Abstract: Using the upper-level and surface meteorological observational data and daily atmospheric component monitoring data from January of 2008 to December of 2014 in Beijing,we statistically analyzed the duration,upper-level circulation pattern at 500 hPa,surface pressure field,and boundary layer structure during heavy pollution days with daily air quality above the 5 levels.The results indicate that heavy air pollution is usually accompanied with zonal circulation at 500 hPa,which counts up 58.4% of the total heavy pollution days.Heavy air pollution occurs most frequently under the weather type of low surface pressure system with convergence,with an occurrence frequency of 38.3%,followed by the rear of the surface high-pressure system,with an occurrence frequency of 18.8%.When heavy air pollution occurs in Beijing,the zonal circulation dominates at 500 hPa and the southerly warm flow dominates at 850 hPa.In addition,the surface pressure patterns include low-pressure system with convergence,the rear and bottom of high-pressure system,weak air pressure,the front of high pressure,depression trough,weak high pressure,saddle pressure field,and topographic trough over the northern China.Under such weather situations,thermal inversion,weak wind speed at low level,and high relative humidity exist in the boundary layer for a long period.Based on the aforementioned weather characteristics,a conceptual model of heavy pollution is established.

Key words: Heavy air pollution, Circulation situation, Surface pressure field, Boundary layer,Conceptual model