A modeling study of severe winter haze events in Beijing and its neighboring regions

• High PM2.5 concentration in Beijing in January 2013 was restricted within 500 m.
• Meteorology played an important role in haze formation.
• Transport pathways of haze aerosols to Beijing were revealed.
• Local sources contributed 44% of PM2.5 in Beijing during January 2013.

A Regional Air Quality Model System (RAQMS) with a series of ground observations and satellite retrievals was utilized to investigate haze events in January 2013, in which the observed maximum daily mean surface PM2.5 concentrations in Beijing exceeded 400 μg m− 3. The model predicted a monthly mean PM2.5 concentration of 200–300 μg m− 3 over southern Beijing, Tianjin, and the southern Hebei province, with most aerosols restricted within 500 m above ground. Meteorology played an important role in haze formation. Aerosols were transported into Beijing and Tianjin from the south to the north at altitudes below 1.5 km during haze episodes. Sensitivity simulations revealed that nearly 44% of surface PM2.5 in Beijing was contributed by local emissions with the remaining 56% coming from surrounding sources in terms of monthly mean and the outside contribution increased to 62% during episode 2, suggesting the important influence of regional trans-boundary transport on Beijing haze pollution.