Long-term measurement of PM2.5 mass concentration using an electrostatic particle concentrator-based quartz crystal microbalance integrated with carbon dioxide aerosol jets for PM sensing in remote areas

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 100-nm and 2-mu m 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/mu g) with R-2 = 0.9904, corresponding to 0.05667 [(Hz/min)/(mu g/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.