Piezoelectric microbalance for monitoring the mass concentration of suspended particles

Abstract

Preliminary experiments prove the feasibility of a piezoelectric microbalance for measuring the mass concentration of suspended particles. The piezoelectric microbalance consists of a vibrating piezoelectric crystal driven by a standard oscillator circuit. The crystal oscillates with very high stability at its resonant mechanical frequency. A collector, such as an impactor or an electrostatic precipitator, deposits the suspended particles onto the electrode surface of the vibrating crystal. The resonant frequency shifts downward linearly with the added mass of the particles. Frequency is monitored with a standard digital counter. The time rate of change of the frequency is proportional to the mass concentration of ambient suspended particles. Experimental and theoretical mass sensitivities (for example, 5.5 Hz μg −1) are identical. Experiments demonstrate the high sensitivity and time resolution of the instrument. With an electrostatic-precipitator collector sampling at 1 1. min −1 the total mass concentration of suspended atmospheric particles (say, 100 μg m −3) is measured to within ±5 per cent in only 41 s. Experiments with an impactor collector show the diurnal variation of the mass concentration of suspended indoor particles, showing decay in the evening and a steep rise during working hours.