Technical noteDevelopment of a polarization optical particle counter capable of aerosol type classification
Introduction
Optical particle counters are widely used for continuous aerosol particle monitoring because of their ease of operation and relatively low cost (Chun et al., 2001, Iwasaka et al., 2003, Kim et al., 2004). These devices measure light scattered from an individual aerosol particle illuminated by a laser beam. The aerosol number concentration is determined from the number of scattered light pulses, while the aerosol particle size is determined from the pulse height. A general optical particle counter represents the aerosol number concentration in discrete size bins. However, these particle counters cannot distinguish between different aerosol types (i.e., Asian dust versus anthropogenic pollution). When the incident light is polarized, the polarization properties of light scattered by a particle can be used to determine the particle's shape. For example, if the particle is spherical, the particle-scattered light has the same polarization plane as the incident light. Conversely, non-spherical particles depolarize the scattered light, resulting in a decrease in the original polarization and an increase in polarization in the perpendicular plane. Previously, polarization properties of light scattered by non-spherical particles have been measured (Muñoz and Hovenier, 2011, Perry et al., 1978, Volten et al., 2001, West et al., 1997). Recently, Glen and Brooks (2013) measured optical properties of dust using the Cloud and Aerosol Spectrometer with Polarization (CASPOL).
In this study, we developed a bench-top polarization optical particle counter (POPC) that measures the P and S polarization components of particle-scattered light to individually monitor the abundance of each aerosol type, especially mineral dust such as Asian dust. The P component is horizontal (with respect to the plane of the scattering angle), and the S component lies in the vertical plane. Each aerosol particle type is classified using a combination of the particle's shape information estimated by measuring each polarization component and the particle's size. To establish the classification rule for each aerosol type, POPC measurements were taken under various atmospheric conditions. Further, we report the time series for the mass concentration of each aerosol type.
Section snippets
Design of the polarization optical particle counter
Fig. 1 shows a schematic diagram of the POPC. The optical geometry was determined using a simulation based on the Mie scattering theory for spherical polystyrene latex beads. Three parameters – the acceptance angle of the polarization detector, the plane of the incident light's polarization, and the scattering angle – were considered. The simulation results are described in Section 3.1. The light source is a 50-mW semiconductor laser of wavelength 780 nm operating in continuous-wave mode. The
Optical geometry
First, we determined the acceptance angle of the polarization detector. Fig. 3 shows the simulation results for the intensity of light scattered with a scattering angle of 120° and for the polarization ratio of the spheres scattering light at various acceptance angles of the polarization detector. Higher intensity of scattered light provides a better signal-to-noise ratio and a lower polarization ratio for sphere scattering and is thus suitable for detecting the depolarization of light
Conclusions
We developed a polarization optical particle counter (POPC) by measuring polarization properties for extracting individual particles’ shape information. The POPC measurements were conducted in Fukuoka City in 2012. The classification rule for each of three aerosol types – air pollution, mineral dust, and sea-salt particles – was determined on the basis of the POPC results. The air pollution mass concentration derived from the POPC results was correlated with black carbon concentration. In the
Acknowledgments
This research was supported by JSPS KAKENHI Grant numbers 20120006 and 24560660 and by the Sumitomo Foundation for Environmental Science.
References (12)
- et al.
Characteristic number size distribution of aerosol during Asian dust period in Korea
Atmos. Environ.
(2001) - et al.
Laboratory measurements of single light scattering by ensembles of randomly oriented small irregular particles in air. A review
J. Quant. Spectrosc. Radiat. Transf.
(2011) - et al.
HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website
(2012) - et al.
A new method for measuring optical scattering properties of atmospherically relevant dusts using the Cloud and Aerosol Spectrometer with Polarization (CASPOL)
Atmos. Chem. Phys.
(2013) Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles
(1999)- et al.
Nature of atmospheric aerosols over the desert areas in the Asian continent: chemical state and number concentration of particles measured at Dunhuang, China
Water, Air, & Soil Pollution: Focus
(2003)
Cited by (47)
Development and evaluation of an online monitoring single-particle optical particle counter with polarization detection
2024, Journal of Environmental Sciences (China)Bayesian inference approach for Full Poincaré Mueller polarimetry
2024, Optics and Laser TechnologyFree-tropospheric aerosols contribute to large aerosol optical depth at a remote, inland location with insignificant anthropogenic emissions
2023, Journal of Atmospheric and Solar-Terrestrial PhysicsSimultaneous measurements of PM<inf>1</inf> and PM<inf>10</inf> aerosol scattering properties and their relationships in urban Beijing: A two-year observation
2021, Science of the Total EnvironmentCitation Excerpt :In addition, previous studies found that the b was also sensitive to composition (Boss et al., 2004; Twardowski et al., 2011) with higher values for dust and particles from forest fires. Mineral dust particles are irregular shape relative to equivalent spherical particles (Kobayashi et al., 2014), which typically will have an increased backscattering ratio (Holland and Gagne, 1970). Higher b in spring could be also related to irregular scattering aerosol such as dust (Zhao et al., 2019).