Field measurement of new polarization optical particle counter capable of measuring particle shape information

Polarization optical particle counter (POPC) capable of measuring particle shape or sphericity was developed, which uses detection of polarization of light scattered by particles. Sensors that detected P and S polarization components are incorporated at scattering angle of 120 deg in addition to ordinary sensor located at the angle of 60 deg. The particle size is derived from the pulse height detected with the ordinary sensor. The size thresholds were determined by measurements of PSL standard particles. Polarization of particle is calculated as the ratio of S polarization component to the sum of all components. The POPC field observations were started in Fukuoka, Japan on 15 March 2012 and in Matsue, Japan on 28 February 2012. Air pollution with high particle number concentration was observed in Fukuoka on 11 April. In that time, the particles with polarization lower than 0.3 constituted 90% of particles in the size range from 0.5 μm to 3 μm and 70-90% of those from 3 μm to 5 μm. Most of the atmospheric aerosol particles were consisted of spherical particles. Significant Asian dust transport was observed in Fukuoka on from 31 March to 1 April. The particles with polarization lower than 0.3 constituted 80% of those from 0.5 μm to 1 μm, 55-65% of those from 1 μm to 5μm. The increase of the ratio of non-spherical particles to all particles in Asian dust event was confirmed. The POPC data were separated into size bins with polarization bins, which are totally 50 bins. The data of each bin were categorized into three groups based on correlation analysis. The groups are as follows: 1) 0.5-1 μm with 0-0.2 in polarization, 2) 0.5-1 μm with 0.2-0.8, 1-3 μm with 0.1-0.8, 3-5 μm with 0.2-0.7, and 5-10 μm with 0.3-0.5, 3) 1-10 μm with 0-0.1 in polarization. It is possible that the second group is related with dust concentration. The POPC thus is capable of measuring particle shape as well as size for each individual particle.