Polarization properties of aerosol particles over western Japan: classification, seasonal variation, and implications for air quality

Xiaole Pan,Kazuo Osada,Zhe Wang,Hiroshi Kobayashi,Shigekazu Yamamoto,Nobuo Sugimoto,Itsushi Uno,Yukari Hara,Zifa Wang

doi:10.5194/acp-16-9863-2016

Abstract

Abstract. Ground-based observation of the polarization properties of aerosol particles using a polarization optical particle counter (POPC) was made from 27 October 2013, to 31 December 2015, at a suburban site in the Kyushu area of Japan. We found that the depolarization ratio (DR, the fraction of s-polarized signal in the total backward light scattering signal) of aerosol particles showed prominent seasonal variability, with peaks in spring (0.21–0.23) and winter (0.19–0.23), and a minimum value (0.09–0.14) in summer. The aerosol compositions in both fine mode (aerodynamic diameter of particle, Dp < 2.5 µm) and coarse mode (2.5 µm < Dp < 10 µm), and the size-dependent polarization characteristics were analyzed for long-range transport dust particles, sea salt, and anthropogenic pollution-dominant aerosols. The DR value increased with increasing particle size, and DR = 0.1 was a reliable threshold value to identify the sphericity of supermicron (Dp > 1 µm) particles. Occurrence of substandard air quality days in Kyushu was closely related with mixed type (coexistence of anthropogenic pollutants and dust particles in the atmosphere), especially in winter and spring, indicating that dust events in the Asian continent played a key role in the cross-boundary transport of continental pollution. Backward trajectory analysis demonstrated that air masses originating from the western Pacific contained large amounts of spherical particles due to the influence of sea salt, especially in summer; however, for air masses from the Asian continent, the dependence of number fraction of spherical particles on air relative humidity was insignificant, indicating the predominance of less-hygroscopic substances (e.g., mineral dust), although the mass concentrations of anthropogenic pollutants were elevated.

Highlights

The East Asian region is characterized by serious regional anthropogenic pollution, due to the mass consumption of fossil fuel in China (Kurokawa et al, 2013)
The peak in the fine mode was mostly attributed to anthropogenic pollution, while the coarse mode peak (4– 8 μm) was related to mineral dust
The volume concentration of particles in both fine and coarse modes decreased significantly because the air masses mostly originated from the western Pacific Ocean where anthropogenic emissions were limited

Summary

Introduction

The East Asian region is characterized by serious regional anthropogenic pollution, due to the mass consumption of fossil fuel in China (Kurokawa et al, 2013). The environmental and climate effects of these anthropogenic and mineral dust aerosols are notably different because of their distinct chemical and physical properties, size distributions, and lifetimes in the troposphere (Pan et al, 2009). Dust aerosols can trap substantial amounts of pollutants (e.g., nitrate), forming a so-called “polluted dust” when they are transported through the planetary boundary layer (PBL) of polluted areas (Wang et al, 2002; Zhang et al, 2005, 2006). There is substantial variability in their resulting hygroscopic properties, which contributes to considerable uncertainty in predicting their climate effects with models.

Results

Discussion

Conclusion