Characteristics of individual particles in a severe short-period haze episode induced by biomass burning in Beijing

Abstract

Atmospheric particles were investigated from a haze episode in autumn 2012 in Beijing that was caused by transported pollutants emitted from biomass burning during the crop harvest season in the North China Plain. Four samples from haze and one sample from clean atmosphere were collected in sequence by a multiple-stage cascade impactor. Based on morphology and elemental compositions, the particles were classified into five types: accumulation-mode secondary particles with and without coating, naked and core–shell soot, and other particles. The ratios of accumulation-mode secondary particles to soot containing particles were 4.0, 2.3, 1.7, 1.8, and 5.5, i.e., secondary particles in haze were proportionally less abundant than those in clean air, which was caused by the different dominant secondary formation mechanisms during hazy and clean periods. Meanwhile, the ratios of particles with coating to those without coating were 0.5, 1.0, 0.7, 0.6, and 0.2, implying that the particles in haze were likely more hygroscopic than those in clean air. In haze, the size distributions of particles were multi-modal, with main modes of approximately 0.55–0.85 μm, suggesting that the particles were from multiple sources due to the transported biomass burning plumes mixed with urban air. The size distribution was unimodal in clean air, and the mode was approximately 0.35 μm, with a mean equivalent diameter of 0.45 μm. The core–shell ratio distribution for soot particles collected in haze induced by biomass-burning aerosols was quite different from those in clean air, and soot particles were more aged in haze samples.