Formation mechanism of continuous extreme haze episodes in the megacity Beijing, China, in January 2013

• Characteristics of continuous extreme haze episodes are clarified.
• Formation mechanism of regional haze is investigated in detail.
• Haze events were grouped into three classes.
• Relationship between PBL and haze formation was clarified

The primary objective of this study is to investigate the formation and evolution mechanism of the regional haze in Beijing by analyzing the process of a severe haze episode that occurredfrom1 to 31 January 2013. The mass concentration of PM2.5 and its chemical components were simultaneously measured at the Beijing urban atmospheric environmental monitoring station. The haze was characterized by a high frequency, a long duration, a large influential region and an extremely high PM2.5 values (> 500 μg/m3). The primary factors driving the haze formation were stationary atmospheric flows (in both vertical and horizontal directions), while a temperature inversion, a lower planetary boundary layer and a higher RH accelerated the formation of the regional haze. In one incident, the temperature inversion layer occurred at a height of 130 m above ground level, which prevented the air pollutants from being dispersed vertically. The regional transport of pollutants also played an important role in the formation of the haze. Wind from the south of Beijing increased from 58% in January 2012 to 63% in January 2013. Because the area to the south of Beijing is characterized by high industrial development, the unusual wind direction favored the regional transport of pollutants and severely exacerbated the haze. SO42−, NO3− and NH4+ are the three major water-soluble ions that contributed to the formation of the haze. The high variability in Cl− and K+ indicated that large quantities of coal combustion and biomass burning occurred during the haze.