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

Özet

Atmospheric particles were investigated from a haze episode in autumn 2012 in Beijing that was causedby transported pollutants emitted from biomass burning during the crop harvest season in the NorthChina Plain. Four samples from haze and one sample from clean atmosphere were collected in sequenceby a multiple-stage cascade impactor. Based on morphology and elemental compositions, the particleswere classified into five types: accumulation-mode secondary particles with and without coating, nakedand coreeshell soot, and other particles. The ratios of accumulation-mode secondary particles to sootcontaining particles were 4.0, 2.3, 1.7, 1.8, and 5.5, i.e., secondary particles in haze were proportionallyless abundant than those in clean air, which was caused by the different dominant secondary formationmechanisms during hazy and clean periods. Meanwhile, the ratios of particles with coating to thosewithout coating were 0.5, 1.0, 0.7, 0.6, and 0.2, implying that the particles in haze were likely morehygroscopic than those in clean air. In haze, the size distributions of particles were multi-modal, withmain modes of approximately 0.55e0.85 mm, suggesting that the particles were from multiple sourcesdue to the transported biomass burning plumes mixed with urban air. The size distribution was unimodalin clean air, and the mode was approximately 0.35 mm, with a mean equivalent diameter of 0.45 mm.The coreeshell ratio distribution for soot particles collected in haze induced by biomass-burning aerosolswas quite different from those in clean air, and soot particles were more aged in haze samples.