A Multi-angle Optical Particle Sizer to Improve In Situ PM2.5 Characterisation

SETH ARTHUR-HASTIE, Katherine Manfred, The University of York

     Abstract Number: 92
     Working Group: Instrumentation and Methods

Abstract
Anthropogenic aerosols (PM2.5) have significant effects on both atmospheric conditions and respiratory health, dependent on physical properties such as size and morphology. On-line/in situ characterisation techniques rely on an assumption of spherical particles. Off-line techniques such as electron microscopy can characterise non-spherical morphology but cannot effectively characterise properties that vary over a short timescale.

The Multi-angle Particle Sizer (MPS) is a new instrument designed to perform in situ PM2.5 analysis without the need to assume particle morphology or refractive index. The MPS follows many of the same design principles as widely used optical particle sizers (OPS) that measure aerosol size distributions using light scattered by individual particles. Like traditional OPSs, the MPS provides a simple, compact, sensitive, and inexpensive characterisation technique.

The MPS differs from conventional OPS by using three photomultiplier tubes (PMTs) to capture different angular regions of light scattering. A 445 nm laser beam scatters off a particle jet in a small spherical chamber. Portions of scattering are directed by three bi-concave lenses (48-degree arcs centred at 58.5 , 90.0 and 121.5 degrees) to the PMTs.

Absolute scattering intensities provide information on particle size, while comparisons of the amount of light scattered over different angles allows for distinction between spherical and non-spherical particles. Recent instruments have utilised this principle to characterise non-spherical particles, but the MPS’ novel modular, 3D-printed design provides the flexibility to be adapted for different applications with minimal re-design.

Based on MPS geometry, lookup tables have been produced to allow identification of typical particle morphologies using common modelling theories, including Mie scattering (spheres), T-matrix (spheroidal particles) and Rayleigh-Debye-Gans (RDG) for fractal particles. These tables are composed of absolute scattering and the relative scattering captured from the three angle ranges.

Progress in the measurement and classification of non-spherical particles in the laboratory will be presented.