The main objective of this study is to investigate the formation andevolution mechanism of the regional haze in megacity Beijing by analyzing theprocess of a severe haze that occurred 20–27 September 2011. Massconcentration and size distribution of aerosol particles as well as aerosoloptical properties were concurrently measured at the Beijing urbanatmospheric environment monitoring station. Gaseous pollutants (SO,NO-NO-NO, O, CO) and meteorological parameters (windspeed, wind direction, and relative humidity) were simultaneously monitored.Meanwhile, aerosol spatial distribution and the height of planetary boundarylayer (PBL) were retrieved from the signal of satellite and LIDAR (lightdetection and ranging). Concentrations of NO, NO, SO, O,and CO observed during 23–27 September had exceeded the national ambient airquality standards for residents. The mass concentration of PMgradually accumulated during the measurement and reached at220 μg m on 26 September, and the corresponding atmosphericvisibility was only 1.1 km. The daily averaged AOD in Beijing increased from~ 0.16 at λ = 500 nm on 22 September and reached ~ 3.5on 26 September. The key factors that affected the formation and evolution ofthis haze episode were stable anti-cyclone synoptic conditions at thesurface, decreasing of the height of PBL, heavy pollution emissions fromurban area, number and size evolution of aerosols, and hygroscopic growth foraerosol scattering. This case study may provide valuable information for thepublic to recognize the formation mechanism of the regional haze event overthe megacity, which is also useful for the government to adopt scientificapproach to forecast and eliminate the occurrence of regional haze in China.
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