Single Aerosol Particle Detection by Acoustic Impaction

A new measurement method has been developed that enables an acoustic detection of individual coarse mode particles (aerodynamic particle diameter > $1 \boldsymbol~ \mu \text{m}$ ) by impaction on a piezo transducer. The aerosol is accelerated and each momentum transfer by a particle is measured as a characteristic pulse in the transducer signal whose amplitude is directly proportional to the particle mass. The current single particle mass detection limit is approximately 50 picograms, which corresponds to an aerodynamic particle diameter of $\boldsymbol \sim 5 \boldsymbol~ \mu \text{m}$ . The measurement technique allows a direct in-situ mass measurement of single coarse mode particles that is unique because it is based on first principles, is relatively simple and less prone to measurement artefacts compared to other methods. This particle mass measurement is independent of assumptions on particle properties like shape, density or refractive index. This technology is of interest for scientific, industrial and health-related monitoring applications as different sources of atmospheric aerosols can be identified via size-resolved mass measurements. Challenges to be overcome include a further lowering of the detection limit, eliminating systematic errors from bouncing phenomena, optimizing the correct mass assignment as well as improving the robustness against sensor vibrations and acoustic noise. In addition, the complete sensor should be portable and affordable. With the future goal to detect submicron particles, coincidence and a reduction of the impactor’s cutoff diameter additionally become important issues.