Abstract: Based on air quality monitoring data and conventional meteorological observations,the characteristics of a heavy haze event in Changsha region on January 27 to 29,2017 was analyzed,including the vertical and horizontal dispersion,surface humidity and dynamical conditions.The results showed that a southern branch trough deepens and moves eastwards during the pollution period,and strong southwesterly flows ahead of the trough transport water vapor from the Bay of Bengal to Changsha,which further increases air humidity in this region.Meanwhile,warm advection ahead of a ridge that continuously moves eastwards greatly impacts on the increase of temperature in the middle and lower levels of atmosphere,which is favorable for the formation of the stable atmospheric stratification.Changsha is located at the bottom of a weak high pressure and controlled by a weak saddle pattern of pressure field,leading to the development of a warm inverted surface trough.Weak winds at the surface and in the low-level atmosphere due to the weak high pressure suppress the horizontal dispersion of air pollutants and favors the surface radiation cooling at night.Stable atmospheric circulation provides steady and stable atmospheric environment for the formation of severe haze pollution.Temperature inversion and stable stratification weaken vertical turbulence exchange and turbulent convection.The weak dilution of particle favors the persistence and enhancement of this pollution event.In addition,Changsha is located between the Luoxiao mountains and the Xuefeng mountains.Such a local surrounding terrain blocks the subsequently horizontal transport of air pollutants after accumulating in Changsha region due to the descending airflows,which is also a major reason for the occurrence of this haze pollution event.

Key words: Changsha region, Severe haze pollution, Meteorological cause