Hz/min)/(µg/m 3 )] in mass concentration sensitivity for the PM2.5 representatives. The influence of particle sizes on qEPC-Snow's frequency behaviors will also be discussed in detail.INDEX TERMS Carbon dioxide snow cleaning, electrostatic particle concentrator, PM2.5 mass concentration measurement, quartz crystal microbalance, remote sensing