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

Fine particulate matter (PM
_2.5
) is a major environmental health risk. Several instruments based on the quartz crystal microbalance (QCM) have been developed for PM
_2.5
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 PM
_2.5
instrument called qEPC-Snow. This instrument consists of a QCM crystal embedded in an electrostatic particle concentrator (EPC) for collection and sensing of PM
_2.5
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-
$\mu \text{m}$
polystyrene latex microspheres as PM
_2.5
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/
$\mu \text{g}$
) with R
²
= 0.9904, corresponding to 0.05667 [(Hz/min)/(
$\mu \text{g}/\text{m}^{3})$
] in mass concentration sensitivity for the PM
_2.5
representatives. The influence of particle sizes on qEPC-Snow’s frequency behaviors will also be discussed in detail.