Abstract
Abstract. Four extreme haze episodes occurred in October 2014 in the North China Plain (NCP). To clarify the formation mechanism of hazes in autumn, strengthened observations were conducted in Beijing from 5 October to 2 November. The meteorological parameters, satellite data, chemical compositions and optical properties of aerosols were obtained. The hazes originated from the NCP, developing in the southwest and northeast directions, with the highest concentration of PM2.5 of 469 μg m−3 in Beijing. The NCP was dominated by a weak high pressure system during the haze episode, which resulted in low surface wind speed and relatively stagnant weather. Moreover, the wind slowed down around Beijing city. The secondary aerosols NO3− was always higher than that of SO42−, which indicated the motor vehicles played a more important part in the hazes in October 2014, even though the oxidation rate from SO2 to SO42− was faster than that of NOx to NO3−. Sudden increases of the concentrations of organic matter, Cl− and BC (black carbon) before each haze episode implied that regional transport of pollutants by biomass burning was important for haze formation during autumn. A satellite map of fire points and the backward trajectories of the air masses also indicated this pollution source. The distinct decrease in the PBL (planetary boundary layer) height during four haze episodes restrained the vertical dispersion of the air pollutants. Water vapor also played a vital role in the formation of hazes by accelerating the chemical transformation of secondary pollutants, leading to hygroscopic growth of aerosols and altering the thermal balance of the atmosphere.
Highlights
Haze is an atmospheric phenomenon composed of smog, dust, and vapor suspended in air, with a horizontal visibility lower than 10 km and an relationship between f (RH) lower than 90 % (Watson, 2002; Wu et al, 2007; Liu et al, 2013a)
The mass concentration of PM2.5 was measured by TEOM at the Beijing Normal University (BNU) site, whereas the mass concentration of gases (SO2, NO2, CO and O3) was monitored by the National Environmental Bureau, and the data were gathered from the Internet
Most studies concerning haze events in the North China Plain (NCP) were performed in the winter and summer, concluding that intense secondary formation, huge regional transport of pollutants, stationary meteorological conditions and large local emissions were most considered to be major factors leading to severe hazes
Summary
Haze is an atmospheric phenomenon composed of smog, dust, and vapor suspended in air, with a horizontal visibility lower than 10 km and an RH lower than 90 % (Watson, 2002; Wu et al, 2007; Liu et al, 2013a). Three widespread and persistent haze episodes were recorded in China in January 2013 (Yang et al, 2015; Huang et al, 2014), October 2013 and February 2014 Two of these episodes seriously influenced the NCP (one in January 2013, and the other in February 2014), which were characterized by long durations, large regions of influence and high concentrations of PM2.5 (particulate matter with an aerodynamic diameter equal or less than 2.5 μm) (Yang et al, 2015). In Beijing city, the RH in autumn is high (Zhao et al, 2011), and biomass burning in the vicinity is prevalent due to autumn harvest (Wang et al, 2014) These special conditions may result in different formation mechanisms of haze in autumn. Comprehensive measurements were conducted from 5 October to 2 November in Beijing to investigate the characteristics and specific mechanism of continuous extreme hazes in the autumn
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