Coupled Aerodynamic and Optical Sizing for Coarse Particles

Airborne particles are of concern due to their public health and environmental impacts. Concentrations, especially in urban areas, vary geographically and temporally, presenting a monitoring challenge. Affordable and accurate sensors that can be distributed in large numbers are required to understand their effects. While low-cost, optically-based particle sensors are currently employed, their measurement accuracy is often questionable. The uncertainty stems from the measurement dependence on particle refractive index, which is generally not known, as well as from variation in environmental conditions. Proposed here is to take an affordable, well-refined optical sensor and adapt it to incorporate aerodynamic sizing for coarse particles, roughly 1–10 µm in diameter, while maintaining optical information. Aerodynamic sizing removes the refractive index dependence of optical sizing. Further, it is the parameter of interest for lung deposition in the coarse range. As the aerodynamic size incorporates the particle density, the estimate of suspended particle mass has far fewer assumptions than that derived from optical scattering. The new system will generate both optical and aerodynamic sizing for each particle. This ‘double sizing spectroscopy’ is expected to distinguish between different types of particles. Our Phase I project demonstrated feasibility of aerodynamic sizing by means of a newly designed inlet nozzle coupled to an optical particle sensor. The measured response to monodispersed particles of varying size and density exhibited the time-of-flight response expected from numerical modeling. In Phase II we will further refine the system components, develop on-board analysis of measured pulses in real-time, and assess the robustness and efficacy of the integrated system in long-term environmental testing. It is reasonable to anticipate that the resulting instrument will weigh less than 1kg and be at least a factor of 5 less expensive than currently available research-grade instrumentation. Our instrument would be suitable for fence-line monitoring, for fixed-site measurements in a distributed network, as well as aboard mobile platforms. Furthermore, accurate coarse particle measurement is also needed by industry. In electronics manufacturing, particle contamination can compromise product quality. The ability of our sensor to provide information on particle type would be valuable to track particle sources and for their mitigation.