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Four extreme haze episodes occurred in October 2014 in theNorth China Plain (NCP). To clarify the formation mechanism of hazes inautumn, strengthened observations were conducted in Beijing from 5 Octoberto 2 November. The meteorological parameters, satellite data, chemicalcompositions and optical properties of aerosols were obtained. The hazesoriginated 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 duringthe haze episode, which resulted in low surface wind speed and relativelystagnant weather. Moreover, the wind slowed down around Beijing city. Thesecondary aerosols NO3− was always higher than that ofSO42−, which indicated the motor vehicles played a more importantpart in the hazes in October 2014, even though the oxidation rate fromSO2 to SO42− was faster than that of NOx toNO3−. Sudden increases of the concentrations of organic matter,Cl− and BC (black carbon) before each haze episode implied thatregional transport of pollutants by biomass burning was important for hazeformation during autumn. A satellite map of fire points and the backwardtrajectories of the air masses also indicated this pollution source. Thedistinct decrease in the PBL (planetary boundary layer) height during fourhaze episodes restrained the vertical dispersion of the air pollutants.Water vapor also played a vital role in the formation of hazes byaccelerating the chemical transformation of secondary pollutants, leading tohygroscopic growth of aerosols and altering the thermal balance of theatmosphere

Characteristics and formation mechanism of continuous hazes in China: a case study during the autumn of 2014 in the North China Plain