On the importance of the flow field in inexpensive optical aerosol particle counting and sizing.

Optical Particle Sizers (OPSs) are widely used for measuring size distributions of particles larger than ca. 0.2 μm. To do so, they use mirrors or lenses to gather light scattered by particles passing through a focused beam, directing it to a photo-detector that produces electric pulses from the scattering events. Considering their ability to provide near real-time measurements with minimal attendance and maintenance, and to expand the networks of Particulate Matter (PM) monitoring, several manufacturers have developed low-cost and compact OPS systems. Despite that low-cost OPSs are already available in the market and employed for monitoring PM concentrations, their reported values typically deviate from those of high-end instruments, warranting further efforts to improve their performance. In this work, we designed and built a custom-made yet inexpensive OPS optical system, and studied its performance using a combination of computational and experimental methods at different flow conditions. Our results demonstrate the importance of the flow field within the OPS optical system, and how this can affect its counting and sizing ability. The overall performance of our OPS optical system is very similar to that of high-end instruments, exhibiting a counting efficiency of 50% for particles having a diameter of 320 nm, and a sizing resolution of below 15% for 500-nm particles, complying with the ISO 21501-1 and 21501-4 standards.