Coupled Aerodynamic and Optical Sizing for Coarse Particles

Coarse airborne particles, roughly defined as those in the size range of 1 ľ 10 µm and larger, are important to human health and to Earthĺs climate. On urban scales, data on coarse particle characteristics are needed to evaluate human exposures. On the global scale, data are needed in models of cloud formation and Earthĺs radiation balance. Yet to date there are no affordable instruments for accurately monitoring the size and concentration of these particles. Available optical methods are inherently limited due to dependence on refractive index, a quantity which is not generally known. Another approach is aerodynamic sizing, which is precise and independent of refractive index, but existing instruments are too expensive for wide-spread monitoring. Proposed is an affordable aerodynamic particle sizing instrument that additionally provides optical characterization. Coarse particles will be sized aerodynamically. Within each aerodynamic size bin, particles will be classified based on their optical signal. The combined aerodynamic and optical information on a single particle basis will enable classification of particles, thus distinguishing soil dust from biological material of similar optical size, but lower density. This instrument will provide a robust tool for assessing accurate size distributions and concentrations of coarse particulate matter. An existing optical particle spectrometer will be modified for aerodynamic particle sizing through redesign of the inlet nozzle and signal processing. Airborne particles will be sharply accelerated, such that smaller particles follow the flow while the larger particles lag. The aerodynamic size is determined by the speed of the particles exiting the nozzle while the optical characteristics determine the intensity of scattered light. Both parameters will be captured for each particle, to provide coupled aerodynamic and optical sizing. The modified instrument will be tested with mono-disperse coarse particles to assess the precision for aerodynamic sizing of various nozzle designs, and to test the ability to distinguish particle types. Our instrument targets an Ĺaffordableĺ cost point in a compact unit while maintaining the accuracy of research-grade instruments. It will be suitable for fence- line monitoring, for fixed-site measurements in a distributed network, as well as aboard mobile platforms that travel throughout an urban area. It will provide critically needed, real-time data for assessing exposures to coarse airborne particles.